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We model the mass distribution of long gamma-ray burst (GRB) host galaxies given recent results suggesting that GRBs occur in low-metallicity environments. By utilizing measurements of the redshift evolution of the mass-metallicity relationship for galaxies, along with a sharp host metallicity cutoff suggested by Modjaz and collaborators, we estimate an upper limit on the stellar mass of a galaxy that can efficiently produce a GRB as a function of redshift. By employing consistent abundance indicators, we find that subsolar metallicity cutoffs effectively limit GRBs to low-stellar mass spirals and dwarf galaxies at low redshift. At higher redshifts, as the average metallicity of galaxies in the Universe falls, the mass range of galaxies capable of hosting a GRB broadens, with an upper bound approaching the mass of even the largest spiral galaxies. We compare these predicted limits to the growing number of published GRB host masses and find that extremely low-metallicity cutoffs of 0.1 to 0.5 Z are effectively ruled out by a large number of intermediate mass galaxies at low redshift. A mass function that includes a smooth decrease in the efficiency of producing GRBs in galaxies of metallicity above 12+log(O/H)KK04 = 8.7 can, however, accommodate a majority of the measured host galaxy masses. We find that at z [?] 1, the peak in the observed GRB host mass distribution is inconsistent with the expected peak in the mass of galaxies harboring most of the star formation. This suggests that GRBs are metallicity-biased tracers of star formation at low and intermediate redshifts, although our model predicts that this bias should disappear at higher redshifts due to the evolving metallicity content of the universe.

Several correlations among parameters derived from modeling the high-energy properties of gamma-ray bursts (GRBs) have been reported. We show that well known examples of these have common features indicative of strong contamination by selection effects. We focus here on the impact of detector threshold truncation on the spectral peak versus isotropic equivalent energy release (E pk-E iso) relation, extended to a large sample of 218 Swift and 56 HETE-2 GRBs with and without measured redshift. The existence of faint Swift events missing from pre-Swift surveys calls into question inferences based on pre-Swift surveys which must be subject to complicated incompleteness effects. We demonstrate a generalized method for treating data truncation in correlation analyses and apply this method to Swift and pre-Swift data. Also, we show that the E pk-E g ("Ghirlanda") correlation is effectively independent of the GRB redshifts, which suggests that its existence has little to do with intrinsic physics. We suggest that a physically based correlation, manifest observationally, must show significantly reduced scatter in the rest frame relative to the observer frame and must not persist if the assumed redshifts are scattered. As with the E pk-E g correlation, we find that the pre-Swift, bright GRB E pk-E iso correlation of Amati does not rigorously satisfy these conditions.

We examine the rest-frame energetics of 76 gamma-ray bursts (GRBs) with known redshift that were detected by the Swift spacecraft and monitored by the satellite's X-Ray Telescope (XRT). Using the bolometric fluence values estimated by Butler and coworkers and the last XRT observation for each event, we set a lower limit to their collimation-corrected energy Eg and find that 68 % of our sample is at high enough redshift and/or low enough fluence to accommodate a jet break occurring beyond the last XRT observation and still be consistent with the pre-Swift Eg distribution for long GRBs. We find that relatively few of the X-ray light curves for the remaining events show evidence for late-time decay slopes that are consistent with that expected from post-jet break emission. The breaks in the X-ray light curves that do exist tend to be shallower and occur earlier than the breaks previously observed in optical light curves, yielding a Eg distribution that is far lower than the pre-Swift distribution. If these early X-ray breaks are not due to jet effects, then a small but significant fraction of our sample have lower limits to their collimation-corrected energy that place them well above the pre-Swift Eg distribution. Either scenario would necessitate a much wider post-Swift Eg distribution for long cosmological GRBs compared to the narrow standard energy deduced from pre-Swift observations. We note that almost all of the pre-Swift Eg estimates come from jet breaks detected in the optical whereas our sample is limited entirely to X-ray wavelengths, furthering the suggestion that the assumed achromaticity of jet breaks may not extend to high energies.

We calculate durations and spectral parameters for 218 Swift bursts detected by the BAT instrument between and including gamma-ray bursts (GRBs) 041220 and 070509, including 77 events with measured redshifts. Incorporating prior knowledge into the spectral fits, we are able to measure the characteristic nFn spectral peak energy Epk, obs and the isotropic equivalent energy Eiso (1-104 keV) for all events. This complete and rather extensive catalog, analyzed with a unified methodology, allows us to address the persistence and origin of high-energy correlations suggested in pre-Swift observations. We find that the Epk, obs-Eiso correlation is present in the Swift sample; however, the best-fit power-law relation is inconsistent with the best-fit pre-Swift relation at >5 s significance. It has a factor [?]2 larger intrinsic scatter, after accounting for large errors on Epk, obs. A large fraction of the Swift events are hard and subluminous relative to (and inconsistent with) the pre-Swift relation, in agreement with indications from BATSE GRBs without redshift. Moreover, we determine an experimental threshold for the BAT detector and show how the Epk, obs-Eiso correlation arises artificially due to partial correlation with the threshold. We show that pre-Swift correlations found by Amati et al., Yonetoku et al., and Firmani et al., and independently by others are likely unrelated to the physical properties of GRBs and are likely useless for tests of cosmology. Also, an explanation for these correlations in terms of a detector threshold provides a natural and quantitative explanation for why short-duration GRBs and events at low redshift tend to be outliers to the correlations.

The early, highly time-variable X-ray emission immediately following gamma-ray bursts (GRBs) exhibits strong spectral variations that are unlike the temporally smoother emission that dominates after t ~ 103 s. The ratio of hard-channel (1.3-10.0 keV) to soft-channel (0.3-1.3 keV) counts in the Swift X-ray telescope provides a new measure delineating the end time of this emission. We define TH as the time at which this transition takes place and measure for 59 events a range of transition times that spans 10 2 to 104 s, on average 5 times longer than the prompt T90 duration observed in the gamma-ray band. It is very likely that the mysterious light-curve plateau phase and the later power-law temporal evolution, both of which typically occur at times greater than TH and hence exhibit very little hardness ratio evolution, are both produced by external shocking of the surrounding medium and not by the internal shocks thought responsible for the earlier emission. We use the apparent lack of spectral evolution to discriminate among proposed models for the plateau phase emission. We favor energy injection scenarios with a roughly linearly increasing input energy versus time for six well-sampled events with nearly flat light curves at t [?] 103-104 s. Also, using the transition time TH as the delineation between the GRB and afterglow emission, we calculate that the kinetic energy in the afterglow shock is typically a factor of 10 lower than that released in the GRB. Three very bright events suggest that this presents a missing X-ray flux problem rather than an efficiency problem for the conversion of kinetic energy into the GRB. Lack of hardness variations in these three events may be due to a very highly relativistic outflow or due to a very dense circumburst medium. There are a handful of rare cases of very late time t > 104 s hardness evolution, which may point to residual central engine activity at very late time.

We study the duration and variability of late-time X-ray flares following gamma-ray bursts (GRBs) observed by the narrow-field X-ray telescope (XRT) aboard the Swift spacecraft. These flares are thought to be indicative of late-time activity by the central engine that powers the GRB and produced by means similar to those which produce the prompt emission. We use a nonparametric procedure to study the overall temporal properties of the flares and a structure function analysis to look for an evolution of the fundamental variability timescale between the prompt and late-time emission. We find a strong correlation in 28 individual X-ray flares in 18 separate GRBs between the flare duration and their time of peak flux since the GRB trigger. We also find a qualitative trend of decreasing variability as a function of time since trigger, with a characteristic minimum variability timescale Dt/t = 0.1 for most flares. The correlation between pulse width and time is consistent with the effects of internal shocks at ever-increasing collision radii, but could also arise from delayed activity by the central source. Contemporaneous detections of high-energy emission by GLAST could test between these two scenarios, as any late-time X-ray emission would undergo inverse Compton scattering as it passes through the external shock. The profile of this high-energy component should depend on the distance between the emitting region and the external shock.

We report simultaneous multicolor near-infrared (NIR) observations of the supernova associated with X-ray flash 060218 during the first 16 days after the high-energy event. We find that the light curve rises and peaks relatively fast compared to other Type Ic supernovae (SNe Ic), with the characteristic broad NIR peak seen in all three bands. We find that the rise profile before the peak is largely independent of NIR wavelength, each band appearing to transition into a plateau phase around day 10-13. Since the light curve is in the plateau phase when our observations end at day 16, we can only place limits on the peak absolute magnitudes, but we estimate that SN 2006aj is one of the lowest NIR luminosity X-ray flash/gamma-ray burst (XRF/GRB) associated SNe observed to date. The broad peaks observed in the JHKs bands point to a large increase in the NIR contribution of the total flux output from days 10-16. This evolution can be seen in the broad color and spectral energy distribution diagrams constructed using UBVRIJHKs monochromatic flux measurements for the first 16 days of the event. Ultimately, a 10 day rise time would make SN 2006aj an extremely fast rise SN Ic event, faster than SN 1998bw and SN 2003dh, which combined with its underluminous nature indicates a lower amount of 56Ni ejected by the progenitor compared to other XRF/GRB-SNe. Furthermore, the lack of significant color change during the rise portion of the burst points to little or no spectral evolution over the first 10 days of activity in the NIR.

We show that the X-ray and g-ray spectra of Swift GRBs and their afterglows are consistent with the emission characteristic of an expanding, relativistic fireball. The classical afterglow due to the impact of the fireball on the external medium is often not observed until 1 to several hours after the GRB. Focusing on GRBs 061121, 060614, and 060124, but generalizing to the full (>50 Ms XRT exposure) Swift sample up to and including GRB 061210, we show that the early emission in >90% of early afterglows has a characteristic nFn spectral energy Epeak, which likely evolves from the g-rays through the soft X-ray band on timescales of 102-104 s after the GRB. The observed spectra are strongly curved when plotted with logarithmic axes and have often been incorrectly fitted in other studies with a time-varying soft X-ray absorption. The spectral evolution inferred from fitting instead models used to fit GRBs demonstrates a common evolution--a power-law hardness-intensity correlation and hard-to-soft evolution--for GRBs and the early X-ray afterglows and X-ray flares. Combined with studies of short-timescale variability, our findings indicate a central engine active for longer than previously suspected. The GRB spectra are observed to become very soft at late times due to an intrinsic spectral evolution and due to the surprising faintness of some afterglows. We discuss models for the early X-ray emission.