Table of contents

A transient gravitational-wave signal, GW150914, was identified in the twin Advanced LIGO detectors on 2015 September 2015 at 09:50:45 UTC. To assess the implications of this discovery, the detectors remained in operation with unchanged configurations over a period of 39 days around the time of the signal. At the detection statistic threshold corresponding to that observed for GW150914, our search of the 16 days of simultaneous two-detector observational data is estimated to have a false-alarm rate (FAR) of $\lt 4.9\times {10}^{-6}\,{\mathrm{yr}}^{-1}$, yielding a p-value for GW150914 of $\lt 2\times {10}^{-7}$. Parameter estimation follow-up on this trigger identifies its source as a binary black hole (BBH) merger with component masses $({m}_{1},{m}_{2})=({36}_{-4}^{+5},{29}_{-4}^{+4})\,{M}_{\odot }$ at redshift $z={0.09}_{-0.04}^{+0.03}$ (median and 90% credible range). Here, we report on the constraints these observations place on the rate of BBH coalescences. Considering only GW150914, assuming that all BBHs in the universe have the same masses and spins as this event, imposing a search FAR threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of merger rates between $2\mbox{--}53\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$ (comoving frame). Incorporating all search triggers that pass a much lower threshold while accounting for the uncertainty in the astrophysical origin of each trigger, we estimate a higher rate, ranging from $13\mbox{--}600\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$ depending on assumptions about the BBH mass distribution. All together, our various rate estimates fall in the conservative range $2\mbox{--}600\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$.

The origin of spin of low-mass supermassive black holes (SMBHs) is still a puzzle at present. We report here a study on the host galaxies of a sample of radio-selected nearby (z < 0.05) Seyfert 2 galaxies with a BH mass of 10^6–7 M_⊙. By modeling the SDSS r-band images of these galaxies through a two-dimensional bulge+disk decomposition, we identify a new dependence of SMBH's radio power on host bulge surface brightness profiles, in which more powerful radio emission comes from an SMBH associated with a more disk-like bulge. This result means low-mass and high-mass SMBHs are spun up by two entirely different modes that correspond to two different evolutionary paths. A low-mass SMBH is spun up by a gas accretion with significant disk-like rotational dynamics of the host galaxy in the secular evolution, while a high-mass one by a BH–BH merger in the merger evolution.

The trans-Neptunian region of the solar system exhibits an intricate dynamical structure, much of which can be explained by an instability-driven orbital history of the giant planets. However, the origins of a highly inclined, and in certain cases retrograde, population of trans-Neptunian objects remain elusive within the framework of this evolutionary picture. In this work, we show that the existence of a distant, Neptune-like planet that resides on an eccentric and mildly inclined orbit fully accounts for the anomalous component the trans-Neptunian orbital distribution. Adopting the same parameters for Planet Nine as those previously invoked to explain the clustering of distant Kuiper Belt orbits in physical space, we carry out a series of numerical experiments that elucidate the physical process though which highly inclined Kuiper Belt objects with semimajor axes smaller than a < 100 au are generated. The identified dynamical pathway demonstrates that enigmatic members of the Kuiper Belt such as Drac and Niku are derived from the extended scattered disk of the solar system.

A planet orbiting in the "habitable zone" of our closest neighboring star, Proxima Centauri, has recently been discovered, and the next natural question is whether or not Proxima b is "habitable." Stellar winds are likely a source of atmospheric erosion that could be particularly severe in the case of M dwarf habitable zone planets that reside close to their parent star. Here, we study the stellar wind conditions that Proxima b experiences over its orbit. We construct 3D MHD models of the wind and magnetic field around Proxima Centauri using a surface magnetic field map for a star of the same spectral type and scaled to match the observed ∼600 G surface magnetic field strength of Proxima. We examine the wind conditions and dynamic pressure over different plausible orbits that sample the constrained parameters of the orbit of Proxima b. For all the parameter space explored, the planet is subject to stellar wind pressures of more than 2000 times those experienced by Earth from the solar wind. During an orbit, Proxima b is also subject to pressure changes of 1–3 orders of magnitude on timescales of a day. Its magnetopause standoff distance consequently undergoes sudden and periodic changes by a factor of 2–5. Proxima b will traverse the interplanetary current sheet twice each orbit, and likely crosses into regions of subsonic wind quite frequently. These effects should be taken into account in any physically realistic assessment or prediction of its atmospheric reservoir, characteristics, and loss.

We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS  J0644−5953 (Pictor II or Pic II) is a low surface brightness ($\mu ={28.5}_{-1}^{+1}\,\mathrm{mag}\,\,\mathrm{arcsec}{}^{-2}$ within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of ${45}_{-4}^{+5}\,\mathrm{kpc}$. The physical size (${r}_{1/2}={46}_{-11}^{+15}\,\mathrm{pc}\,$) and low luminosity (${M}_{V}=-{3.2}_{-0.5}^{+0.4}\,\mathrm{mag}\,$) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS  J0644−5953 (Pic II) is located ${11.3}_{-0.9}^{+3.1}\,\mathrm{kpc}\,$ from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS  J0644−5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.

We present subarcsecond images of submillimeter CO and continuum emission from a local galaxy forming massive star clusters: the blue compact dwarf galaxy II Zw 40. At ∼04 resolution (20 pc), the CO(3-2), CO(1-0), 3 mm, and 870 μm continuum maps illustrate star formation on the scales of individual molecular clouds. Dust contributes about one-third of the 870 μm continuum emission, with free–free accounting for the rest. On these scales, there is not a good correspondence between gas, dust, and free–free emission. Dust continuum is enhanced toward the star-forming region as compared to the CO emission. We suggest that an unexpectedly low and spatially variable gas-to-dust ratio is the result of rapid and localized dust enrichment of clouds by the massive clusters of the starburst.

We report Venus image observations around the two maximum elongations of the planet at 2015 June and October. From these images we describe the global atmospheric dynamics and cloud morphology in the planet before the arrival of JAXA's Akatsuki mission on 2015 December 7. The majority of the images were acquired at ultraviolet wavelengths (380–410 nm) using small telescopes. The Venus dayside was also observed with narrowband filters at other wavelengths (890 nm, 725–950 nm, 1.435 μm CO2 band) using the instrument PlanetCam-UPV/EHU at the 2.2 m telescope in Calar Alto Observatory. In all cases, the lucky imaging methodology was used to improve the spatial resolution of the images over the atmospheric seeing. During the April–June period, the morphology of the upper cloud showed an irregular and chaotic texture with a well-developed equatorial dark belt (afternoon hemisphere), whereas during October–December the dynamical regime was dominated by planetary-scale waves (Y-horizontal, C-reversed, and ψ-horizontal features) formed by long streaks, and banding suggesting more stable conditions. Measurements of the zonal wind velocity with cloud tracking in the latitude range from 50°N to 50°S shows agreement with retrievals from previous works.

In order to discuss the potential impact of solar "superflares" on space weather, we investigated statistical relations among energetic proton peak flux with energy higher than 10 MeV (F_p), CME speed near the Sun (V_CME) obtained by Solar and Heliospheric Observatory/LASCO coronagraph, and flare soft X-ray peak flux in the 1–8 Å band (F_SXR) during 110 major solar proton events recorded from 1996 to 2014. The linear regression fit results in the scaling relations ${V}_{\mathrm{CME}}\propto {F}_{\mathrm{SXR}}^{\alpha }$, ${F}_{p}\propto {F}_{\mathrm{SXR}}^{\beta }$, and ${F}_{p}\propto {V}_{\mathrm{CME}}^{\gamma }$ with α = 0.30 ± 0.04, β = 1.19 ± 0.08, and γ = 4.35 ± 0.50, respectively. On the basis of simple physical assumptions, on the other hand, we derive scaling relations expressing CME mass (M_CME), CME speed, and energetic proton flux in terms of total flare energy (E_flare) as ${M}_{\mathrm{CME}}\propto {E}_{\mathrm{flare}}^{2/3}$, ${V}_{\mathrm{CME}}\propto {E}_{\mathrm{flare}}^{1/6}$, and ${F}_{p}\propto {E}_{\mathrm{flare}}^{5/6}\propto {V}_{\mathrm{CME}}^{5}$, respectively. We then combine the derived scaling relations with observation and estimated the upper limit of V_CME and F_p to be associated with possible solar superflares.

Laboratory studies are presented, showing for the first time that thermally driven reactions in solid H₂O + SO₂ + O₃ mixtures can occur below 150 K, with the main sulfur-containing product being bisulfate (${{\mathrm{HSO}}_{4}}^{-}$). Using a technique not previously applied to the low-temperature kinetics of either interstellar or solar-system ice analogs, we estimate an activation energy of 32 kJ mol⁻¹ for ${{\mathrm{HSO}}_{4}}^{-}$ formation. These results show that at the temperatures of the Jovian satellites, SO₂ and O₃ will efficiently react making detection of these molecules in the same vicinity unlikely. Our results also explain why O₃ has not been detected on Callisto and why the SO₂ concentration on Callisto appears to be highest on that world's leading hemisphere. Furthermore, our results predict that the SO₂ concentration on Ganymede will be lowest in the trailing hemisphere, where the concentration of O₃ is the highest. Our work suggests that thermal reactions in ices play a much more important role in surface and sub-surface chemistry than generally appreciated, possibly explaining the low abundance of sulfur-containing molecules and the lack of ozone observed in comets and interstellar ices.

Microstructure emission, involving short timescale, often quasi-periodic, intensity fluctuations in subpulse emission, is well known in normal period pulsars. In this Letter, we present the first detections of quasi-periodic microstructure emission from millisecond pulsars (MSPs), from Giant Metrewave Radio Telescope observations of two MSPs at 325 and 610 MHz. Similar to the characteristics of microstructure observed in normal period pulsars, we find that these features are often highly polarized and exhibit quasi-periodic behavior on top of broader subpulse emission, with periods of the order of a few μs. By measuring their widths and periodicities from single pulse intensity profiles and their autocorrelation functions, we extend the microstructure timescale–rotation period relationship by more than an order of magnitude down to rotation periods ∼5 ms, and find it to be consistent with the relationship derived earlier for normal pulsars. The similarity of behavior is remarkable, given the significantly different physical properties of MSPs and normal period pulsars, and rules out several previous speculations about the possible different characteristics of microstructure in MSP radio emission. We discuss the possible reasons for the non-detection of these features in previous high time resolution MSP studies along with the physical implications of our results, both in terms of a geometric beam sweeping model and temporal modulation model for micropulse production.

It has been known that some solar activity indicators show a double-peak feature in their evolution through a solar cycle, which is not conspicuous in sunspot number. In this Letter, we investigate the high solar dynamic activity in the declining phase of the sunspot cycle by examining the evolution of polar and low-latitude coronal hole (CH) areas, splitting and merging events of CHs, and coronal mass ejections (CMEs) detected by SOHO/LASCO C3 in solar cycle 23. Although the total CH area is at its maximum near the sunspot minimum, in which polar CHs prevail, it shows a comparable second maximum in the declining phase of the cycle, in which low-latitude CHs are dominant. The events of CH splitting or merging, which are attributed to surface motions of magnetic fluxes, are also mostly populated in the declining phase of the cycle. The far-reaching C3 CMEs are also overpopulated in the declining phase of the cycle. From these results we suggest that solar dynamic activities due to the horizontal surface motions of magnetic fluxes extend far in the declining phase of the sunspot cycle.

PSR J1723−2837 is a "redback" millisecond pulsar (MSP) with a low-mass companion in a 14.8 hr orbit. The system's properties closely resemble those of "transitional" MSPs that alternate between spin-down and accretion-powered states. In this Letter, we report on long-term photometry of the 15.5 mag companion to the pulsar. We use our data to illustrate that the star experiences sporadic activity, which we attribute to starspots. We also find that the companion is not tidally locked and infer ${P}_{{\rm{s}}}/{P}_{{\rm{b}}}=0.9974(7)$ for the ratio between the rotational and orbital periods. Finally, we place constraints on various parameters, including the irradiation efficiency and pulsar mass. We discuss similarities with other redback MSPs and conclude that starspots may provide the most likely explanation for the often seen irregular and asymmetric optical light curves.

Starting in 2014 December, Kepler K2 observed Neptune continuously for 49 days at a 1 minute cadence. The goals consisted of studying its atmospheric dynamics, detecting its global acoustic oscillations, and those of the Sun, which we report on here. We present the first indirect detection of solar oscillations in intensity measurements. Beyond the remarkable technical performance, it indicates how Kepler would see a star like the Sun. The result from the global asteroseismic approach, which consists of measuring the oscillation frequency at maximum amplitude ν_max and the mean frequency separation between mode overtones Δν, is surprising as the ν_max measured from Neptune photometry is larger than the accepted value. Compared to the usual reference ν_max,⊙ = 3100 μHz, the asteroseismic scaling relations therefore make the solar mass and radius appear larger by 13.8 ± 5.8% and 4.3 ± 1.9%, respectively. The higher ν_max is caused by a combination of the value of ν_max,⊙, being larger at the time of observations than the usual reference from SOHO/VIRGO/SPM data (3160 ± 10 μHz), and the noise level of the K2 time series, being 10 times larger than VIRGO's. The peak-bagging method provides more consistent results: despite a low signal-to-noise ratio (S/N), we model 10 overtones for degrees ℓ = 0, 1, 2. We compare the K2 data with simultaneous SOHO/VIRGO/SPM photometry and BiSON velocity measurements. The individual frequencies, widths, and amplitudes mostly match those from VIRGO and BiSON within 1σ, except for the few peaks with the lowest S/N.

The formation pathways of different types of organic molecules in protostellar envelopes and other regions of star formation are subjects of intense current interest. We present here observations of C₄H and CH₃OH, tracing two distinct groups of interstellar organic molecules, toward 16 protostars in the Ophiuchus and Corona Australis molecular clouds. Together with observations in the literature, we present C₄H and CH₃OH data from single-dish observations of 40 embedded protostars. We find no correlation between the C₄H and CH₃OH column densities in this large sample. Based on this lack of correlation, a difference in line profiles between C₄H and CH₃OH, and previous interferometric observations of similar sources, we propose that the emission from these two molecules is spatially separated, with the CH₃OH tracing gas that has been transiently heated to high (∼70–100 K) temperatures and the C₄H tracing the cooler large-scale envelope where CH₄ molecules have been liberated from ices. These results provide insight in the differentiation between hotcorino and warm carbon-chain chemistry in embedded protostars.