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A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.

We present faint galaxy counts from deep VRI images obtained with the Keck Telescope. These images reach R ~ 27 in median seeing FWHM ~ 05-06, and we detect a integrated galaxy number density of 7 × 10⁵ deg^-2, equivalent to 3 × 10¹⁰ galaxies in the observable universe. In addition we present median galaxy colors as a function of magnitude; bluing trends are visible in all colors to R ~ 24.5. Fainter than R ~ 24.5, however, the typical V - R color becomes redder again, V - I remains constant, and R - I becomes yet bluer. These trends are consistent with the VRI count slopes, implying a decrease in the V slope at the faintest levels, which our data support. Taking advantage of our good seeing we also present median half-light radii for faint galaxies; these show a steady decline at fainter magnitudes, leading to an intrinsic half-light radius of ~02 for a typical R ~ 26 galaxy. Irrespective of the redshift distribution, the extremely high galaxy surface densities and their small intrinsic sizes are consistent with a scenario in which the majority of the very faint field population are dwarf galaxies or subgalactic units.

We report high resolution imaging of the CO (3-2) emission in the luminous infrared source FSC 10214+4724 at z = 2.2853. Maps at resolutions of 23 × 30 and 45-90 km s^-1 show the CO emission peak coinciding with those of Hα and radio continuum. Two components are identified in the CO emission: an unresolved core and an extended source (~19 × 44)—elongated southeast-northwest. The implied size is 9 × 24 kpc for H₀ = 75 km s^-1 Mpc^-1 and q₀ = 0.5. The extent of the CO emission is significantly greater than that of either the optical or the radio continuum (~1'') and the CO extent varies between the line core and the wings. If the gravitational lensing is occuring in FSC 10214+4724 (as has been suggested based on the near infrared morphology), the magnification factor for the CO emission is likely to be lower than for the near-infrared and the observed CO extent is probably an upper limit to the true source size.

The mass of molecular gas implied by the observed CO line flux is ~2 × 10¹¹M_☉, assuming no lens magnification. In order that the derived gas mass does not exceed the dynamical mass, the gas would have to be in a nearly face-on (i < 20°) disk. However, this configuration appears inconsistent with the elongated morphology of the CO (which suggests an inclined disk or an interacting galaxy system). We suggest that the "dynamical" mass might be reconciled with the derived gas mass if substantial support for the gas is provided by radiation pressure from the nucleus of FSC 10214+4724. For the observed ratio of infrared luminosity-to-gas-mass (10³L_☉M_☉⁻¹), we find that a column density ≤2.4 × 10²² H₂ cm^-2 (A_V ~ 24) could be supported; this is consistent with the average gas column that is observed. The very high luminosity-to-mass ratio strongly also favors a nonthermal origin for the luminosity since a starburst would require consumption of the entire ISM on a timescale significantly shorter than the dynamical times (2 × 10⁷-2 × 10⁸ yr) estimated from the CO data. These conclusions are not critically dependent on the presence or abscence of gravitational lensing since the molecular emission, the far-infrared flux, and the "dynamical" mass are similarly effected and the ratios therefore remained almost unchanged.

Optical observations of an extended red halo surrounding the galaxy 1E 1111.9-3754 sparked considerable interest as potentially the first detection of low-mass star formation in an X-ray cooling flow. To test this possibility, we obtained near-infrared surface photometry of this galaxy at J (1.25 μm) and K (2.15 μm) and compare this data with models of a low-mass stellar accretion population. Our surface brightness measurements are consistent with a normal r^1/4 elliptical galaxy profile and show no evidence of an extended infrared halo, which argues against the low-mass stellar accretion hypothesis.

We imaged the brown dwarf candidate 0918-0023B of Jones, Miller, & Glazebrook at 2.2 μm on the Keck telescope under outstanding seeing of 04 full width at half-maximum intensity. The candidate object is clearly extended in comparison to the primary star 0918-0023A and several other stars in the field. So, 0918-0023B is certainly a distant galaxy and not a brown dwarf. Seven years after its discovery on the IRTF, GD 165B remains the only low-mass star/brown dwarf candidate with J - K color > 1.5 mag.

We have obtained 8-13 μm spectra of two asymptotic giant branch stars in the Magellanic Clouds (MCs) and report the first detection of dust features in AGB stars in the MCs. The long-period variable (LPV) TRM 60 (P = 1260 days) in the LMC displays silicate absorption, and the LPV GM 103 (P = 1070 days) in the SMC shows silicate emission. The presence of silicates confirms previous observations using different techniques that these stars are oxygen rich. By modeling the spectral energy distributions and 8-13 μm spectra with a dust radiative transfer model we find that although the two stars have similar high luminosities their dust optical depths differ by a factor of 10.

A similar analysis was carried out for a Galactic OH/IR star of comparable period. The results for these three stars suggest that the mass-loss rate increases with metallicity in AGB stars.

For almost 20 years models of the Galaxy have included a dark halo responsible for supporting a substantial fraction of the local rotation velocity and a flat rotation curve at large distances. Estimates of the local halo density range from 2 × 10^-25 g cm^-3 to 10 × 10^-25 g cm^-3. By careful modeling of the Galaxy, taking account of the evidence that dark halos are flattened and recent microlensing data, we arrive at a more quantitative estimate, 9.2^{+ 3.8}_−3.1 × 10^-25 g cm^-3. Microlensing toward the LMC indicates that only a small fraction, less than ~30%, can be in the form of MACHOs, which is consistent with the idea that most of the halo consists of cold dark matter particles.

We have observed the molecular globule G70.7+1.2 at 1375 MHz using the C configuration of the VLA, and have imaged a peculiar H168α recombination line detected at the Arecibo radio telescope. The narrow width of the recombination line (Δv ~ 3 km s^-1) indicates gas cooler than 185 K and suggests that the globule harbors the coldest known H II region. Previous work showed that the recombination line came either from newly ionized gas outside a bow shock produced by supersonic motion of an early-type star through the molecular globule, or from a cold H II region inside the globule. The 20'' angular resolution of the VLA image of G70.7+1.2 was sufficient to resolve the separation between the nonthermal radio-emitting bow shock and the thermal H II region. The spectral line images show that the radio recombination line comes from cold gas near the outer boundary of the bow shock and that the line intensity is enhanced by stimulated amplification of the nonthermal continuum emanating from the bow shock.

We report the detection of broad (FWHM 270 km s^-1) H I · 21 cm absorption toward the compact (<15 h^-1 pc) radio nucleus of the nearby powerful radio galaxy Cygnus A. The absorption corresponds to a column density of hydrogen atoms of at least 2.54 ± 0.44 × 10¹⁹T_spin cm^-2. Observations of OH and H₂CO yielded upper limits. While other possibilities exist, we argue that the observed H I absorption plausibly occurs within a circumnuclear obscuring torus which is thought to block our direct view of a quasar nucleus in this object.

We have attempted to constrain the properties of the obscuring gas by combining our H I result with upper limits on molecular absorption and estimates of the total obscuring column density from X-ray observations. One possibility is that the majority of the gas is in a hot (≈ 8000 K) mainly atomic phase; we derive limits on the size of such an atomic torus. Alternatively, the H I absorption might be caused by atomic gas within a warm (≈ 1000 K) mainly molecular torus. In this case, the nondetection of molecular absorption can possibly be explained by radiative excitation due to the central radio source.

Follow-up VLBI observations are planned which will further constrain the properties of the absorbing gas and distinguish between the competing models.

The spectrum of 1 Sco has been recorded in the region of the 1434 Å line of Pb II at S/N = 220, using the ECH-A mode of the HST/GHRS. Absorption by interstellar Pb II is seen with an equivalent width W_λ = 0.3 ± 0.2 (2 σ) mÅ, at a radial velocity which agrees satisfactorily with that of the strongest component of the interstellar Na I D₁ line seen in high-resolution optical spectra. If a theoretical oscillator strength f = 0.865 is adopted, the resulting logarithmic depletion is D = -0.97 (+0.22, -0.48) with respect to the meteoritic abundance of Pb. This depletion, which is consistent with Cardelli's result toward ζ Oph, is stronger than that expected from the low condensation temperature, T_c = 496 K, of Pb, in light of the general correlation between the depletions and condensation temperatures of 29 other elements. Some previous ideas about the formation and the evolution of interstellar grains are briefly considered.

High-resolution imaging of the protostar HH 24 MMS at wavelengths of 7 mm and 3.4 mm shows the dust emission to originate from two components: an unresolved disk and an extended envelope. The envelope is an order of magnitude more massive than the disk, suggesting that HH 24 MMS is very young, since the fraction of circumstellar material in an extended component probably decreases with the age of the forming star. For the disk, the frequency dependence of the dust mass opacity coefficient, β, is 0.68 ± 0.12, significantly lower than the interstellar medium value of 2. In the envelope β is less well constrained but must lie in the range 0 to 1.9. Emission from the disk dominates at wavelengths longer than 3 mm, but the far-infrared emission is relatively weak. This suggests that the envelope is optically thick at wavelengths as short as 60 μm and obscures the disk.

Clumpy, hydrogen-depleted material in the planetary nebula Abell 30 was ejected by its central star ~1000 yr ago. We present observations of Abell 30 showing compact, hydrogen-poor knots with wind-blown tails, obtained with the Wide Field Planetary Camera (WFPC2) on board the Hubble Space Telescope, which offer an unprecedented view of the interaction of a stellar wind with an ambient inhomogeneous medium. We can see how dense clumps of material, left within an expanding bubble blown by a stellar wind, are being photoevaporated by the stellar radiation and then swept back and accelerated by the wind. This accelerated material mixes with the wind, slowing it and increasing its density. The observed extent of this mass loading in Abell 30 supports claims that the mass-loading process is generally important in highly inhomogeneous astrophysical flows. In particular, mass loading of the magnitude observed in Abell 30 may explain the detection of X-ray-emitting gas in planetary nebulae.

Inverse-Compton (IC) scattering of cosmic microwave background photons into the X-ray band by relativistic electrons in diffuse radio lobes is an obligatory process. We present the strongest evidence to date for the detection of these IC X-rays. They appear in a deep image of the nearby radio galaxy Fornax A (=NGC 1316) obtained with the Position Sensitive Proportional Counter on board the ROSAT satellite. The spatial correspondence of the X-ray and radio emission is excellent. The absence of Faraday depolarization in the lobes argues strongly against the possibility that the X-rays are due to an entrained thermal plasma. The detected level of X-rays is somewhat higher than, but consistent with, that expected from the assumption of equipartition between magnetic fields and relativistic electrons in the lobes. This observation provides the first direct estimate of the magnetic field of a radio lobe, B_IC ≃ 2-3 μG, indicating that the integrated energy output of this active galactic nucleus is near the minimum energy level.

Simple expressions are derived for the torque exerted by an accretion disk on a rotating, magnetized object (which may, for example, be a neutron star, a white dwarf, or a T Tauri star). In the equilibrium state (for which there is no net torque on the star), the inner edge of the Keplerian disk R₀ is located very close to the corotation radius R_c, for physically plausible assumptions about the dissipation time of the wound-up field component; the equilibrium value ω_crit of the "fastness parameter" ω ≡ (R₀/R_c)^3/2 lies in the range 0.875-0.95. Empirical constraints requiring ω_crit to be significantly less than unity are thus incompatible with the magnetically threaded disk model.

Using daily averages of the solar wind speed and mass density measured at Earth together with an improved method for extrapolating the observed photospheric field to 1 AU, we construct scatter plots relating the coronal field strength B₀ to the mass flux density ρ₀v₀ and the total energy flux density F_w0 at the coronal base. On average, both ρ₀v₀ and F_w0 increase roughly linearly with B₀; they also show a monotonic increase with the coronal flux-tube expansion factor. However, the wind speed at Earth, determined by the ratio F_w0/(ρ₀v₀), is essentially independent of B₀, while tending to decrease with increasing expansion factor (as found in earlier studies).

We present a model of a collision between a plasma cloud and a current loop by performing simulations with a three-dimensional electromagnetic particle code. The loop heating and particle acceleration in such collision processes are crucially important in understanding the triggering mechanisms of solar flares. Theories and observations previously studied have revealed that plasmoids produced in the coalescence process of current loops escape from the interaction region. Furthermore, they may move to another magnetic loop, colliding with it and triggering a flare. Based on this idea, we investigate this simulation study as a flare model. A cloud is pushed across an ambient magnetic field to a current loop along the field. Simulation results show that a great deal of the released energy from the cloud is transferred into loop kinetic energy, resulting in heating of the loop. If the initial kinetic energy of the cloud is large enough to compress and bend the ambient magnetic field distinctly, nonthermal electrons are produced in the loop, which have a broken power law spectrum. Those heated electrons are responsible for soft X-ray and hard X-ray emissions from the loop. In addition, electrons in the cloud are also heated. The high-energy electrons in the loop and the cloud are almost identical, with the same maximum energies, of the order of 30 times the thermal energy. The high-energy electrons in the cloud correspond to a hard X-ray source above the loop apex.