Universities Space Research Association

About: Universities Space Research Association is a nonprofit organization based out in Columbia, Maryland, United States. It is known for research contribution in the topics: Gamma-ray burst & Galaxy. The organization has 1921 authors who have published 5412 publications receiving 255681 citations. The organization is also known as: USRA.

Evidence for a supernova in reanalyzed optical and near-infrared images of GRB 970228

Abstract: We present B-, V-, RC-, IC-, J-, H-, K-, and K'-band observations of the optical transient (OT) associated with GRB 970228, based on a reanalysis of previously used images and unpublished data. In order to minimize calibration differences, we have collected and analyzed most of the photometry and consistently determined the magnitude of the OT relative to a set of secondary field stars. We confirm our earlier finding that the early decay of the light curves (before 1997 March 6) was faster than that at intermediate times (between 1997 March 6 and April 7). At late times the light curves resume a fast decay (after 1997 April 7). The early-time observations of GRB 970228 are consistent with relativistic blast-wave models, but the intermediate- and late-time observations are hard to understand in this framework. The observations are well explained by an initial power-law decay with α = -1.51 ± 0.06 modified at later times by a Type Ic supernova light curve. Together with the evidence for GRB 980326 and GRB 980425, this gives further support for the idea that at least some γ-ray bursts are associated with a possibly rare type of supernova.

Jet Breaks in Short Gamma-Ray Bursts. II. The Collimated Afterglow of GRB 051221A

Abstract: We report the results of the Chandra observations of the Swift-discovered short gamma-ray burst GRB 050724. Chandra observed this burst twice, about 2 days after the burst and a second time 3 weeks later. The first Chandra pointing occurred at the end of a strong late-time flare. About 150 photons were detected during this 49.3 ks observation in the 0.4-10.0 keV range. The spectral fit is in good agreement with spectral analysis of earlier Swift XRT data. In the second Chandra pointing the afterglow was clearly detected with eight background-subtracted photons in 44.6 ks. From the combined Swift XRT and Chandra-ACIS-S light curve we find significant flaring superposed on an underlying power-law decay slope of α = 0.98. There is no evidence for a break between about 1 ks after the burst and the last Chandra pointing about 3 weeks after the burst. The nondetection of a jet break places a lower limit of 25° on the jet opening angle, indicating that the outflow is less strongly collimated than most previously reported long GRBs. This implies that the beaming corrected energy of GRB 050724 is at least 4 × 1049 ergs.

On X-Ray Variability in Seyfert Galaxies

Abstract: This paper presents a quantification of the X-ray variability amplitude for 79 ASCA observations of 36 Seyfert 1 galaxies. We find that consideration of sources with the narrowest permitted lines in the optical band introduces scatter into the established correlation between X-ray variability and nuclear luminosity. Consideration of the X-ray spectral index and variability properties together shows distinct groupings in parameter space for broad and narrow-line Seyfert 1 galaxies, confirming previous studies. A strong correlation is found between hard X-ray variability and FWHM Hbeta. A range of nuclear mass and accretion rate across the Seyfert population can explain the differences observed in X-ray and optical properties. An attractive alternative model, which does not depend on any systematic difference in central mass, is that the circumnuclear gas of NLSy1s is different to BLSy1s in temperature, optical depth, density or geometry.

Cataclysmic Variables from The Sloan Digital Sky Survey. I. The First Results

Abstract: The commissioning year of the Sloan Digital Sky Survey (SDSS) has demonstrated that many cataclysmic variables (CVs) have been missed in previous surveys with brighter limits. We report the identification of 22 CVs, of which 19 are new discoveries and three are known systems (SW UMa, BH Lyn, and OU Vir). A compendium of positions, colors, and characteristics of these systems obtained from the SDSS photometry and spectroscopy is presented, along with data obtained during follow-up studies with the Apache Point Observatory and Manastash Ridge Observatory telescopes. We have determined orbital periods for three of the new systems, two show dwarf nova outbursts, and the third is a likely magnetic system with eclipses of its region of line emission. Based on these results, we expect the completed survey to locate at least 400 new CVs. Most of these will be faint systems with low accretion rates that will provide new constraints on binary evolution models.

Radio to Gamma-Ray Emission from Shell-type Supernova Remnants: Predictions from Non-linear Shock Acceleration Models

Abstract: Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency and spectral considerations, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range; these are consistent with Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission at super-TeV energies; such sources will generally be gamma-ray dim at GeV energies.