A Search for Discrete X-Ray Spectral Features in a Sample of Bright γ-Ray Burst Afterglows
We present uniform, detailed spectral analyses of gamma-ray burst (GRB) X-ray afterglows observed with ASCA, Beppo-SAX, Chandra, and XMM-Newton, and critically evaluate the statistical significances of X-ray emission and absorption features in these spectra. The sample consists of 21 X-ray afterglow observations up to and including that of GRB040106 with spectra of sufficient statistical quality to allow meaningful line searches, chosen here somewhat arbitrarily to be detections with more than 100 total (source plus background) counts. This sample includes all nine X-ray afterglows with published claims of line detections. Moderate resolution spectra are available for 16 of the 21 sources, and for the remaining five the Chandra transmission grating spectrometers obtained high-resolution data. All of the data are available from the public archive. We test a simple hypothesis in which the observed spectra are produced by a power-law continuum model modified by photoelectric absorption by neutral material both in our Galaxy and possibly also local to the burst. As a sample, these afterglow spectra are consistent with this relatively simple model. However, since the {chi}{sup 2} statistic is not sensitive to weak and/or localized fluctuations, we have performed Monte Carlo simulations to search for discrete features and to estimate theirmore » significances. Our analysis shows that there are four afterglows (GRB011211, GRB030227, GRB021004, and GRB040106) with line-like features that are significant at the 3 {sigma} level. We cautiously note that, in two cases, the features are associated with an unusual background feature; in the other two, the fractional magnitudes of the lines are small, and comparable to the expected level of systematic uncertainty in the spectral response. In addition, none of the statistically significant features are seen in more than one detector or spectral order where available. We conclude that, to date, no credible X-ray line feature has been detected in a GRB afterglow. Finally, in a majority of cases, we find no evidence for significant absorbing columns local to the GRB host galaxy, implying there is little evidence from X-ray observations that GRB preferentially explode in high-density environments.« less

title: Observational Limits on X-ray Bursts from RRAT J1911+00
abstract: (abridged) The high radio-flux brightness temperature of the recently discovered class of sources known as Rotating RAdio Transients (RRATs) motivates detailed study in the X-ray band. We describe analyses of historical X-ray data, searching for X-ray phenomena (sources, behaviors), finding no sources or behaviors which may unequivocally be associated with RRAT J1911+00. We put forward a candidate X-ray counterpart to RRAT J1911+00, discovered in a Chandra observation in Feb 2001, which fades by a factor >5 prior to April 2004. The X-ray flux and optical (F_X/F_R>12) and near infra-red (F_X/F_J>35) limits, as well as the X-ray flux itself, are consistent with an AGN origin, unrelated to RRAT J1911+00. Searches for msec X-ray bursts found no evidence for such a signal, and we place the first observational upper-limit on the X-ray to radio flux ratio of RRAT bursts: F_X/F_{radio} <6e-11 ergs cm-2 s-1 mJy-1. The upper-limit on the X-ray burst flux (corresponding to <2.2e37 (d/3.3 kpc)^2 erg s-1, 2-10 keV) requires a limit on the spectral energy density power-law slope of \alpha<-0.3 between the radio and X-ray bands. We place a limit on the time-average X-ray burst luminosity, associated with radio bursts, of < 3.4e30 (d/3.3 kpc)^2 erg s-1.

title: RefSponse: A Literature Evaluation System for the Professional Astrophysics Community
abstract: We describe an implementation of a semi-automated review system for the astrophysics literature. Registered users identify names under which they publish, and provide scores for individual papers of their choosing. Scores are held confidentially, and combined in a weighted average grade for each paper. The grade is divided among the co-authors as assigned credit. The credit accumulated by each user (their ``mass'') provides the weight by which their score is averaged into papers' grades. Thus, papers' grades and users' masses are mutually dependent and evolve in time as scores are added. Likewise, a user's influence on the grade of a paper is determined from the perceived original scientific contribution of all the user's previous papers. The implementation, called RefSponse -- currently hosted at this http URL -- includes papers in astro-ph, the ApJ, AJ, A&A, MNRAS, PASP, PASJ, New Astronomy,  ARA&A,  to the present, making extensive use of the NASA/ADS abstract server. We describe some of the possible utilities of this system in enabling progress in the field.

title: Transient Low-Mass X-Ray Binaries in Quiescence
abstract: We summarize the quiescent X-ray observations of transient low-mass X-ray binaries. These observations show that, in quiescence, binaries containing black holes are fainter than those containing neutron stars. This has triggered a number of theoretical ideas about what causes the quiescent X-ray emission. For black hole binaries, the options are accretion onto the black hole or coronal emission from the rapidly rotating stellar companion. There are more possibilities for the neutron stars: accretion, thermal emission from the surface or non-thermal emission from a ``turned-on'' radio pulsar. We review recent theoretical work on these mechanisms and note where current observations can distinguish between them. We highlight the re-analysis of the quiescent neutron star emission by Rutledge and collaborators that showed thermal emission to be a predominant contributor in many of these systems. Our knowledge of these binaries is bound to dramatically improve now that the Chandra and XMM satellites are operating successfully,

title: A strong upper limit on the pulsed radio luminosity of the compact object 1RXS J141256.0+792204
abstract: The ROSAT X-ray source 1RXS J141256.0+792204 has recently been identified as a likely compact object whose properties suggest it could be a very nearby radio millisecond pulsar at d = 80-260 pc.  We investigated this hypothesis by searching for radio pulsations using the Westerbork Synthesis Radio Telescope.  We observed 1RXS J141256.0+792204 at 385 and 1380 MHz, recording at high time and frequency resolution in order to maintain sensitivity to millisecond pulsations. These data were searched both for dispersed single pulses and using Fourier techniques sensitive to constant and orbitally modulated periodicities.  No radio pulsations were detected in these observations, resulting in pulsed radio luminosity limits of L400^max â‰ˆ 0.3 (d/250 pc)2 mJy kpc2 and L1400^max â‰ˆ 0.03 (d/250 pc)2 mJy kpc2 at 400 and 1400 MHz respectively.  The lack of detectable radio pulsations from 1RXS J141256.0+792204 brings into question its identification as a nearby radio pulsar, though, because the pulsar could be beamed away from us, this hypothesis cannot be strictly ruled out.

title: Quasi-periodic oscillations in black hole candidates as an indicator of transition between low and high states
abstract: By comparing positions on a spectral color-color diagram from 10 black hole candidates (BHCs) observed with Ginga (1354-64, 1826-24, 1630-47, LMC X-1, LMC X-3, GS 2000+25, GS 2023+33, GS 1124-68, Cyg X-1, and GX 339-4) with the observed broadband noise (BBN) (0.001-64 Hz) and quasi-periodic oscillation (QPO) variability, we find that the "very high state" is spectrally intermediate to the soft/high state and hard/low state. We find a transition point in spectral hardness where the dependence of the BHC QPO centroid frequency (of GS 1124-68 and GX 339-4) on spectral hardness switches from a correlation to an anticorrelation; where the BBN variability switches from high state to low state; and where the spectral hardness of the QPO relative to that of the BBN variability is a maximum. This coincidence of changing behavior in both the QPO and the broadband variability leads us to hypothesize that the QPO is due to interaction between the physical components which dominate the behaviors of BHCs when they occupy the hard/low and soft/high states. We conclude that these QPOs should be observed from BHCs during transition between these two states. Comparison with QPO and BBN behavior observed during the 1996 transition of Cyg X-1 supports this hypothesis. We also report 1-3 Hz QPOs observed in GS 2000+25 and Cyg X-1 in the hard/low state, and we compare these to the QPOs observed in GS 1124-68 and GX 339-4.

title: Hubble Residuals of Nearby Type Ia Supernovae are Correlated with Host Galaxy Masses
abstract: From Sloan Digital Sky Survey u'g'r'i'z' imaging, we estimate the stellar masses of the host galaxies of 70 low-redshift Type Ia supernovae (SNe Ia, 0.015 <z< 0.08) from the hosts' absolute luminosities and mass-to-light ratios. These nearby SNe were discovered largely by searches targeting luminous galaxies, and we find that their host galaxies are substantially more massive than the hosts of SNe discovered by the flux-limited Supernova Legacy Survey (SNLS). Testing four separate light curve fitters, we detect ~2.5σ correlations of Hubble residuals with both host galaxy size and stellar mass, such that SNe Ia occurring in physically larger, more massive hosts are ~10% brighter after light curve correction. The Hubble residual is the deviation of the inferred distance modulus to the SN, calculated from its apparent luminosity and light curve properties, away from the expected value at the SN redshift. Marginalizing over linear trends in Hubble residuals with light curve parameters shows that the correlations cannot be attributed to a light-curve-dependent calibration error. Combining 180 higher-redshift ESSENCE, SNLS, and HigherZ SNe with 30 nearby SNe whose host masses are less than 1010.8 M ☉ in a cosmology fit yields 1 + w = 0.22+0.152 –0.108, while a combination where the 30 nearby SNe instead have host masses greater than 1010.8 M ☉ yields 1 + w = –0.03+0.217 –0.143. Progenitor metallicity, stellar population age, and dust extinction correlate with galaxy mass and may be responsible for these systematic effects. Host galaxy measurements will yield improved distances to SNe Ia.