he burst alert telescope (BAT) is one of three instruments on the Swift MIDEX spacecraft to study gamma-ray bursts (GRBs). The BAT first detects the GRB and localizes the burst direction to an ...accuracy of 1-4 arcmin within 20 s after the start of the event. The GRB trigger initiates an autonomous spacecraft slew to point the two narrow field-of-view (FOV) instruments at the burst location within 20-70 s so to make follow-up X-ray and optical observations. The BAT is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane. The detector plane is composed of 32,768 pieces of CdZnTe (4×4×2 mm), and the coded-aperture mask is composed of 52,000 pieces of lead (5×5×1 mm) with a 1-m separation between mask and detector plane. The BAT operates over the 15-150 keV energy range with 7 keV resolution, a sensitivity of 10^sup -8^ erg s^sup -1^ cm^sup -2^, and a 1.4 sr (half-coded) FOV. We expect to detect > 100 GRBs/year for a 2-year mission. The BAT also performs an all-sky hard X-ray survey with a sensitivity of 2 m Crab (systematic limit) and it serves as a hard X-ray transient monitor.PUBLICATION ABSTRACT
In this concluding part of the series of three papers dedicated to the Swift BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and statistical properties of the source sample. ...Using a dedicated method to extract time-averaged spectra of BAT sources, we show that Galactic sources have, generally, softer spectra than extragalactic objects and that Seyfert 2 galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert galaxies is consistent with a power-law with a photon index of 2.00 plus or minus 0.07. The cumulative flux-number relation for the extragalactic sources in the 14-170 keV band is best described by a power-law with a slope alpha = 1.55 plus or minus 0.20 and a normalization of 9.6 plus or minus 1.9 x 10 super(-3) AGNs deg super(-2) (or 396 plus or minus 80 AGNs all-sky) above a flux level of 2 x 10 super(-11) ergs cm super(-2) s super(-1) ( similar to 0.85 mcrab). The integration of the cumulative flux per unit area Indicates that BAT resolves 1%-2% of the X-ray background emission in the 14-170 keV band. A subsample of 24 extragalactic sources above the 4.5 sigma detection limit is used to study the statistical properties of AGNs. This sample is composed of local Seyfert galaxies (z = 0.026, median value) and similar to 10% blazars. We find that 55% of the Seyfert galaxies are absorbed by column densities of N sub(H) > 10 super(22) H atoms cm super(-2) but that none is genuinely bona fide Compton thick. This study shows the capabilities of BAT to probe the hard X-ray sky to the millicrab level.
Swift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB 081028. A rising phase is likely ...present in all GRBs but is usually hidden by the prompt tail emission and constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the afterglow onset (start of the forward shock emission along our line of sight when the outflow is decelerated) as the origin of the observed re-brightening. We investigate alternative scenarios and find that the observations are consistent with the predictions for a narrow jet viewed off-axis. The high on-axis energy budget implied by this interpretation suggests different physical origins of the prompt and (late) afterglow emission. Strong spectral softening takes place from the prompt to the steep decay phase: we track the evolution of the spectral peak energy from the γ-rays to the X-rays and highlight the problems of the high latitude and adiabatic cooling interpretations. Notably, a softening of both the high and low spectral slopes with time is also observed. We discuss the low on-axis radiative efficiency of GRB 081028 comparing its properties against a sample of Swift long GRBs with secure Eγ,iso measurements.
One of the most prominent, yet controversial associations derived from the ensemble of prompt-phase observations of gamma-ray bursts (GRBs) is the apparent correlation in the source frame between the ...peak energy (E peak) of the Delta *nF( Delta *n) spectrum and the isotropic radiated energy, E iso. Since most GRBs have E peak above the energy range (15-150 keV) of the Burst Alert Telescope (BAT) on Swift, determining accurate E peak values for large numbers of Swift bursts has been difficult. However, by combining data from Swift/BAT and the Suzaku Wide-band All-Sky Monitor (WAM), which covers the energy range from 50 to 5000 keV, for bursts which are simultaneously detected, one can accurately fit E peak and E iso and test the relationship between them for the Swift sample. Between the launch of Suzaku in 2005 July and the end of 2009 April, there were 48 GRBs that triggered both Swift/BAT and WAM, and an additional 48 bursts that triggered Swift and were detected by WAM, but did not trigger. A BAT-WAM team has cross-calibrated the two instruments using GRBs, and we are now able to perform joint fits on these bursts to determine their spectral parameters. For those bursts with spectroscopic redshifts, we can also calculate the isotropic energy. Here, we present the results of joint Swift/BAT-Suzaku/WAM spectral fits for 91 of the bursts detected by the two instruments. We show that the distribution of spectral fit parameters is consistent with distributions from earlier missions and confirm that Swift bursts are consistent with earlier reported relationships between E peak and isotropic energy. We show through time-resolved spectroscopy that individual burst pulses are also consistent with this relationship.
We describe and discuss the spectral and temporal characteristics of the prompt emission and X-ray afterglow emission of X-ray flashes (XRFs) and X- ray-rich gamma-ray bursts (XRRs) detected and ...observed by Swift between 2004 December and 2006 September. We compare these characteristics to a sample of conventional classical gamma-ray bursts (C-GRBs) observed during the same period. We confirm the correlation between image and fluence noted by others and find further evidence that XRFs, XRRs, and C-GRBs form a continuum. We also confirm that our known redshift sample is consistent with the correlation between the peak energy in the GRB rest frame and the isotropic radiated energy, the so-called image -image relation. The spectral properties of X-ray afterglows of XRFs and C-GRBs are similar, but the temporal properties of XRFs and C-GRBs are quite different. We found that the light curves of C-GRB afterglows show a break to steeper indices (shallow-to-steep break) at much earlier times than do XRF afterglows. Moreover, the overall luminosity of XRF X-ray afterglows is systematically smaller by a factor of 2 or more compared to that of C-GRBs. These distinct differences between the X-ray afterglows of XRFs and C-GRBs may be the key to understanding not only the mysterious shallow-to-steep break in X-ray afterglow light curves, but also the unique nature of XRFs.
BAT and XRT observations of two recent well-covered GRBs observed by Swift, GRB 050315 and GRB 050319, clearly show a prompt component joining the onset of the afterglow emission. By fitting a ...power-law form to the g-ray spectrum, we extrapolate the time-dependent fluxes measured by BAT, in the energy band 15-350 keV, into the spectral regime observed by XRT, 0.2-10 keV, and examine the functional form of the rate of decay of the two light curves. We find that the BAT and XRT light curves merge to form a unified curve. There is a period of steep decay up to 6300 s, followed by a flatter decay. The duration of the steep decay, 6100 s in the source frame after correcting for cosmological time dilation, agrees roughly with a theoretical estimate for the deceleration time of the relativistic ejecta as it interacts with circumstellar material. For GRB 050315, the steep decay can be characterized by an exponential form, where the e-folding decay time t sub(e) 24 c 2 s (BAT), and t sub(e) 35 c 2 s (XRT). For GRB 050319, a power-law decay -d ln f/d ln t = n, where n 3 provides a reasonable fit. The early-time X-ray fluxes are consistent with representing the lower energy tail of the prompt emission and provide our first quantitative measure of the decay of the prompt g-ray emission over a large dynamic range in flux. The initial steep decay is expected, due to the delayed high-latitude photons from a curved shell of relativistic plasma illuminated only for a short interval. The overall conclusion is that the prompt phase of GRBs remains observable for hundreds of seconds longer than previously thought.
We report a correlation based on a spectral simulation study of the prompt emission spectra of gamma-ray bursts (GRBs) detected by the Swift Burst Alert Telescope (BAT). The correlation is between ...the E peak energy, which is the peak energy in the nFn spectrum, and the photon index () derived from a simple power-law (PL) model. The E peak- relation, assuming the typical smoothly broken PL spectrum of GRBs, is log E peak = 3.258 - 0.829 (1.3 <= <= 2.3). We take into account not only a range of E peak energies and fluences, but also distributions for both the low-energy photon index and the high-energy photon index in the smoothly broken PL model. The distribution of burst durations in the BAT GRB sample is also included in the simulation. Our correlation is consistent with the index observed by BAT and E peak measured by the BAT, and by other GRB instruments. Since about 85% of GRBs observed by the BAT are acceptably fit with the simple PL model because of the relatively narrow energy range of the BAT, this relationship can be used to estimate E peak when it is located within the BAT energy range.
Swift triggered on a precursor to the main burst of GRB 061121 ( approximately = 1.314), allowing observations to be made from the optical to gamma-ray bands. Many other telescopes, including ...Konus-Wind, XMM-Newton, ROTSE, and the Faulkes Telescope North, also observed the burst. The gamma-ray, X-ray, and UV/optical emission all showed a peak similar to 75 s after the trigger, although the optical and X-ray afterglow components also appear early on, before or during the main peak. Spectral evolution was seen throughout the burst, with the prompt emission showing a clear positive correlation between brightness and hardness. The SED of the prompt emission, stretching from 1 eV up to 1 MeV, is very flat, with a peak in the flux density at similar to 1 keV. The optical to X-ray spectra at this time are better fitted by a broken, rather than single, power law, similar to previous results for X-ray flares. The SED shows spectral hardening as the afterglow evolves with time. This behavior might be a symptom of self-Comptonization, although circumstellar densities similar to those found in the cores of molecular clouds would be required. The afterglow also decays too slowly to be accounted for by the standard models. Although the precursor and main emission show different spectral lags, both are consistent with the lag-luminosity correlation for long bursts. GRB 061121 is the instantaneously brightest long burst yet detected by Swift. Using a combination of Swift and Konus-Wind data, we estimate an isotropic energy of 2.8 x 10 super(53) ergs over 1 keV-10 MeV in the GRB rest frame. A probable jet break is detected at similar to 2 x 10 super(5) s, leading to an estimate of similar to 10 super(51) ergs for the beaming-corrected gamma-ray energy.
InterPlanetary Network (IPN) data are presented for the gamma-ray bursts in the second Fermi Gamma-Ray Burst Monitor (GBM) catalog. Of the 462 bursts in that catalog between 2010 July 12 and 2012 ...July 11, 428, or 93%, were observed by at least 1 other instrument in the 9-spacecraft IPN. Of the 428, the localizations of 165 could be improved by triangulation. For these bursts, triangulation gives one or more annuli whose half-widths vary between about 2 3° and 16°, depending on the peak flux, fluence, time history, arrival direction, and the distance between the spacecraft. We compare the IPN localizations with the GBM 1 , 2 , and 3 error contours and find good agreement between them. The IPN 3 error boxes have areas between about 8 square arcminutes and 380 square degrees, and are an average of 2500 times smaller than the corresponding GBM 3 localizations. We identify four bursts in the IPN/GBM sample whose origins were given as "uncertain," but may in fact be cosmic. This leads to an estimate of over 99% completeness for the GBM catalog.
The long gamma-ray burst GRB 060714 was observed to exhibit a series of five X-ray flares beginning similar to 70 s after the burst trigger T sub(0) and continuing until similar to T sub(0) + 200 s. ...The first two flares were detected by the Burst Alert Telescope (BAT) on the Swift satellite, before Swift had slewed to the burst location, while the last three flares were strongly detected by the X-Ray Telescope (XRT) but only weakly detected by the BAT. This burst provides an unusual opportunity to track a complete sequence of flares over a wide energy range. The flares were very similar In their light curve morphology, showing power-law rise and fall components, and in most cases significant substructure. The flares also showed strong evolution with time, both spectrally and temporally. The small timescale and large amplitude variability observed are incompatible with an external shock origin for the flares, and support instead late-time sporadic activity either of the central source or of localized dissipation events within the outflow. We show that the flares in GRB 060714 cannot be the result of internal shocks in which the contrast in the Lorentz factor of the colliding shells is very small, and that this mechanism faces serious difficulties in most Swift GRBs. The morphological similarity of the flares and the prompt emission and the gradual and continual evolution of the flares with time makes it difficult and arbitrary to draw a dividing line between the prompt emission and the flares.
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