Table of contents

Solar rotational tomography is applied to almost eleven years of Large Angle Spectrometric Coronagraph C2/Solar and Heliospheric Observatory data, revealing for the first time the behavior of the large-scale coronal density structures, also known as streamers, over almost a full solar activity cycle. This study gives an overview of the main results of this project. (1) Streamers are most often shaped as extended, narrow plasma sheets. The sheets can be extremely narrow at times (⩽0.14 × 10⁶ km at 4 R_☉). This is over twice their heliocentric angular thickness at 1 AU. (2) At most times outside the height of solar maximum, there are two separate stable large helmet streamer belts extending from mid-latitudes (in both north and south). At solar minimum, the streamers converge and join near the equator, giving the impression of a single large helmet streamer. Outside of solar minimum, the two streamers do not join, forming separate high-density sheets in the extended corona (one in the north, another in the south). At solar maximum, streamers rise radially from their source regions, while during the ascending and descending activity phases, streamers are skewed toward the equator. (3) For most of the activity cycle, streamers share the same latitudinal extent as filaments on the disk, showing that large-scale stable streamers are closely linked to the same large-scale photospheric magnetic configuration, which give rise to large filaments. (4) The poleward footpoints of the streamers are often above crown polar filaments and the equatorial footpoints are above filaments or active regions (or above the photospheric neutral lines which underlie these structures). The high-density structures arising from the equatorial active regions either rise and form the equatorial footpoints of mid-latitude quiescent streamers, or form unstable streamers at the equator, not connected to the quiescent streamer structure at higher latitude (so there are often three streamer sheets sharing the same extended longitudinal region). (5) Comparison between the tomography results and a potential field source surface model shows that streamers are not necessarily associated with a magnetic polarity reversal, but rather are regions containing field lines arising from widely separated sources at the Sun. We call these convergence sheets. (6) There is considerable differential rotation of streamers at high latitudes, which makes comparison between disk and coronal structure complicated. The presence of differential rotation has implications for many areas of coronal and heliospheric research.

A calibration is made for the correlation between the X-ray Variability Amplitude (XVA) and black hole (BH) mass. The correlation for 21 reverberation-mapped Active Galactic Nuclei (AGNs) appears very tight, with an intrinsic dispersion of 0.20 dex. The intrinsic dispersion of 0.27 dex can be obtained if BH masses are estimated from the stellar velocity dispersions. We further test the uncertainties of mass estimates from XVAs for objects that have been observed multiple times with good enough data quality. The results show that the XVAs derived from multiple observations change by a factor of 3. This means that BH mass uncertainty from a single observation is slightly worse than either reverberation-mapping or stellar velocity dispersion measurements; however, BH mass estimates with X-ray data only can be more accurate if the mean XVA value from more observations is used. With this calibrated relation, the BH mass and accretion rate may be determined for a large sample of AGNs with the planned International X-ray Observatory mission. Proper interpretation of the first AGN X-ray quasi-periodic oscillation (QPO), seen in the Seyfert galaxy RE J1034+396, depends on its BH mass, which is not currently known very well. Applying this relation, the BH mass of RE J1034+396 is found to be 4⁺³₋₂ × 10⁶ M_☉. The high end of the mass range follows the relationship between the 2f₀ frequencies of high-frequency QPO and the BH masses derived from the Galactic X-ray binaries. We also calculate the high-frequency constant C = 2.37 M_☉ Hz⁻¹ from 21 reverberation-mapped AGNs. As suggested by Gierliński et al., M_BH = C/C_M, where C_M is the high-frequency variability derived from XVA. Given the similar shape of power-law dominated X-ray spectra in ultra-luminous X-ray sources (ULXs) and AGNs, this can be applied to BH mass estimates of ULXs. We discuss the observed QPO frequencies and BH mass estimates in the ULXs M82 X-1 and NGC 5408 X-1 and favor ULXs as intermediate mass BH systems.

Using public Fermi Large Area Telescope (LAT) and Swift Burst Alert Telescope observations, we constructed the first sample of blazars selected at both hard X-rays and gamma rays. Studying its spectral properties, we find a luminosity dependence of the spectral slopes at both energies. Specifically, luminous blazars, generally classified as flat spectrum radio quasars, have hard continua in the medium-hard X-ray range but soft continua in the LAT gamma-ray range (photon indices Γ_X ≲ 2 and Γ_G ≳ 2), while lower luminosity blazars, classified as BL Lacs, have opposite behavior, i.e., soft X-ray and hard gamma-ray continua (Γ_X ≳ 2.4 and Γ_G < 2). The trends are confirmed by detailed Monte Carlo simulations explicitly taking into account the observational biases of both instruments. Our results support the so-called blazar sequence which was originally based on radio samples of blazars and radio luminosities. We also argue that the X-ray-to-gamma-ray continua of blazars may provide independent insights into the physical conditions around the jet, complementing/superseding the ambiguities of the traditional classification based on optical properties.

The beamed radio emission from relativistic plasma (particles or bunches), constrained to move along the curved trajectories, occurs in the direction of velocity. We have generalized the coherent curvature radiation model to include the detailed geometry of the emission region in pulsar magnetosphere and deduced the polarization state in terms of Stokes parameters. By considering both the uniform and modulated emissions, we have simulated a few typical pulse profiles. The antisymmetric type of circular polarization survives only when there is modulation or discrete distribution in the emitting sources. Our model predicts a correlation between the polarization angle swing and sign reversal of circular polarization as a geometric property of the emission process.

We present the discovery of a bright (J = 13.1 mag) nearby L6 dwarf found in a search for L-type ultracool subdwarfs in the Sloan Digital Sky Survey (SDSS) Data Release 7. SDSS J141624.08+134826.7 exhibits blue near-infrared colors compared to other optically typed L6 objects, but its optical and near-infrared spectra do not show metal-poor features characteristic of known L-type ultracool subdwarfs. Instead, SDSS J141624.08+134826.7 is probably a nearby example of the class of L dwarfs with low condensate opacities that exhibit unusually blue near-infrared colors for a given spectral type. Its deep 1.4 and 1.9 μm H₂O absorption bands, weak 2.3 μm CO feature, strong 0.99 μm FeH band, and shallow optical TiO and CaH bands resemble the spectra of other blue L dwarfs which are believed to have unusually thin or large-grained cloud structure. The luminosity of SDSS J141624.08+134826.7 implies that it is either a high-mass brown dwarf or a low-mass star, depending on its age, and its UVW space motion suggests a thin-disk membership. With a spectrophotometric distance of 8.4 ± 1.9 pc, SDSS J141624.08+134826.7 is one of the nearest L dwarfs to the Sun and is therefore an excellent target for high resolution imaging, spectroscopic, and astrometric follow-up observations.

Two recent empirical developments in the study of extragalactic globular cluster (GC) populations are the color–magnitude relation of the blue GCs (the "blue tilt") and the nonlinearity of the dependence of optical GC colors on metallicity. The color–magnitude relation, interpreted as a mass–metallicity relation, is thought to be a consequence of self-enrichment. Nonlinear color–metallicity relations have been shown to produce bimodal color distributions from unimodal metallicity distributions. We simulate GC populations including both a mass–metallicity scaling relation and nonlinear color–metallicity relations motivated by theory and observations. Depending on the assumed range of metallicities and the width of the GC luminosity function (GCLF), we find that the simulated populations can have bimodal color distributions with a "blue tilt" similar to observations, even though the metallicity distribution appears unimodal. The models that produce these features have the relatively high mean GC metallicities and nearly equal blue and red peaks characteristic of giant elliptical galaxies. The blue tilt is less apparent in the models with metallicities typical of dwarf ellipticals; the narrower GCLF in these galaxies has an even bigger effect in reducing the significance of their color–magnitude slopes. We critically examine the evidence for nonlinearity versus bimodal metallicities as explanations for the characteristic double-peaked color histograms of giant ellipticals and conclude that the question remains open. We discuss the prospects for further theoretical and observational progress in constraining the models presented here and for uncovering the true metallicity distributions of extragalactic GC systems.

Time-resolved low resolution Hubble Space Telescope ultraviolet spectra together with ground-based optical photometry and spectra are used to constrain the temperatures and pulsation properties of six cataclysmic variables containing pulsating white dwarfs (WDs). Combining our temperature determinations for the five pulsating WDs that are several years past outburst with past results on six other systems shows that the instability strip for accreting pulsating WDs ranges from 10,500 to 15,000 K, a wider range than evident for ZZ Ceti pulsators. Analysis of the UV/optical pulsation properties reveals some puzzling aspects. While half the systems show high pulsation amplitudes in the UV compared to their optical counterparts, others show UV/optical amplitude ratios that are less than one or no pulsations at either wavelength region.

The N v emission line of active galactic nuclei shows peculiar behavior in the line–continuum correlation, which may be indicative of an extra line component in addition to that from the normal broad emission line region. In this paper, we investigate possible contribution to the N v emission via resonant scattering of both continuum and Lyα in a broad absorption line (BAL) outflow by performing the Sobolev Monte Carlo simulations. The contribution is dependent on the covering factor, optical depth, and velocity profile of the outflow, as well as the equivalent width (EW) of Lyα. Adopting model parameters constrained by observations, we find that the measured N v EW in the spectra of non-BAL quasi-stellar objects (QSOs) could have been enhanced by a factor of 1.82–2.73 on average, while there is only moderate absorption of Lyα along the BAL outflow direction. Our model can produce a relatively narrow scattering line profile. About 80% of the total scattered flux falls within the central ±4500 km s^-1. We find that the resonant scattering can produce a prominent polarized emission line around N v. Both the broad excess emission and the unusually large polarized flux observed around N v in BAL QSOs are considered as strong evidence for the scattering enhancement. Future spectropolarimetric observations and spectroscopic monitoring of luminous QSOs may offer crucial tests for this interpretation, and provide useful information on the physical and geometrical properties of QSO outflows. We argue that the scattering offers a promising and robust process for producing the peculiar behavior of N v emission compared to the other processes proposed previously.

We address the origin of the enhancement of ∼40 keV to 5 MeV ions at the solar wind termination shock. Using self-consistent two-dimensional hybrid simulations (kinetic proton, fluid electron) of a shock moving through a plasma similar to that observed in the outer heliosphere, we conclude that the observed ion enhancements are consistent with accelerated "core" interstellar pickup ions (those that have not previously undergone any significant energization) by the termination shock via a combination of shock drift acceleration and particle scattering in meandering magnetic fields in the vicinity of the shock. In addition to the consequences for our understanding of anomalous cosmic rays, this work is also relevant to the more-general long-standing problem of accelerating low-energy particles by shocks that move nearly normal to a mean magnetic field.

The highly irradiated transiting exoplanet, HAT-P-7b, currently provides one of the best opportunities for studying planetary emission in the optical and infrared wavelengths. We observe six near-consecutive secondary eclipses of HAT-P-7b at optical wavelengths with the EPOXI spacecraft. We place an upper limit on the relative eclipse depth of 0.055% (95% confidence). We also analyze Spitzer observations of the same target in the infrared, obtaining secondary eclipse depths of 0.098% ± 0.017%, 0.159% ± 0.022%, 0.245% ± 0.031%, and 0.225% ± 0.052% in the 3.6, 4.5, 5.8, and 8.0 μm IRAC bands, respectively. We combine these measurements with the recently published Kepler secondary eclipse measurement and generate atmospheric models for the dayside of the planet that are consistent with both the optical and infrared measurements. The data are best fit by models with a temperature inversion, as expected from the high incident flux. The models predict a low optical albedo of ≲0.13, with subsolar abundances of Na, K, TiO, and VO. We also find that the best-fitting models predict that 10% of the absorbed stellar flux is redistributed to the nightside of the planet, which is qualitatively consistent with the inefficient day–night redistribution apparent in the Kepler phase curve. Models without thermal inversions fit the data only at the 1.25σ level, and also require an overabundance of methane, which is not expected in the very hot atmosphere of HAT-P-7b. We also analyze the eight transits of HAT-P-7b present in the EPOXI data set and improve the constraints on the system parameters, finding a period of P = 2.2047308 ± 0.0000025 days, a stellar radius of R_⋆ = 1.824 ± 0.089 R_☉, a planetary radius of R_p = 1.342 ± 0.068 R_Jup, and an inclination of i = 85.7^+3.5_−2.2 deg.

The first detection of ammonia (NH₃) is reported from the Magellanic Clouds. Using the Australia Telescope Compact Array, we present a targeted search for the (J, K) = (1,1) and (2,2) inversion lines toward seven prominent star-forming regions in the Large Magellanic Cloud (LMC). Both lines are detected in the massive star-forming region N 159 W, which is located in the peculiar molecular ridge south of 30 Doradus, a site of extreme star formation strongly influenced by an interaction with the Milky Way halo. Using the ammonia lines, we derive a kinetic temperature of ∼16 K, which is 2–3 times below the previously derived dust temperature. The ammonia column density, averaged over ∼17'', is ∼6 × 10¹² cm⁻² (<1.5 × 10¹³ cm⁻² over 9'' in the other six sources) and we derive an ammonia abundance of ∼4 × 10⁻¹⁰ with respect to molecular hydrogen. This fractional abundance is 1.5–5 orders of magnitude below those observed in Galactic star-forming regions. The nitrogen abundance in the LMC (∼10% solar) and the high UV flux, which can photo-dissociate the particularly fragile NH₃ molecule, both must contribute to the low fractional NH₃ abundance, and we likely only see the molecule in an ensemble of the densest, best shielded cores of the LMC.

Photodissociation of CH₃NH₂⁺ has been studied using the DC sliced ion imaging technique and ab initio calculations in order to understand the formation of HCNH⁺, an important molecule in Titan's ionosphere. Our experimental and theoretical observations show that hydrogen loss from CH₃NH₂⁺ has two channels: one giving rise to the triplet species CH₃NH⁺, while the other product is CH₂NH₂⁺. The latter then decomposes further to form HCNH⁺. H₂ loss from CH₃NH₂⁺ has only one channel, yielding CH₂NH⁺. This species further loses H to form HCNH⁺. The branching ratio of the H, H₂, and H+H₂ loss channels is found to be 4.2:1:2.5. This is ascribed to the fact that, at these energies, the H loss has one stable triplet product channel, while most of the H₂ loss product further decomposes to HCNH⁺.

We observed the neutron star (NS) ultra-compact X-ray binary 4U 0614+091 quasi-simultaneously in the radio band (Very Large Array), mid-infrared (IR)/IR (Spitzer/Multiband Imaging Photometer for Spitzer and Infrared Array Camera), near-IR/optical (Small and Moderate Aperture Research Telescope System), optical–UV (Swift/UV–Optical Telescope), soft and hard X-rays (Swift/X-ray Telescope and Rossi-X-ray Timing Explorer). The source was steadily in its "hard state." We detected the source in the whole range, for the first time in the radio band at 4.86 and 8.46 GHz and in the mid-IR at 24 μm, up to 100 keV. The optically thick synchrotron spectrum of the jet is consistent with being flat from the radio to the mid-IR band. The flat jet spectrum breaks in the range ∼(1–4) × 10¹³ Hz to an optically thin power-law synchrotron spectrum with spectral index ∼−0.5. These observations allow us to estimate a lower limit on the jet radiative power of ∼3 × 10³² erg s⁻¹ and a total jet power L_J ∼ 10³⁴μ⁻¹_0.05E^0.53_c erg s⁻¹ (where E_c is the high-energy cutoff of the synchrotron spectrum in eV and μ_0.05 is the radiative efficiency in units of 0.05). The contemporaneous detection of the optically thin part of the compact jet and the X-ray tail above 30 keV allows us to assess the contribution of the jet to the hard X-ray tail by synchrotron self-Compton (SSC) processes. We conclude that, for realistic jet size, boosting, viewing angle, and energy partition, the SSC emission alone, from the post-shock, accelerated, non-thermal population in the jet, is not a viable mechanism to explain the observed hard X-ray tail of the NS 4U 0614+091.

We used magnetohydrodynamic (MHD) simulations of interstellar turbulence to study the probability distribution functions (PDFs) of increments of density, velocity, and magnetic field. We found that the PDFs are well described by a Tsallis distribution, following the same general trends found in solar wind and electron MHD studies. We found that the PDFs of density are very different in subsonic and supersonic turbulence. In order to extend this work to ISM observations, we studied maps of column density obtained from three-dimensional MHD simulations. From the column density maps, we found the parameters that fit to Tsallis distributions and demonstrated that these parameters vary with the sonic and Alfvén Mach numbers of turbulence. This opens avenues for using Tsallis distributions to study the dynamical and perhaps magnetic states of interstellar gas.

We present the analysis of the interstellar γ-ray emission measured by the Fermi Large Area Telescope toward a region in the second Galactic quadrant at 100° ⩽ l ⩽ 145° and −15° ⩽ b ⩽ +30°. This region encompasses the prominent Gould Belt clouds of Cassiopeia, Cepheus, and the Polaris flare, as well as atomic and molecular complexes at larger distances, like that associated with NGC 7538 in the Perseus arm. The good kinematic separation in velocity between the local, Perseus, and outer arms, and the presence of massive complexes in each of them, make this region well suited to probe cosmic rays (CRs) and the interstellar medium beyond the solar circle. The γ-ray emissivity spectrum of the gas in the Gould Belt is consistent with expectations based on the locally measured CR spectra. The γ-ray emissivity decreases from the Gould Belt to the Perseus arm, but the measured gradient is flatter than expectations for CR sources peaking in the inner Galaxy as suggested by pulsars. The X_CO = N(H₂)/W_CO conversion factor is found to increase from (0.87 ± 0.05) × 10²⁰ cm⁻² (K km s⁻¹)⁻¹ in the Gould Belt to (1.9 ± 0.2) × 10²⁰ cm⁻² (K km s⁻¹)⁻¹ in the Perseus arm. We derive masses for the molecular clouds under study. Dark gas, not properly traced by radio and microwave surveys, is detected in the Gould Belt through a correlated excess of dust and γ-ray emission: its mass amounts to ∼50% of the CO-traced mass.

We report the first systematic survey of molecular lines (including HCO⁺ (1–0) and ¹²CO, ¹³CO, C¹⁸O (1–0) lines at the 3 mm band) toward a new sample of 88 massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects or EGOs) identified from the Spitzer GLIMPSE survey in the northern hemisphere with the Purple Mountain Observatory 13.7 m radio telescope. By analyzing the asymmetries of the optically thick line HCO⁺ for 69 of 72 EGOs with HCO⁺ detection, we found 29 sources with "blue asymmetric profiles" and 19 sources with "red asymmetric profiles." This results in a blue excess of 0.14, seen as a signature of collapsing cores in the observed EGO sample. We found that the sources not associated with infrared dark clouds (IRDCs) show a higher blue excess (0.41) than those associated with IRDCs (−0.08), and "possible" outflow candidates show a higher blue excess (0.29) than "likely" outflow candidates (0.05). A higher blue excess (0.19) and a lower blue excess (0.07) were also measured in ultracompact H ii regions and 6.7 GHz class II methanol maser sources, respectively. These suggest that the relatively small blue excess (0.14) in our full sample is due to the fact that the observed EGOs are mostly dominated by outflows and at an earlier evolutionary phase associated with IRDCs and 6.7 GHz methanol masers. The physical properties of clouds surrounding EGOs derived from CO lines are similar to those of massive clumps wherein the massive star-forming cores associated with EGOs possibly embedded. The infall velocities and mass infall rates derived for 20 infall candidates are also consistent with the typical values found in MYSOs. Thus, our observations further support the speculation of Cyganowski et al. that EGOs trace a population with ongoing outflow activity and at the active rapid accretion stage of massive protostellar evolution from a statistical view, although there may be limitations due to a single-pointing survey with a large beam.

In this paper, we present a very important and quite controversial observational property of penumbral filaments that may exhibit twisting motions and change their chirality. Using high-resolution observations from the Solar Optical Telescope on board Hinode, we study the unwinding and twisting motions in the penumbral filaments of active region NOAA 10930. Penumbral filaments, including those to the northwest of the magnetic polarity inversion line (PIL) and those inside the PIL region, are found to make unwinding motions associated with the decrease of their right-handed twists. After unwinding, the filaments inside the PIL region are found to twist continuously to develop left-handed twists in them. Moreover, we find the filament elongating, expanding, splitting, and the screw pitch decreasing in the unwinding process and, getting shorter, shrinking, merging, and the screw pitch increasing in the twisting process. These observational results indicate that penumbral filaments are more inclined to be twisted magnetic flux tubes in nature.

We report on a series of three-dimensional magnetohydrodynamic simulations of active galactic nucleus (AGN) jet propagation in realistic models of magnetized galaxy clusters. We are primarily interested in the details of energy transfer between jets and the intracluster medium (ICM) to help clarify what role such flows could have in the reheating of cluster cores. Our simulated jets feature a range of intermittency behaviors, including intermittent jets that periodically switch on and off and one model jet that shuts down completely, naturally creating a relic plume. The ICM into which these jets propagate incorporates tangled magnetic field geometries and density substructure designed to mimic some likely features of real galaxy clusters. We find that our jets are characteristically at least 60% efficient at transferring thermal energy to the ICM. Irreversible heat energy is not uniformly distributed, however, instead residing preferentially in regions very near the jet/cocoon boundaries. While intermittency affects the details of how, when, and where this energy is deposited, all of our models generically fail to heat the cluster cores uniformly. Both the detailed density structure and nominally weak magnetic fields in the ICM play interesting roles in perturbing the flows, particularly when the jets are non-steady. Still, this perturbation is never sufficient to isotropize the jet energy deposition, suggesting that some other ingredient is required for AGN jets to successfully reheat cluster cores.

Orbital variability has been found in the X-ray hardness of the black hole candidate Cygnus X-1 during the soft/high X-ray state using light curves provided by the Rossi X-ray Timing Explorer's All-Sky Monitor. We are able to set broad limits on how the mass-loss rate and X-ray luminosity vary between the hard and soft states. The folded light curve shows diminished flux in the soft X-ray band at ϕ = 0 (defined as the time of the superior conjunction of the X-ray source). Models of the orbital variability provide slightly superior fits when the absorbing gas is concentrated in neutral clumps and better explain the strong variability in hardness. In combination with the previously established hard/low state dips, our observations give a lower limit to the mass-loss rate in the soft state ($\dot{M}<2\times 10^{-6}\, M_{\odot }$ yr⁻¹) than the limit in the hard state ($\dot{M}<4\times 10^{-6}\, M_{\odot }$ yr⁻¹). Without a change in the wind structure between X-ray states, the greater mass-loss rate during the low/hard state would be inconsistent with the increased flaring seen during the high-soft state.

We present a near-infrared extinction study of nine dense cores at evolutionary stages between starless and Class I. Our results show that the density structure of all but one observed core can be modeled with a single power law ρ ∝ r^p between ∼0.2R–R of the cores. The starless cores in our sample show two different types of density structures, one follows p ∼ −1.0 and the other follows p ∼ −2.5, while the protostellar cores all have p ∼ −2.5. The similarity between the prestellar cores with p ∼ −2.5 and protostellar cores implies that those prestellar cores could be evolving toward the protostellar stage. The slope of p ∼ −2.5 is steeper than that of an singular isothermal sphere, which may be interpreted with the evolutionary model of cores with finite mass.

We report Chandra X-ray constraints for 20 of the 52 high-redshift ultraluminous infrared galaxies (ULIRGs) identified in the Spitzer Extragalactic First Look Survey with f_ν(24 μm)>0.9 mJy, $\log ({\nu f_{\nu }(24\,\mu {\rm m})\over \nu f_{\nu }(R)})>1$, and $\log ({\nu f_{\nu }(24\,\mu {\rm m})\over \nu f_{\nu }(8\,\mu {\rm m})})>0.5$. Notably, decomposition of Spitzer mid-infrared IRS spectra for the entire sample indicates that they are comprised predominantly of weak polycyclic aromatic hydrocarbon ULIRGs dominated by hot-dust continua, characteristic of active galactic nuclei (AGNs) activity. Given their redshifts, they have AGN bolometric luminosities of ≈10⁴⁵–10⁴⁷ erg s⁻¹ comparable to powerful quasi-stellar objects (QSOs). This, coupled with their high IR-to-optical ratios and often significant silicate absorption, strongly argues in favor of these mid-IR objects being heavily obscured QSOs. Here we use Chandra observations to further constrain their obscuration. At X-ray energies, we marginally detect two ULIRGs, while the rest have only upper limits. Using the IRS-derived 5.8 μm AGN continuum luminosity as a proxy for the expected X-ray luminosities, we find that all of the observed sources must individually be highly obscured, while X-ray stacking limits on the undetected sources suggest that the majority, if not all, are likely to be at least mildly Compton-thick (N_H ≳ 10²⁴ cm⁻²). With a space density of ≈1.4 × 10⁻⁷ Mpc⁻³ at z ∼ 2, such objects imply an obscured AGN fraction (i.e., the ratio of AGNs above and below N_H = 10²² cm⁻²) of ≳1.7:1 even among luminous QSOs. Given that we do not correct for mid-IR extinction effects and that our ULIRG selection is by no means complete for obscured AGNs, we regard our constraints as a lower limit to the true obscured fraction among QSOs at this epoch. Our findings, which are based on extensive multi-wavelength constraints including Spitzer IRS spectra, should aid in the interpretation of similar objects from larger or deeper mid-IR surveys, where considerable uncertainty about the source properties remains and comparable follow-up is not yet feasible.

On 2009 January 22 numerous strong bursts were detected from the anomalous X-ray pulsar 1E 1547.0–5408. Swift/XRT and XMM-Newton/EPIC observations carried out in the following two weeks led to the discovery of three X-ray rings centered on this source. The ring radii increased with time following the expansion law expected for a short impulse of X-rays scattered by three dust clouds. Assuming different models for the dust composition and grain size distribution, we fit the intensity decay of each ring as a function of time at different energies, obtaining tight constraints on the distance of the X-ray source. Although the distance strongly depends on the adopted dust model, we find that some models are incompatible with our X-ray data, restricting to 4–8 kpc the range of possible distances for 1E 1547.0–5408. The best-fitting dust model provides a source distance of 3.91 ± 0.07 kpc, which is compatible with the proposed association with the supernova remnant G327.24–0.13, and implies distances of 2.2 kpc, 2.6 kpc and 3.4 kpc for the dust clouds, in good agreement with the dust distribution inferred by CO line observations toward 1E 1547.0–5408. However, dust distances in agreement with CO data are also obtained for a set of similarly well-fitting models that imply a source distance of ∼5 kpc. A distance of ∼4–5 kpc is also favored by the fact that these dust models are already known to provide good fits to the dust-scattering halos of bright X-ray binaries. Assuming N_H = 10²² cm⁻² in the dust cloud responsible for the brightest ring and a bremsstrahlung spectrum with kT = 100 keV, we estimate that the burst producing the X-ray ring released an energy of 10⁴⁴–10⁴⁵ erg in the 1–100 keV band, suggesting that this burst was the brightest flare without any long-lasting pulsating tail ever detected from a magnetar.

Recent observations of cosmic ray (CR) electrons from several instruments have revealed various degrees of deviation in the measured electron energy distribution from a simple power law, in the form of an excess around 0.1–1 TeV energies. An even more prominent deviation and excess has been observed in the fraction of CR positrons around 10 and 100 GeV energies. These observations have received considerable attention and many theoretical models have been proposed to explain them. The models rely on either dark matter annihilation/decay or specific nearby astrophysical sources, and involve several additional assumptions regarding dark matter distribution or particle acceleration. In this paper, we show that the observed excesses in the electron spectrum may be easily re-produced without invoking any unusual sources other than the general diffuse Galactic components of CRs. The model presented here assumes a power-law injection of electrons (and protons) by supernova remnants (SNRs), and evaluates their expected energy spectrum based on a simple kinetic equation describing the propagation of charged particles in the interstellar medium (ISM). The primary physical effect involved is the Klein–Nishina suppression of the electron cooling rate around TeV energies. With a very reasonable choice of the model parameters characterizing the local ISM, we can reproduce the most recent observations by the Fermi and HESS experiments. Interestingly, in our model the injection spectral index of CR electrons becomes comparable to, or even equal to that of CR protons. The Klein–Nishina effect may also affect the propagation of the secondary e^± pairs, and therefore modify the CR positron-to-electron ratio. We have explored this possibility by considering two mechanisms for production of e^± pairs within the Galaxy. The first is due to the decay of π^±'s produced by interaction of CR nuclei with ambient protons. The second source discussed here is due to the annihilation of the diffuse Galactic γ-rays on the stellar photon field. We find that high positron fraction increasing with energy, as claimed by the PAMELA experiment, cannot be explained in our model with the conservative set of the model parameters. We are able, however, to reproduce the PAMELA (as well as the Fermi and HESS) results assuming high values of the starlight and interstellar gas densities, which would be more appropriate for vicinities of SNRs. A possible solution to this problem may be that CRs undergo most of their interactions near their sources due to the efficient trapping in the far upstream of supernova shocks by self-generated, CR-driven turbulence.

We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope. A 1000 km s^-1 collision (t_col = 5 × 10⁶ yr) has produced a group-wide shock, and for the first time the large-scale distribution of warm molecular hydrogen emission is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 × 10⁸ M_☉ of warm H₂ spread over ∼480 kpc² and additionally report the discovery of a second major shock-excited H₂ feature, likely a remnant of previous tidal interactions. This brings the total H₂ line luminosity of the group in excess of 10⁴² erg s^-1. In the main shock, the H₂ line luminosity exceeds, by a factor of 3, the X-ray luminosity from the hot shocked gas, confirming that the H₂-cooling pathway dominates over the X-ray. [Si ii]34.82 μm emission, detected at a luminosity of 1/10th of that of the H₂, appears to trace the group-wide shock closely, and in addition, we detect weak [Fe ii]25.99 μm emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 km s^-1 < V_s < 300 km s^-1) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, polycyclic aromatic hydrocarbon and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H₂ power is driven by turbulent energy transfer from motions in a post-shocked layer which suppresses star formation. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.

With the Keck Interferometer, we have studied at 2 μm the innermost regions of several nearby, young, dust-depleted "transitional" disks. Our observations target five of the six clearest cases of transitional disks in the Taurus/Auriga star-forming region (DM Tau, GM Aur, LkCa 15, UX Tau A, and RY Tau) to explore the possibility that the depletion of optically thick dust from the inner disks is caused by stellar companions rather than the more typical planet-formation hypothesis. At the 99.7% confidence level, the observed visibilities exclude binaries with flux ratios of at least 0.05 and separations ranging from 2.5 to 30 mas (0.35–4 AU) over ≳94% of the area covered by our measurements. All targets but DM Tau show near-infrared (NIR) excess in their spectral energy distribution (SED) higher than our companion flux ratio detection limits. While a companion has previously been detected in the candidate transitional disk system CoKu Tau/4, we can exclude similar mass companions as the typical origin for the clearing of inner dust in transitional disks and of the NIR excess emission. Unlike CoKu Tau/4, all our targets show some evidence of accretion. We find that all but one of the targets are clearly spatially resolved, and UX Tau A is marginally resolved. Our data are consistent with hot material on small scales (0.1 AU) inside of and separated from the cooler outer disk, consistent with the recent SED modeling. These observations support the notion that some transitional disks have radial gaps in their optically thick material, which could be an indication for planet formation in the habitable zone (∼ a few AU) of a protoplanetary disk.

There is a large observational scatter toward low velocities in the stellar mass Tully–Fisher (TF) relation if disturbed and compact objects are included. However, this scatter can be eliminated if one replaces rotation velocity with S_0.5, a quantity that includes a velocity dispersion term added in quadrature with the rotation velocity. In this work, we use a large suite of hydrodynamic N-body galaxy merger simulations to explore a possible mechanism for creating the observed relations. Using mock observations of the simulations, we test for the presence of observational effects and explore the relationship between S_0.5 and intrinsic properties of the galaxies. We find that galaxy mergers can explain the scatter in the TF as well as the tight S_0.5–stellar mass relation. Furthermore, S_0.5 is correlated with the total central mass of a galaxy, including contributions due to dark matter.

We present an analysis of Spitzer IRS spectroscopy of 83 active galaxies from the extended 12 μm sample. We find rank correlations between several tracers of star formation which suggest that (1) the polycyclic aromatic hydrocarbon feature is a reliable tracer of star formation, (2) there is a significant contribution to the heating of the cool dust by stars, and (3) the H₂ emission is also primarily excited by star formation. The 55–90 versus 20–30 spectral index plot is also a diagnostic of the relative contribution of starburst to active galactic nuclei (AGNs). We see there is a large change in spectral index across the sample: Δα ∼ 3 for both indices. Thus, the contribution to the IR spectrum from the AGN and starburst components can be comparable in magnitude but the relative contribution also varies widely across the sample. We find rank correlations between several AGN tracers. We find correlations of the ratios [O iii]λ5007/[O iv] 26 μm and [O iii]λ5007/[Ne v] 14 μm with the silicate strength which we adopt as an orientation indicator. This suggests that some of the [O iii]λ5007 emission in these Seyferts is subject to orientation dependent obscuration as found by Haas et al. for radio galaxies and quasars. There is no correlation of [Ne v] equivalent width with the silicate 10 μm strength, indicating that the [Ne v] emission is not strongly orientation dependent. This suggests that the obscuring material (e.g., torus) is not very optically thick at 14 μm consistent with the results of Buchanan et al. We search for correlations between AGN and starburst tracers and we conclude that the AGN and starburst tracers are not correlated. This is consistent with our conclusion that the relative strength of the AGN and starburst components varies widely across the sample. Thus, there is no simple link between AGN fueling and black hole growth and star formation in these galaxies. The density diagnostic [Ne v] 14/24 μm and [S iii] 18/33 μm line ratios are consistent with the gas being near the low density limit, i.e., ∼10³ cm⁻³ for [Ne v] and n_e∼ few hundred cm⁻³ for [S iii]. The distribution of silicate 10 μm and 18 μm strengths is consistent with the clumpy torus models of Sirocky et al. We find a rank correlation between the [Ne v] 14 μm line and the 6.7 μm continuum which may be due to an extended component of hot dust. The Sy 2's with a hidden broad-line region (HBLR) have a higher ratio of AGN-to-starburst contribution to the spectral energy distribution than Sy 2's without an HBLR. This may contribute to the detection of the HBLR in polarized light. The Sy 2's with an HBLR are more similar to the Sy 1's than they are to the Sy 2's without an HBLR.

G54.1+0.3 is a young pulsar wind nebula (PWN), closely resembling the Crab, for which no thermal shell emission has been detected in X-rays. Recent Spitzer observations revealed an infrared (IR) shell containing a dozen point sources arranged in a ring-like structure, previously proposed to be young stellar objects. An extended knot of emission located in the NW part of the shell appears to be aligned with the pulsar's X-ray jet, suggesting a possible interaction with the shell material. Surprisingly, the IR spectrum of the knot resembles the spectrum of freshly formed dust in Cas A, and is dominated by an unidentified dust emission feature at 21 μm. The spectra of the shell also contain various emission lines and show that some are significantly broadened, suggesting that they originate in rapidly expanding supernova (SN) ejecta. We present the first evidence that the PWN is driving shocks into expanding SN ejecta and we propose an alternative explanation for the origin of the IR emission in which the shell is composed entirely of SN ejecta. In this scenario, the freshly formed SN dust is being heated by early-type stars belonging to a cluster in which the SN exploded. Simple dust models show that this interpretation can give rise to the observed shell emission and the IR point sources.

We present new Spitzer Infrared Spectrograph (IRS) observations of three intermediate polars: V1223 Sgr, EX Hya, and V603 Aql. We detected a strong, fading flare event from V1223 Sgr. During this event, the flux declined by a factor of 13 in 30 minutes. Given the similarity in the slope of its mid-infrared spectrum during this event to that of AE Aqr, we suggest that this event was caused by transient synchrotron emission. Thus, V1223 Sgr becomes the third cataclysmic variable known to be a synchrotron source. We were unable to confirm the mid-infrared excess noted by Harrison et al. (Paper I) for EX Hya, suggesting that this object is either not a synchrotron source, or is slightly variable. Due to a very high background, V603 Aql was not detected in the long-wavelength regions accessible to the IRS. Given the recent detection of SS Cygni at radio wavelengths during outburst, we extract archival Spitzer IRS spectra for this source obtained during two successive maxima. These spectra do not show a strong excess, but without simultaneous data at shorter wavelengths, it is not possible to determine whether there is any contribution to the mid-infrared fluxes from a synchrotron jet.

Using the Advanced Camera for Surveys on Hubble Space Telescope, we have surveyed the far-ultraviolet (FUV) and near-ultraviolet (NUV) populations in the core region of M80. The color–magnitude diagram (CMD) reveals large numbers of blue and extreme horizontal branch stars and blue stragglers, as well as ≈60 objects lying in the region of the CMD where accreting and detached white dwarf binaries are expected. Overall, the blue straggler stars are the most centrally concentrated population, with their radial distribution suggesting a typical blue straggler mass of about 1.2 M_☉. However, counterintuitively, the faint blue stragglers are significantly more centrally concentrated than the bright ones and a Kolmogorov–Smirnov test suggest only a 3.5% probability that both faint and bright blue stragglers are drawn from the same distribution. This may suggest that (some) blue stragglers get a kick during their formation. We have also been able to identify the majority of the known X-ray sources in the core with FUV bright stars. One of these FUV sources is a likely dwarf nova that was in eruption at the time of the FUV observations. This object is located at a position consistent with Nova 1860 AD, or T Scorpii. Based on its position, X-ray and UV characteristics, this system is almost certainly the source of the nova explosion. The radial distribution of the X-ray sources and of the cataclysmic variable candidates in our sample suggest masses >1 M_☉.

We present the results of searches for high-energy muon neutrinos from 41 gamma-ray bursts (GRBs) in the northern sky with the IceCube detector in its 22 string configuration active in 2007/2008. The searches cover both the prompt and a possible precursor emission as well as a model-independent, wide time window of −1 hr to +3 hr around each GRB. In contrast to previous searches with a large GRB population, we do not utilize a standard Waxman–Bahcall GRB flux for the prompt emission but calculate individual neutrino spectra for all 41 GRBs from the burst parameters measured by satellites. For all of the three time windows, the best estimate for the number of signal events is zero. Therefore, we place 90% CL upper limits on the fluence from the prompt phase of 3.7 × 10⁻³ erg cm⁻² (72 TeV–6.5 PeV) and on the fluence from the precursor phase of 2.3 × 10⁻³ erg cm⁻² (2.2–55 TeV), where the quoted energy ranges contain 90% of the expected signal events in the detector. The 90% CL upper limit for the wide time window is 2.7 × 10⁻³ erg cm⁻² (3 TeV–2.8 PeV) assuming an E⁻² flux.

We present a 2 month Suzaku X-ray monitoring of the Seyfert 1 galaxy NGC 5548. The campaign consists of seven observations (with exposure time of ∼30 ks each), separated by ∼1 week. This paper focus on the X-ray Imaging Spectrometer data of NGC 5548. We analyze the response in the opacity of the gas that forms the well-known ionized absorber in this source for ionizing flux variations. Despite variations by a factor of ∼4 in the impinging continuum, the soft X-ray spectra of the source show little spectral variations, suggesting no response from the ionized absorber. A detailed time modeling of the spectra confirms the lack of opacity variations for an absorbing component with high ionization (U_X ≈ −0.85), and high outflow velocity (v_out ≈ 1040 km s⁻¹), as the ionization parameter was found to be consistent with a constant value during the whole campaign. Instead, the models suggest that the ionization parameter of a low ionization (U_X ≈ −2.8), low velocity (v_out ≈ 590 km s⁻¹) absorbing component might be changing linearly with the ionizing flux, as expected for gas in photoionization equilibrium. However, given the lack of spectral variations among observations, we consider the variations in this component as tentative. Using the lack of variations, we set an upper limit of n_e < 2.0 × 10⁷ cm⁻³ for the electron density of the gas forming the high ionization, high velocity component. This implies a large distance from the continuum source (R>0.033 pc; R>5000R_S). If the variations in the low ionization, low velocity component are real, they imply n_e>9.8 × 10⁴ cm⁻³ and R < 3 pc. We discuss our results in terms of two different scenarios: a large-scale outflow originating in the inner parts of the accretion disk, or a thermally driven wind originating much farther out. Given the large distance of the wind, the implied mass outflow rate is also large ($\dot{M}_w > 0.08 \dot{M}_{\rm accr}$; the mass outflow is dominated by the high ionization component). The associated total kinetic energy deployed by the wind in the host galaxy (>1.2 × 10⁵⁶ erg) can be enough to disrupt the interstellar medium, possibly quenching or regulating large-scale star formation. However, the total mass and energy ejected by the wind may still be lower than the one required for cosmic feedback, even when extrapolated to quasar luminosities. Such feedback would require that we are observing the wind before it is fully accelerated.

We present a gravitational lensing and photometric study of the exceptional strong lensing system SDSS J1538+5817, identified by the Sloan Lens Advanced Camera for Survey. The lens is a luminous elliptical galaxy at redshift z_l = 0.143. Using Hubble Space Telescope public images obtained with two different filters, the presence of two background sources lensed, respectively, into an Einstein ring and a double system is ascertained. Our new spectroscopic observations, performed at the Nordic Optical Telescope, reveal unequivocally that the two sources are located at the same redshift z_s = 0.531. We investigate the total (luminous and dark) mass distribution of the lens between 1 and 4 kpc from the galaxy center by means of parametric and non-parametric lensing codes that describe the multiple images as point-like objects. Bootstrapping and Bayesian analyses are performed to determine the uncertainties on the quantities relevant to the lens mass characterization. Several disparate lensing models provide results that are consistent, given the errors, with those obtained from the best-fit model of the lens mass distribution in terms of a singular power-law ellipsoid model. In particular, the lensing models agree on: (1) reproducing accurately the observed positions of the images; (2) predicting a nearly axisymmetric total mass distribution, centered and oriented as the light distribution; (3) measuring a value of 8.11^+0.27_−0.59 × 10¹⁰ M_☉ for the total mass projected within the Einstein radius of 2.5 kpc; and (4) estimating a total mass density profile slightly steeper than an isothermal one $(\rho (r) \propto r^{-2.33^{+0.43}_{-0.20}})$. A fit of the Sloan Digital Sky Survey multicolor photometry with composite stellar population models provides a value of 20⁺¹₋₄ × 10¹⁰ M_☉ for the total mass of the galaxy in the form of stars and of 0.9^+0.1_−0.2 for the fraction of projected luminous over total mass enclosed inside the Einstein radius. By combining lensing (total) and photometric (luminous) mass measurements, we differentiate the lens mass content in terms of luminous and dark matter components. This two-component modeling, which is viable only in extraordinary systems like SDSS J1538+5817, leads to a description of the global properties of the galaxy dark matter halo. Extending these results to a larger number of lens galaxies would considerably improve our understanding of galaxy formation and evolution processes in the ΛCDM scenario.

We present new Green Bank Telescope (GBT) 21 cm neutral hydrogen (H i) observations of a complete distance-limited sample of 22 Hickson Compact Groups (HCGs) with at least four true members. We detected an average H i mass of 8 × 10⁹ M_☉ (median = 6 × 10⁹ M_☉), which is significantly larger than previous single-dish measurements. Consequently, the H i deficiencies for these HCGs have been reduced, although not completely eliminated. Spectral comparison of the GBT data with complementary Very Large Array data shows significant H i excess in the GBT spectra. The observed excess is primarily due to the high surface brightness (HSB) sensitivity of the GBT detecting diffuse, low column density H i in these groups. The excess gas forms a faint diffused neutral medium which is an intermediate stage in the evolution of HSB H i tidal debris in the intragroup medium (IGM) before it is fully ionized. The excess gas mass fraction, (M(H i)_GBT − M(H i)_VLA)/M(H i)_GBT, for our complete sample varies from 5% to 81% with an average of 36% (median = 30%). The excess gas mass fraction is highest in slightly H i deficient groups where the tidal debris has had enough time to evolve. We also find the excess gas content increases with the evolutionary phase of the group described in Verdes-Montenegro et al. Theoretical calculations indicate that an H i cloud of radius ⩾ 200 pc would survive in an IGM of 2 × 10⁶ K for more than the typical dynamical lifetime of a group. However, smaller clouds get evaporated and assimilated into the hot IGM in a much shorter timescale.

We use the publicly available subhalo catalogs from the via Lactea II simulation along with a Gpc-scale N-body simulation to understand the impact of inhomogeneous reionization on the satellite galaxy population of the Milky Way. The large-volume simulation is combined with a model for reionization that allows us to predict the distribution of reionization times for Milky Way mass halos. Motivated by this distribution, we identify candidate satellite galaxies in the simulation by requiring that any subhalo must grow above a specified mass threshold before it is reionized; after this time the photoionizing background will suppress both the formation of stars and the accretion of gas. We show that varying the reionization time over the range expected for Milky Way mass halos can change the number of satellite galaxies by roughly 2 orders of magnitude. This conclusion is in contradiction with a number of studies in the literature, and we conclude that this is a result of inconsistent application of the results of Gnedin; subtle changes in the assumptions about how reionization affects star formation in small galaxies can lead to large changes in the effect of changing the reionization time on the number of satellites. We compare our satellite galaxies to observations using both abundance matching and stellar population synthesis methods to assign luminosities to our subhalos and account for observational completeness effects. Additionally, if we assume that the mass threshold is set by the virial temperature $T_{\rm vir}= 8 \times 10^3{\;\rm K}$ we find that our model accurately matches the v_max distribution, radial distribution, and luminosity function of observed Milky Way satellites for a reionization time z_reion = 8⁺³₋₂, assuming that the via Lactea II subhalo distribution is representative of the Milky Way. This results in the presence of 540⁺¹⁰⁰₋₃₄₀ satellite galaxies.

We investigate the effects of α-element enhancement and the thermally pulsing-asymptotic giant branch (TP-AGB) stars on the surface brightness fluctuation (SBF) magnitudes and broadband colors of simple stellar populations and compare to the empirical calibrations. We consider a broad range of ages and metallicities using the recently updated Teramo BaSTI isochrones. We find that the α-element-enhanced I-band SBF magnitudes are about 0.35 mag brighter and their integrated V − I colors are about 0.02 mag redder, mostly because of oxygen-enhancement effects on the upper red giant branch and AGB. We also demonstrate, using both the Teramo BaSTI and Padova isochrones, the acute sensitivity of SBF magnitudes to the presence of TP-AGB stars, particularly in the near-IR, but in the I band as well. Empirical SBF trends therefore hold great promise for constraining this important but still highly uncertain stage of stellar evolution. In a similar vein, non-negligible disparities are found among several different models available in the literature due to intrinsic model uncertainties.

The detection of planets around very low-mass stars with the radial velocity (RV) method is hampered by the fact that these stars are very faint at optical wavelengths where the most high-precision spectrometers operate. We investigate the precision that can be achieved in RV measurements of low mass stars in the near-infrared (NIR) Y-, J-, and H-bands, and we compare it to the precision achievable in the optical assuming comparable telescope and instrument efficiencies. For early-M stars, RV measurements in the NIR offer no or only marginal advantage in comparison with optical measurements. Although they emit more flux in the NIR, the richness of spectral features in the optical outweighs the flux difference. We find that NIR measurement can be as precise as optical measurements in stars of spectral type ∼M4, and from there the NIR gains in precision toward cooler objects. We studied potential calibration strategies in the NIR finding that a stable spectrograph with a ThAr calibration can offer enough wavelength stability for m s⁻¹ precision. Furthermore, we simulate the wavelength-dependent influence of activity (cool spots) on RV measurements from optical to NIR wavelengths. Our spot simulations reveal that the RV jitter does not decrease as dramatically toward longer wavelengths as often thought. The jitter strongly depends on the details of the spots, i.e., on spot temperature and the spectral appearance of the spot. At low temperature contrast (∼200 K), the jitter shows a decrease toward the NIR up to a factor of 10, but it decreases substantially less for larger temperature contrasts. Forthcoming NIR spectrographs will allow the search for planets with a particular advantage in mid- and late-M stars. Activity will remain an issue, but simultaneous observations at optical and NIR wavelengths can provide strong constraints on spot properties in active stars.

High-quality observations of B and V light curves obtained at Las Campanas Observatory for local Type Ia Supernovae (SNe Ia) show clear evidence that SNe Ia with the same brightness decline or stretch may have systematic and independent deviations at times t ≲ 5 days before and at times t ≳ 30 days after maximum light. This suggests the existence of two independent secondary parameters that control the shape of SN Ia light curves in addition to the primary light curve parameter, stretch s or Δm₁₅. The secondary parameters may reflect two independent physical effects caused by variations in the initial carbon-to-oxygen (C/O) profile in the progenitor and the initial central density ρ_c in a carbon–oxygen white dwarf exploding as an SN Ia. Theoretical light curves of delayed detonation SN Ia models with varying progenitor masses on the main sequence, varying accretion rates, and varying primordial metallicity reproduce two morphologically different and independent types of variations in observed visual light curves. These calculations predict small variations of ≈0.05 mag in the absolute brightness of SNe Ia which are correlated with the variations of progenitor mass on the main-sequence M_MS, which changes the C/O profile, and ρ_c, which depends on the accretion rate. Such variations in real supernovae will induce systematic errors in SN Ia calibration at high redshifts. A physically motivated three-parameter, s, C/O, ρ_c, template for SNe Ia light curves might take these variations into account. Comparison between the theoretical predictions and the observational results agree qualitatively; however, the observations show variations between the B and V light curves that are not expected from the modeling and may indicate limitations in the details of the theoretical models.

We estimate axial lengths of helical parts in magnetic clouds (MCs) at 1 AU from the magnetic flux (magnetic helicity) conservation between solar active regions (ARs) and MCs with the event list of Leamon et al. Namely, considering poloidal magnetic flux (Φ_P) conservation between MCs and ARs, we estimated L_h in MCs, where L_h is the axial length of an MC where poloidal magnetic flux and magnetic twist exist. It is found that L_h is 0.01–1.25 AU in the MCs. If the cylinder flux rope picture is assumed, this result leads to a possible new picture of the cylinder model whose helical structure (namely, poloidal magnetic flux) localizes in a part of a MC.

We present high angular resolution (down to 03 ≈ 13 AU in diameter) Submillimeter Array observations of the 880 μm (340 GHz) thermal continuum emission from circumstellar dust disks in the nearby HD 98800 and Hen 3-600 multiple star systems. In both cases, the dust emission is resolved and localized around one stellar component—the HD 98800B and Hen 3-600A spectroscopic binaries—with no evidence for circum-system material. Using two-dimensional Monte Carlo radiative transfer calculations, we compare the Submillimeter Array interferometer visibilities and broadband spectral energy distributions with truncated disk models to empirically locate the inner and outer edges of both disks. The HD 98800B disk appears to be aligned with the spectroscopic binary orbit, is internally truncated at a radius of 3.5 AU, and extends to only 10–15 AU from the central stars. The Hen 3-600A disk is slightly larger, with an inner edge at ∼1 AU and an outer radius of 15–25 AU. These inferred disk structures compare favorably with theoretical predictions of their truncation due to tidal interactions with the stellar companions.

A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars. We use as our starting point the evolutionary model following the inside-out collapse of a singular isothermal sphere as presented by Young & Evans. We calculate the radiative transfer of the collapsing core throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures (L_bol, T_bol, L_bol/L_smm) to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions; we include (1) the opacity from scattering in the radiative transfer, (2) a circumstellar disk directly in the two-dimensional radiative transfer, (3) a two-dimensional envelope structure, taking into account the effects of rotation, (4) mass-loss and the opening of outflow cavities, and (5) a simple treatment of episodic mass accretion. We find that scattering, two-dimensional geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. On the other hand, we find that a cycle of episodic mass accretion similar to that predicted by recent theoretical work can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function and stellar initial mass function. Finally, the combination of outflow cavities and episodic mass accretion reduces the connection between observational class and physical stage to the point where neither of the two commonly used observational signatures (T_bol and L_bol/L_smm) can be considered reliable indicators of physical stage.

The Swift Burst Alert Telescope survey of active galactic nuclei (AGNs) is providing an unprecedented view of local AGNs (〈z〉 ≈ 0.03) and their host galaxy properties. In this paper, we present an analysis of the optical spectra of a sample of 64 AGNs from the nine month survey, detected solely based on their 14–195 keV flux. Our analysis includes both archived spectra from the Sloan Digital Sky Survey and our own observations from the 2.1 m Kitt Peak National Observatory telescope. Among our results, we include line ratio classifications utilizing standard emission line diagnostic plots, [O iii] 5007 Å luminosities, and Hβ-derived black hole masses. As in our X-ray study, we find the type 2 sources to be less luminous (in [O iii] 5007 Å and 14–195 keV luminosities) with lower accretion rates than the type 1 sources. We find that the optically classified low-ionization narrow emission line regions, H ii/composite galaxies, and ambiguous sources have the lowest luminosities, while both broad-line and narrow-line Seyferts have similar luminosities. From a comparison of the hard X-ray (14–195 keV) and [O iii] luminosities, we find that both the observed and extinction-corrected [O iii] luminosities are weakly correlated with X-ray luminosity. In a study of the host galaxy properties from both continuum fits and measurements of the stellar absorption indices, we find that the hosts of the narrow-line sources have properties consistent with late-type galaxies.

We have performed three searches for high-frequency signals in the solar neutrino flux measured by the Sudbury Neutrino Observatory, motivated by the possibility that solar g-mode oscillations could affect the production or propagation of solar ⁸B neutrinos. The first search looked for any significant peak in the frequency range 1–144 day⁻¹, with a sensitivity to sinusoidal signals with amplitudes of 12% or greater. The second search focused on regions in which g-mode signals have been claimed by experiments aboard the Solar and Heliospheric Observatory satellite, and was sensitive to signals with amplitudes of 10% or greater. The third search looked for extra power across the entire frequency band. No statistically significant signal was detected in any of the three searches.

We use mid-infrared images from the Spitzer Space Telescope Cygnus X Legacy Survey to search for stellar bow shocks (BSs), a signature of early-type "runaway" stars with high space velocities. We identify ten arc-shaped nebulae containing centrally located stars as candidate BSs. New spectroscopic observations of five stars show that all are late-O to early-B dwarfs, while one is a previously classified B0.2 giant. These stars have moderate radial velocities, differing by ΔV< 10 km s⁻¹ from members of the Cygnus OB2 Association. The spectral energy distributions (SEDs) of the other four stars are consistent with late-O to early-B dwarfs at the nominal ∼1.6 kpc distance of Cyg OB2. Our morphologically selected sample of BS candidates encompasses diverse physical phenomena. Three of the stars appear to be pre-main-sequence objects on the basis of rising SEDs in the mid-IR, and their nebulae may be photon-dominated regions illuminated by the central star but shaped by external sources such as winds from Cyg OB2. Four objects have ambiguous classification. These may be partial dust shells or bubbles. We conclude that three of the objects are probable BSs, based on their morphological similarity to analytic prescriptions. Their nebular morphologies reveal no systematic pattern of orientations that might indicate either a population of stars ejected from or large-scale hydrodynamic outflows from Cyg OB2. The fraction of runaways among OB stars near Cyg OB2 identified either by radial velocity or BS techniques is ∼0.5%, much smaller than the ∼8% estimated among field OB stars. We discuss possible reasons for this difference. We also obtained a heliocentric radial velocity for the previously known BS star, BD+43°3654, of −66.2 ± 9.4 km s⁻¹, solidifying its runaway status and implying a space velocity of 77 ± 10 km s⁻¹. We use the principles of momentum-driven BSs in conjunction with the observed sizes, BS luminosities and SEDs, and dust/polycyclic aromatic hydrocarbon emission models to arrive at a novel method for estimating stellar mass loss rates. Derived mass loss rates range between 10⁻⁷ and few ×10⁻⁶ M_☉ yr⁻¹ for the three O5V–∼B2V stars identified as generating BSs. These values are at the upper range of, but broadly consistent with, estimates from other methods. We calculate a relatively large mass loss rate of 160 × 10⁻⁶ M_☉ yr⁻¹ for O4If star BD+43°3654 using the same method.

We report molecular line and dust continuum observations, made with the Swedish-ESO Submillimeter Telescope, toward four young high-mass star-forming regions associated with highly luminous (${\cal L}> 6\times 10^5\,{\cal L}_{\odot }$) Infrared Astronomical Satellite sources (15290−5546, 15502−5302, 15567−5236, and 16060−5146). Molecular emission was mapped in three lines of CS (J = 2 → 1, 3 → 2, and 5 → 4), two lines of SiO (J = 2 → 1 and 3 → 2), two rotational transitions of CH₃OH (J_k = 3_k → 2_k and 2_k → 1_k), and in the C³⁴S(J = 3 → 2) line. In addition, single spectra at the peak position were taken in the CO(J = 1 → 0), ¹³CO(J = 1 → 0), and C¹⁸O(J = 1 → 0) lines. We find that the luminous star-forming regions are associated with molecular gas and dust structures with radii of typically 0.5 pc, masses of ∼5 × 10³ M_☉, column densities of ∼5 × 10²³ cm⁻², molecular hydrogen densities of typically ∼2 × 10⁵ cm⁻³, and dust temperatures of ∼40 K. The 1.2 mm dust continuum observations further indicate that the cores are centrally condensed, having radial density profiles with power-law indices in the range 1.9–2.3. We find that under these conditions dynamical friction by the gas plays an important role in the migration of high-mass stars toward the central core region, providing an explanation for the observed stellar mass segregation within the cores. The CS profiles show two distinct emission components: a bright component, with line widths of typically 5 km s⁻¹ (FWHM), and a weaker and wider velocity component, which typically extends up to ±13 km s⁻¹ from the ambient cloud velocity. The SiO profiles also show emission from both components, but the intensity of the pedestal feature relative to that of the bright component is stronger than for CS. The narrow SiO component is likely to trace warm ambient gas close to the recently formed massive stars, whereas the high velocity emission indicates mass outflows produced by either the expansion of the H ii regions, stellar winds, and/or collimated outflows. We find that the abundances of CS, CH₃OH, and SiO, relative to H₂, in the warm ambient gas of the massive cores are typically 4 × 10⁻⁸, 6 × 10⁻⁹, and 5 × 10⁻¹¹, respectively.

We present sensitive near-infrared (J, H, and K) imaging observations toward four luminous massive star-forming regions in the Norma Spiral Arm: G324.201+0.119, G328.307+0.432, G329.337+0.147, and G330.949−0.174. We identify three clusters of young stellar objects (YSOs) based on surface density diagnostics. We also find that sources detected only in the H and K bands and with colors corresponding to spectral types earlier than B2, are likely YSOs. We analyze the spatial distribution of stars of different masses and find signatures in two clusters of primordial mass segregation that cannot be explained as due to incompleteness effects. We show that dynamic interactions of cluster members with the dense gas from the parent core can explain the observed mass segregation, indicating that the gas plays an important role in the dynamics of young clusters.

We present accretion rates for a large number of solar-type stars in the Cep OB2 region, based on U-band observations. Our study comprises 95 members of the ∼4 Myr old cluster Tr 37 (including 20 "transition" objects (TOs)), as well as the only classical T Tauri star (CTTS) in the ∼12 Myr old cluster NGC 7160. The stars show different disk morphologies, with the majority of them having evolved and flattened disks. The typical accretion rates are about 1 order of magnitude lower than in regions aged 1–2 Myr, and we find no strong correlation between disk morphology and accretion rates. Although half of the TOs are not accreting, the median accretion rates of normal CTTS and accreting "transition" disks are similar (∼3 × 10⁻⁹ and 2 × 10⁻⁹ M_☉ yr⁻¹, respectively). Comparison with other regions suggests that the TOs observed at different ages do not necessarily represent the same type of objects, which is consistent with the fact that the different processes that can lead to reduced IR excess/inner disk clearing (e.g., binarity, dust coagulation/settling, photoevaporation, giant planet formation) do not operate on the same timescales. Accreting TOs in Tr 37 are probably suffering strong dust coagulation/settling. Regarding the equally large number of non-accreting TOs in the region, other processes, such as photoevaporation, the presence of stellar/substellar companions, and/or giant planet formation, may account for their "transitional" spectral energy distributions and negligible accretion rates.

Thermally broadened Lyα absorbers (BLAs) offer an alternative method to highly ionized metal lines for tracing the warm-hot intergalactic medium (WHIM) at T>10⁵ K. However, observing BLAs requires data of high quality and accurate continuum definition to detect the low-contrast features, and a good knowledge of the velocity structure to differentiate multiple blended components from a single broad line. Even for well-characterized absorption profiles, disentangling the thermal line width from the various thermal and non-thermal contributors to the observed line width is ambiguous. We compile a catalog of reliable BLA candidates along seven active galactic nucleus sight lines from a larger set of Lyα absorbers observed by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST). We compare our measurements based on independent reduction and analysis of the data to those published by other research groups. We examine the detailed structure of each absorber and determine a reliable line width and column density. Purported BLAs are grouped into probable (15), possible (48), and non-BLA (56) categories. Combining the first two categories, we infer a line frequency $(d{\cal N}/dz)_{\rm BLA}=18\pm 11$, comparable to observed O vi absorbers, also thought to trace the WHIM. We discuss the overlap between BLA and O vi absorbers (20%–40%) and the distribution of BLAs in relation to nearby galaxies (O vi detections in BLAs are found closer to galaxies than O vi nondetections). We assume that the line width determined through a multi-line curve of growth (COG) is a close approximation to the thermal line width. Based on 164 measured COG H i line measurements, we statistically correct the observed line widths via a Monte Carlo simulation. Gas temperature and neutral fraction $f_{\rm H\,\mathsc {i}}$ are inferred from these statistically corrected line widths and lead to a distribution of total hydrogen columns. Summing the total column density over the total observed path length, we find a BLA contribution to the closure density of Ω_BLA = 6.3^+1.1_−0.8 × 10⁻³ h⁻¹₇₀ based on 10⁴ Monte Carlo simulations of each BLA system. There are a number of critical systematic assumptions implicit in this calculation, and we discuss how each affects our results and those of previously published work. In particular, the most comparable previous study by Lehner et al. gave Ω_BLA = 3.6 × 10⁻³ h⁻¹₇₀ or 9.1 × 10⁻³ h⁻¹₇₀, depending on which assumptions were made about hydrogen neutral fraction. Taking our value, current O vi and BLA surveys can account for ∼ 20% of the baryons in the local universe while an additional ∼ 29% can be accounted for in the photoionized Lyα forest; about half of all baryons in the low-z universe are found in the intergalactic medium. Finally, we present new, high signal-to-noise ratio observations of several of the BLA candidate lines from Early Release Observations made by the Cosmic Origins Spectrograph on HST.

The Perseus galaxy cluster was observed by the MAGIC Cherenkov telescope for a total effective time of 24.4 hr during 2008 November and December. The resulting upper limits on the γ-ray emission above 100 GeV are in the range of 4.6–7.5 × 10⁻¹² cm⁻² s⁻¹ for spectral indices from −1.5 to −2.5, thereby constraining the emission produced by cosmic rays, dark matter annihilations, and the central radio galaxy NGC 1275. Results are compatible with cosmological cluster simulations for the cosmic-ray-induced γ-ray emission, constraining the average cosmic ray-to-thermal pressure to <4% for the cluster core region (<8% for the entire cluster). Using simplified assumptions adopted in earlier work (a power-law spectrum with an index of −2.1, constant cosmic ray-to-thermal pressure for the peripheral cluster regions while accounting for the adiabatic contraction during the cooling flow formation), we would limit the ratio of cosmic ray-to-thermal energy to E_CR/E_th < 3%. Improving the sensitivity of this observation by a factor of about 7 will enable us to scrutinize the hadronic model for the Perseus radio mini-halo: a non-detection of γ-ray emission at this level implies cosmic ray fluxes that are too small to produce enough electrons through hadronic interactions with the ambient gas protons to explain the observed synchrotron emission. The upper limit also translates into a level of γ-ray emission from possible annihilations of the cluster dark matter (the dominant mass component) that is consistent with boost factors of ∼10⁴ for the typically expected dark matter annihilation-induced emission. Finally, the upper limits obtained for the γ-ray emission of the central radio galaxy NGC 1275 are consistent with the recent detection by the Fermi-LAT satellite. Due to the extremely large Doppler factors required for the jet, a one-zone synchrotron self-Compton model is implausible in this case. We reproduce the observed spectral energy density by using the structured jet (spine-layer) model which has previously been adopted to explain the high-energy emission of radio galaxies.

The quantities and wavelength dependencies of the dust extinction along the lines of sight toward 33 nearby gamma-ray bursts (GRBs) with redshifts z < 2 are derived from fitting their afterglow spectral energy distributions. Unlike previous studies which often assume a specific extinction law like that of the Milky Way (MW) and the Large and Small Magellanic Clouds (LMC/SMC), our approach—we call it the "Drude" approach—is more flexible in determining the true wavelength dependence of the extinction (while the shape of the extinction curve inferred from that relying on a priori assumption of a template extinction law is, of course, fixed). The extinction curves deduced from the Drude approach display a wide diversity of shapes, ranging from relatively flat curves to curves which are featureless and steeply rise toward the far-ultraviolet, and from curves just like that of the MW, LMC, and SMC to curves resembling that of the MW and LMC but lacking the 2175 Å bump. The visual extinction A_V derived from the Drude approach is generally larger by a factor of ∼2–5 than that inferred by assuming a SMC-type template extinction law. Consistent with previous studies, the extinction-to-gas ratio is mostly smaller than that of the MW, and does not seem to correlate with the shape of the extinction curve. It is shown that the standard silicate–graphite interstellar grain model closely reproduces the extinction curves of all 33 GRBs host galaxies. For these 33 bursts at z < 2, we find no evidence for the evolution of the dust extinction, dust sizes, and relative abundances of silicate to graphite on redshifts.

In this paper, we present systematic studies on the B-, R- and K_s-band luminosity–metallicity (L–Z) relations for a set of metal-poor, blue compact dwarf galaxies (BCDs). Metallicity is derived by using both the empirical N2 and the direct T_e methods. Our work reconciles contradictory results obtained by different authors and shows that the L–Z relationship does also hold for BCDs. The empirical N2-based slope of the L–Z relation, for each photometric band, is consistent with the T_e-based one. We confirm that the slope of the L–Z relation is shallower in the near-infrared than that in the optical. Our investigations on the correlations between the L_B–Z relation residuals and different galactic parameters show that the star formation activities could be a cause of the large scatter in the optical L–Z relationships, whereas the internal absorption might be another possible contributing factor.

The origins of the hot solar corona and the supersonically expanding solar wind are still the subject of debate. A key obstacle in the way of producing realistic simulations of the Sun–heliosphere system is the lack of a physically motivated way of specifying the coronal heating rate. Recent one-dimensional models have been found to reproduce many observed features of the solar wind by assuming the energy comes from Alfvén waves that are partially reflected, then dissipated by magnetohydrodynamic turbulence. However, the nonlocal physics of wave reflection has made it difficult to apply these processes to more sophisticated (three-dimensional) models. This paper presents a set of robust approximations to the solutions of the linear Alfvén wave reflection equations. A key ingredient of the turbulent heating rate is the ratio of inward-to-outward wave power, and the approximations developed here allow this to be written explicitly in terms of local plasma properties at any given location. The coronal heating also depends on the frequency spectrum of Alfvén waves in the open-field corona, which has not yet been measured directly. A model-based assumption is used here for the spectrum, but the results of future measurements can be incorporated easily. The resulting expression for the coronal heating rate is self-contained, computationally efficient, and applicable directly to global models of the corona and heliosphere. This paper tests and validates the approximations by comparing the results to exact solutions of the wave transport equations in several cases relevant to the fast and slow solar wind.

We present the two- and three-point real space correlation functions of the five-year Wilkinson Microwave Anisotropy Probe (WMAP) sky maps and compare the observed functions to simulated ΛCDM concordance model ensembles. In agreement with previously published results, we find that the temperature correlation functions are consistent with expectations. However, the pure polarization correlation functions are acceptable only for the 33 GHz band map; the 41, 61, and 94 GHz band correlation functions all exhibit significant large-scale excess structures. Further, these excess structures very closely match the correlation functions of the two (synchrotron and dust) foreground templates used to correct the WMAP data for galactic contamination, with a cross-correlation statistically significant at the 2σ–3σ confidence level. The correlation is slightly stronger with respect to the thermal dust template than with the synchrotron template.

Infrared-Faint Radio Sources (IFRSs) are a class of radio objects found in the Australia Telescope Large Area Survey which have no observable mid-infrared counterpart in the Spitzer Wide-area Infrared Extragalactic (SWIRE) survey. The extended Chandra Deep Field South now has even deeper Spitzer imaging (3.6–70 μm) from a number of Legacy surveys. We report the detections of two IFRS sources in IRAC images. The non-detection of two other IFRSs allows us to constrain the source type. Detailed modeling of the spectral energy distribution of these objects shows that they are consistent with high-redshift (z ≳ 1) active galactic nuclei.

We present new identifications of infrared counterparts to the population of hard X-ray sources near the Galactic center detected by the Chandra X-ray Observatory. We have spectroscopically confirmed 16 new massive stellar counterparts to the X-ray population, including nitrogen-type (WN) and carbon-type (WC) Wolf–Rayet stars, and O supergiants. These discoveries increase the total sample of massive stellar X-ray sources in the Galactic center region to 30 (possibly 31). For the majority of these sources, the X-ray photometry is consistent with thermal emission from plasma having temperatures in the range of kT = 1–8 keV or non-thermal emission having power-law indices in the range of −1 ≲ Γ ≲ 3, and X-ray luminosities in the range of L_X ∼ 10³²–10³⁴ erg s⁻¹ (0.5–8.0 keV). Several sources have exhibited X-ray variability of several factors between observations. These X-ray properties are not a ubiquitous feature of single massive stars but are typical of massive binaries, in which the high-energy emission is generated by the collision of supersonic winds, or by accretion onto a compact companion. However, without direct evidence for companions, the possibility of intrinsic hard X-ray generation from single stars cannot be completely ruled out. The spectral energy distributions of these sources exhibit significant infrared excess, attributable to free–free emission from ionized stellar winds, supplemented by hot dust emission in the case of the WC stars. With the exception of one object located near the outer regions of the Quintuplet cluster, most of the new stars appear isolated or in loose associations. Seven hydrogen-rich WN and O stars are concentrated near the Sagittarius B H ii region, while other similar stars and more highly evolved hydrogen-poor WN and WC stars lie scattered within ≈50 pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable of generating the observed X-rays and the implications these stars have for massive star formation in the Galaxy's Central Molecular Zone.

In order to study the distribution of gas and ionizing radiation around η Car and their implications for its likely companion star, we have examined high-excitation emission lines of [Ne iii], [Fe iii], etc., in spectra obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph instrument during 1998–2004. Our principal results, some of them unexpected, are as follows. (1) The high-excitation fluxes varied systematically and non-trivially throughout η Car's 5.5 year spectroscopic cycle. Instead of rising to a plateau after the 1998 "event," they changed continuously with a maximum in mid-cycle. (2) At one significant location a brief, strong secondary maximum occurred just before the 2003.5 spectroscopic event. (3) These emission lines are strongly concentrated at the "Weigelt knots" several hundred AU northwest of the star. With less certainty, [Ne iii] appears to be somewhat more concentrated than [Fe iii]. (4) A faster, blueshifted component of each feature appears concentrated near the star and elongated perpendicular to the system's bipolar axis. This structure may be related to the equatorial outflow and/or to dense material known to exist along our line of sight to the star. (5) Using the photoionization program Cloudy, we estimated the range of parameters for the hot secondary star that would give satisfactory high-excitation line ratios in the ejecta. T_eff ≈ 39, 000 K and L ∼ 4 × 10⁵ L_☉, for example, would be satisfactory. The allowed region in parameter space is wider (and mostly less luminous) than some previous authors suggested.

The possible role of radio active galactic nucleus (AGN) "feedback" in conventional hierarchical cosmological models has become widely discussed. This paper examines some of the details of how such feedback might work. A basic requirement is the conversion of radio AGN outflow energy into heating of the circumgalactic medium in a time comparable to the relevant cooling times. First, the class of radio AGN relevant to this process is identified as FR-I radio sources. Second, it is argued via comparisons with experimental data that these AGN outflows are strongly decelerated and become fully turbulent sonic or subsonic flows due to their interaction with the surrounding medium. Using this, a three-dimensional time-dependent calculation of the evolution of such turbulent magnetohydrodynamic flows is made to determine the time scale required for conversion of the turbulent energy into heat. This calculation, when coupled with observational data, suggests that the onset of heating can occur ∼10⁸ yr after the fully turbulent flow is established, and this time is less than or comparable to the local cooling times in the interstellar or circumgalactic medium for many of these objects. The location of where heat deposition occurs remains uncertain, but estimates of outflow speeds suggest that heating may occur many tens of kpc from the center of the parent galaxy. Recent observations suggest that such radio AGN outflows may become dispersed on much larger scales than previously thought, thus possibly satisfying the requirement that heating occurs over a large fraction of the volume occupied by the circumgalactic gas.

We confirm our earlier tentative detection of M31* in X-rays and measure its light curve and spectrum. Observations in 2004–2005 find M31* rather quiescent in the X-ray and radio. However, X-ray observations in 2006–2007 show M31* to be highly variable at times. A separate variable X-ray source is found near P1, the brighter of the two optical nuclei. The apparent angular Bondi radius of M31* is the largest of any black hole and large enough to be well resolved with Chandra. The diffuse emission within this Bondi radius is found to have an X-ray temperature ∼0.3 keV and density 0.1 cm⁻³, indistinguishable from the hot gas in the surrounding regions of the bulge given the statistics allowed by the current observations. The X-ray source at the location of M31* is consistent with a point source and a power-law spectrum with energy slope 0.9 ± 0.2. Our identification of this X-ray source with M31* is based solely on positional coincidence.

We present the results of a study on the kiloparsec-scale radio emission in the complete flux density limited MOJAVE sample, comprising 135 radio-loud active galactic nuclei. New 1.4 GHz Very Large Array (VLA) radio images of six quasars and previously unpublished images of 21 blazars are presented, along with an analysis of the high-resolution (VLA A-array) 1.4 GHz emission for the entire sample. While extended emission is detected in the majority of the sources, about 7% of the sources exhibit only radio core emission. We expect more sensitive radio observations, however, to detect faint emission in these sources, as we have detected in the erstwhile "core-only" source, 1548+056. The kiloparsec-scale radio morphology varies widely across the sample. Many BL Lac objects exhibit extended radio power and kiloparsec-scale morphology typical of powerful FRII jets, while a substantial number of quasars possess radio powers intermediate between FRIs and FRIIs. This poses challenges to the simple radio-loud unified scheme, which links BL Lac objects to FRIs and quasars to FRIIs. We find a significant correlation between extended radio emission and parsec-scale jet speeds: the more radio powerful sources possess faster jets. This indicates that the 1.4 GHz (or low-frequency) radio emission is indeed related to jet kinetic power. Various properties such as extended radio power and apparent parsec-scale jet speeds vary smoothly between different blazar subclasses, suggesting that, at least in terms of radio jet properties, the distinction between quasars and BL Lac objects, at an emission-line equivalent width of 5 Å, is essentially an arbitrary one. While the two blazar subclasses display a smooth continuation in properties, they often reveal differences in the correlation test results when considered separately. This can be understood if, unlike quasars, BL Lac objects do not constitute a homogeneous population, but rather include both FRI and FRII radio galaxies for their parent population. It could also just be due to small number statistics. We find that the ratio of the radio core luminosity to the k-corrected optical luminosity (R_v) appears to be a better indicator of orientation for this blazar sample than the traditionally used radio core prominence parameter (R_c). Based on the assumption that the extended radio luminosity is affected by the kiloparsec-scale environment, we define the ratio of extended radio power to absolute optical magnitude (L_ext/M_abs) as a proxy for environmental effects. Trends with this parameter suggest that the parsec-scale jet speeds and the parsec-to-kiloparsec jet misalignments are not affected by the large-scale environment, but are more likely to depend upon factors intrinsic to the active galactic nucleus, or its local parsec-scale environment. The jet speeds could, for instance, be related to the black hole spins, while jet misalignments could arise due to the presence of binary black holes, or kicks imparted to black holes via black hole mergers, consistent with both radio morphologies resembling precessing jet models observed in some MOJAVE blazars and the signature of a 90° bump in the jet misalignment distribution, attributed to low-pitch helical parsec-scale jets in the literature. We suggest that some of the extremely misaligned MOJAVE blazar jets could be "hybrid" morphology sources, with an FRI jet on one side and an FRII jet on the other. It is tempting to speculate that environmental radio boosting (as proposed for Cygnus A) could be responsible for blurring the Fanaroff–Riley dividing line in the MOJAVE blazars, provided a substantial fraction of them reside in dense (cluster) environments.

Gravitational interactions between galaxies are widely believed to be the principal mechanism responsible for triggering non-thermal activity in galactic nuclei. We investigate the connection between interacting galaxies and active galactic nuclei (AGNs) in the local universe by comparing the clustering properties of their environments on 0.5 Mpc scales, as quantified by the amplitude of the spatial cross-correlation function. If a direct evolutionary relationship exists, the samples should be situated in environments that are statistically similar. It was previously found that a sample of 33 Seyfert galaxies with z ⩽ 0.05 is located in environments comparable to those of isolated field galaxies. The analysis presented here reveals that a well-matched sample of 52 strongly interacting galaxy systems are preferentially situated in regions more consistent with an Abell Richness Class between 0 and 1. The apparent dissimilarity in the environments thus provides a strong argument against a causal link between major galaxy interactions and Seyfert activity. In contrast, we find that the environments of luminous quasars with z ⩽ 1 exhibit a range of richness levels that are more consistent with the interacting galaxies, suggesting that these objects could be triggered by interactions. Together the results presented here indicate that the relative importance of different mechanisms in initiating nuclear activity may vary with AGN luminosity.

We analyze the X-ray scattering halos around three Galactic Anomalous X-ray Pulsars in order to constrain the distance and the optical extinction of each source. We obtain surface brightness distributions from EPIC-pn data obtained with XMM-Newton, compare the profiles of different sources, and fit them with a model based on the standard theory of X-ray scattering by dust grains, both for a uniform distribution of dust along the line of sight, and for dust distributions constrained by previous measurements. Somewhat surprisingly, we find that for all three sources, the uniform distribution reproduces the observed surface brightness as well as or better than the distributions that are informed by previous constraints. Nevertheless, the inferred total dust columns are robust, and serve to confirm that previous measurements based on interstellar edges in high-resolution X-ray spectra and on modeling of broadband X-ray spectra were reliable. Specifically, we find A_V ≃ 4, 6, and 8 mag for 4U 0142+61, 1E 1048.1 − 5937, and 1RXS J170849.0 − 400910, respectively. For 1E 1048.1 − 5937, this is well in excess of the extinction expected toward an H i bubble along the line of sight, thus casting further doubt on the suggested association with the source.

The abundance of potassium is derived from X-ray lines observed during flares by the RESIK instrument on the solar mission CORONAS-F between 3.53 Å and 3.57 Å. The lines include those emitted by He-like K and Li-like K dielectronic satellites, which have been synthesized using the chianti atomic code and newly calculated atomic data. There is good agreement between observed and synthesized spectra, and the theoretical behavior of the spectra with varying temperature estimated from the ratio of the two GOES channels is correctly predicted. The observed fluxes of the He-like K resonance line per unit emission measure give log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9. This is higher than photospheric abundance estimates by a factor 5.5, a slightly greater enhancement than for other elements with first ionization potential (FIP) less than ∼10 eV. There is, then, the possibility that enrichment of low-FIP elements in coronal plasmas depends weakly on the value of the FIP which for K is extremely low (4.34 eV). Our work also suggests that fractionation of elements to form the FIP effect occurs in the low chromosphere rather than higher up, as in some models.

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope discovered a rapid (∼5 days duration), high-energy (E > 100 MeV) gamma-ray outburst from a source identified with the blazar PKS 1502+106 (OR 103, S3 1502+10, z = 1.839) starting on 2008 August 5 (∼23 UTC, MJD 54683.95), and followed by bright and variable flux over the next few months. Results on the gamma-ray localization and identification, as well as spectral and temporal behavior during the first months of the Fermi all-sky survey, are reported here in conjunction with a multiwaveband characterization as a result of one of the first Fermi multifrequency campaigns. The campaign included a Swift ToO (followed up by a 16 day observation on August 7–22, MJD 54685–54700), VLBA (within the MOJAVE program), Owens Valley Radio Observatory (OVRO) 40 m, Effelsberg-100 m, Metsähovi-14 m, RATAN-600, and Kanata–Hiroshima radio/optical observations. Results from the analysis of archival observations by INTEGRAL, XMM-Newton, and Spitzer space telescopes are reported for a more complete picture of this new gamma-ray blazar. PKS 1502+106 is a sub-GeV peaked, powerful flat spectrum radio quasar (luminosity at E > 100 MeV, L_γ, is about 1.1 × 10⁴⁹ erg s⁻¹, and black hole mass likely close to 10⁹ M_☉), exhibiting marked gamma-ray bolometric dominance, in particular during the asymmetric outburst (L_γ/L_opt ∼ 100, and 5 day averaged flux F_{E > 100 MeV} = 2.91 ± 1.4 × 10⁻⁶ ph cm⁻² s⁻¹), which was characterized by a factor greater than 3 of flux increase in less than 12 hr. The outburst was observed simultaneously from optical to X-ray bands (F_{0.3 − 10 keV} = 2.18^+0.15_−0.12 × 10⁻¹² erg cm⁻² s⁻¹, and hard photon index ∼1.5, similar to past values) with a flux increase of less than 1 order of magnitude with respect to past observations, and was likely controlled by Comptonization of external-jet photons produced in the broad-line region (BLR) in the gamma-ray band. No evidence of a possible blue bump signature was observed in the optical–UV continuum spectrum, while some hints for a possible 4 day time lag with respect to the gamma-ray flare were found. Nonetheless, the properties of PKS 1502+106 and the strict optical/UV, X-, and gamma-ray cross-correlations suggest the contribution of the synchrotron self-Compton (SSC), in-jet, process should dominate from radio to X-rays. This mechanism may also be responsible for the consistent gamma-ray variability observed by the LAT on longer timescales, after the ignition of activity at these energies provided by the BLR-dissipated outburst. Modulations and subsequent minor, rapid flare events were detected, with a general fluctuation mode between pink-noise and a random-walk. The averaged gamma-ray spectrum showed a deviation from a simple power law, and can be described by a log-parabola curved model peaking around 0.4–0.5 GeV. The maximum energy of photons detected from the source in the first four months of LAT observations was 15.8 GeV, with no significant consequences on extragalactic background light predictions. A possible radio counterpart of the gamma-ray outburst can be assumed only if a delay of more than three months is considered on the basis of opacity effects at cm and longer wavelengths. The rotation of the electric vector position angle observed by VLBA from 2007 to 2008 could represent a slow field ordering and alignment with respect to the jet axis, likely a precursor feature of the ejection of a superluminal radio knot and the high-energy outburst. This observing campaign provides more insight into the connection between MeV–GeV flares and the moving, polarized structures observed by the VLBI.

The MAGIC collaboration has searched for high-energy gamma-ray emission of some of the most promising pulsar candidates above an energy threshold of 50 GeV, an energy not reachable up to now by other ground-based instruments. Neither pulsed nor steady gamma-ray emission has been observed at energies of 100 GeV from the classical radio pulsars PSR J0205+6449 and PSR J2229+6114 (and their nebulae 3C58 and Boomerang, respectively) and the millisecond pulsar PSR J0218+4232. Here, we present the flux upper limits for these sources and discuss their implications in the context of current model predictions.

We present a study of the centroid frequencies and phase lags of quasi-periodic oscillations (QPOs) as functions of photon energy for GRS 1915+105. It is found that the centroid frequencies of the 0.5–10 Hz QPOs and their phase lags are both energy dependent, and there exists an anticorrelation between the QPO frequency and phase lag. These new results challenge the popular QPO models, because none of them can fully explain the observed properties. We suggest that the observed QPO phase lags are partially due to the variation of the QPO frequency with energy, especially for those with frequency higher than 3.5 Hz.

We present high-resolution continuum images of the W51e2 complex processed from archival data of the Submillimeter Array (SMA) at 0.85 and 1.3 mm and the Very Large Array at 7 and 13 mm. We also made line images and profiles of W51e2 for three hydrogen radio recombination lines (RRLs; H26α, H53α, and H66α) and absorption of two molecular lines of HCN(4–3) and CO(2–1). At least four distinct continuum components have been detected in the 3'' region of W51e2 from the SMA continuum images at 0.85 and 1.3 mm with resolutions of 03 × 02 and 14 × 07, respectively. The west component, W51e2-W, coincides with the ultracompact H ii region reported from previous radio observations. The H26α line observation reveals an unresolved hyper-compact ionized core (<006 or <310 AU) with a high electron temperature of 1.2 × 10⁴ K, with the corresponding emission measure EM>7 × 10¹⁰ pc cm^-6 and the electron density N_e>7 × 10⁶ cm⁻³. The inferred Lyman continuum flux implies that the H ii region W51e2-W requires a newly formed massive star, an O8 star or a cluster of B-type stars, to maintain the ionization. W51e2-E, the brightest component at 0.85 mm, is located 09 east from the hyper-compact ionized core. It has a total mass of ∼140 M_☉ according to our spectral energy distribution analysis and a large infall rate of >1.3 × 10⁻³ M_☉ yr⁻¹ inferred from the absorption of HCN. W51e2-E appears to be the accretion center in W51e2. Given the fact that no free–free emission and no RRLs have been detected, the massive core of W51e2-E appears to host one or more growing massive proto-stars. Located 2'' northwest from W51e2-E, W51e2-NW is detected in the continuum emission at 0.85 and 1.3 mm. No continuum emission has been detected at λ⩾ 7 mm. Along with the maser activities previously observed, our analysis suggests that W51e2-NW is at an earlier phase of star formation. W51e2-N is located 2'' north of W51e2-E and has only been detected at 1.3 mm with a lower angular resolution (∼1''), suggesting that it is a primordial, massive gas clump in the W51e2 complex.

We use the data of the Wisconsin Hα Mapper (WHAM) to test the hypothesis of whether the amplitudes and spectra of density fluctuations measured by WHAM can be matched to the data obtained for interstellar scintillations and scattering. To do this, first of all, we adjusted the mean level of signal in the adjacent patches of the data. Then assuming that the spectrum is Kolmogorov, we successfully matched the amplitudes of turbulence obtained from the WHAM data and the interstellar density fluctuations reported in the existing literature. As a result, we conclude that the existing data is consistent with the Kolmogorov cascade which spans from 10⁶ to 10¹⁷ m.

We present results from Suzaku and Swift observations of the nearby radio galaxy 3C 33, and investigate the nature of absorption, reflection, and jet production in this source. We model the 0.5–100 keV nuclear continuum with a power law that is transmitted either through one or more layers of pc-scale neutral material, or through a modestly ionized pc-scale obscurer. The standard signatures of reflection from a neutral accretion disk are absent in 3C 33: there is no evidence of a relativistically blurred Fe Kα emission line, and no Compton reflection hump above 10 keV. We find the upper limit to the neutral reflection fraction is R < 0.41 for an e-folding energy of 1 GeV. We observe a narrow, neutral Fe Kα line, which is likely to originate at least 2000 R_s from the black hole. We show that the weakness of reflection features in 3C 33 is consistent with two interpretations: either the inner accretion flow is highly ionized, or the black-hole spin configuration is retrograde with respect to the accreting material.

A Bayesian network approach for short-term solar flare level prediction has been proposed based on three sequences of photospheric magnetic field parameters extracted from Solar and Heliospheric Observatory/Michelson Doppler Imager longitudinal magnetograms. The magnetic measures, the maximum horizontal gradient, the length of neutral line, and the number of singular points do not have determinate relationships with solar flares, so the solar flare level prediction is considered as an uncertainty reasoning process modeled by the Bayesian network. The qualitative network structure which describes conditional independent relationships among magnetic field parameters and the quantitative conditional probability tables which determine the probabilistic values for each variable are learned from the data set. Seven sequential features—the maximum, the mean, the root mean square, the standard deviation, the shape factor, the crest factor, and the pulse factor—are extracted to reduce the dimensions of the raw sequences. Two Bayesian network models are built using raw sequential data (BN_R) and feature extracted data (BN_F), respectively. The explanations of these models are consistent with physical analyses of experts. The performances of the BN_R and the BN_F appear comparable with other methods. More importantly, the comprehensibility of the Bayesian network models is better than other methods.

It has been suggested for quite a long time that galaxy mergers trigger activities of supermassive black holes (SMBHs) on the grounds of imaging observations of individual galaxies. To quantitatively examine this hypothesis, we calculate quasar luminosity functions (LFs) by manipulating the observed galaxy LFs (z ≲ 2) and theoretical merger rates from semi-analytical formulations. We find that the model reproduces the observed quasar LFs provided that the mass ratio (q) of the secondary galaxy to the newly formed one changes with cosmic time. The results show that the fraction of major mergers decreases from f_maj ∼ 0.2 at z ∼ 2 to f_maj → 0 at z ∼ 0. As a consequence, the newly formed SMBHs from major mergers at z ∼ 2 may acquire a maximal spin due to the orbital angular momentum of the merging holes. Subsequently, random accretion led by minor mergers rapidly drives the SMBHs to spin down. Such an evolutionary trend of the SMBH spins is consistent with the fact that radiative efficiency of accreting SMBHs strongly declines with cosmic time, reported by Wang et al. This suggests that minor mergers are important in triggering activities of SMBHs at low redshift, while major mergers may dominate at high redshift.