Showing papers by "Pawan Kumar published in 2006"

Swift observations of the X-ray - Bright GRB 050315

Abstract: This paper discusses Swift observations of the � -ray burst GRB 050315 (z ¼ 1:949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t � 5 ) for � 200 s and several breaks. However, both the prompt hard X-ray/� -ray emission (observed by the BAT) and the first � 300 s of X-ray emission (observed bytheXRT)canbeexplainedbyexponentialdecays,withsimilardecayconstants.ExtrapolatingtheBATlightcurve into the XRT band suggests that the rapidly decaying, early X-ray emission was simply a continuation of the fading promptemission;thisstrongsimilaritybetweentheprompt � -rayandearlyX-rayemissionmayberelatedtothesimple temporal and spectral character of this X-ray–rich GRB. Theprompt (BAT) spectrum was steep down to � 15keVand appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline, the X-ray afterglow did not fade for � 1:2 ; 10 4 s, after which time it decayed with at emporal index of� � 0:7, followed by a second break at � 2:5 ; 10 5 s to a slope of � � 2. The apparent ‘‘plateau’’ in the X-raylight curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break, then the jet opening

Swift panchromatic observations of the bright gamma-ray burst GRB 050525a

Abstract: The bright gamma- ray burst GRB 050525a has been detected with the Swift observatory, providing unique multiwavelength coverage from the very earliest phases of the burst. The X- ray and optical / UV afterglow decay light curves both exhibit a steeper slope similar to 0.15 days after the burst, indicative of a jet break. This jet break time combined with the total gamma- ray energy of the burst constrains the opening angle of the jet to be 3 degrees.2. We derive an empirical `` timelag'' redshift from the BAT data of (z) over bar z 0: 69 +/- 0: 02, in good agreement with the spectroscopic redshift of 0.61. Prior to the jet break, the X- ray data can be modeled by a simple power law with index alpha = - 1: 2. However, after 300 s the X- ray flux brightens by about 30% compared to the power- law fit. The optical / UV data have a more complex decay, with evidence of a rapidly falling reverse shock component that dominates in the first minute or so, giving way to a flatter forward shock component at later times. The multiwavelength X- ray/ UV/ optical spectrum of the afterglow shows evidence for migration of the electron cooling frequency through the optical range within 25,000 s. The measured temporal decay and spectral indexes in the X- ray and optical/ UV regimes compare favorably with the standard fireball model for gamma- ray bursts assuming expansion into a constant- density interstellar medium.

Distribution of gamma-ray burst ejecta energy with Lorentz factor

Abstract: The early X-ray afterglow for a significant number of gamma-ray bursts detected by the Swift satellite is observed to have a phase of very slow flux decline with time (F{sub {nu}} {proportional_to} t{sup -{alpha}} with 02 {approx}< {alpha} {approx}< 08) for 10{sup 25} s{approx}< t {approx}< 10{sup 4} s, while the subsequent decline is the usual 1 {approx}< {alpha}{sub 3} {approx}< 15 behavior, that was seen in the pre-Swift era We show that this behavior is a natural consequence of a small spread in the Lorentz factor of the ejecta, by a factor of {approx} 2-4, where the slower ejecta gradually catch-up with the shocked external medium, thus increasing the energy of forward shock and delaying its deceleration The end of the ''shallow'' flux decay stage marks the beginning of the Blandford-McKee self similar external shock evolution This suggests that most of the energy in the relativistic outflow is in material with a Lorentz factor of {approx} 30-50

GRB 060313: A New Paradigm for Short-Hard Bursts?

Abstract: We report the simultaneous observations of the prompt emission in the gamma-ray and hard X-ray bands by the Swift BATand the Konus-Wind instruments of the short-hard burst, GRB 060313. The observations reveal multiple peaks in both the gamma-ray and hard X-ray bands suggesting a highly variable outflow from the central explosion. We also describe the early-time observations of the X-ray and UV/optical afterglows by the Swift XRT and UVOT instruments. The combination of the X-ray and UV/optical observations provides the most comprehensive light curves to date of a short-hard burst at such an early epoch. The afterglows exhibit complex structure with different decay indices and flaring. This behavior can be explained by the combination of a structured jet, radiative loss of energy, and decreasing microphysics parameters occurring in a circumburst medium with densities varying by a factor of approximately two on a length scale of 10 17 cm. These density variations are normally associated with the environment of a massive star and inhomogeneities in its windy medium. However, the mean density of the observed medium (n � 10 � 4 cm 3 ) is much less than that expected for a massive star. Although the collapse of a massive star as theoriginofGRB060313isunlikely,themergerofacompactbinaryalsoposesproblemsforexplainingthebehavior of this burst. Two possible suggestions for explaining this scenario are that some short bursts may arise from a mechanism that does not invoke the conventional compact binary model, or that soft late-time central engine activity is producing UV/optical but no X-ray flaring.

No universality for the electron power-law index (p) in gamma-ray bursts and other relativistic sources

Abstract: The gamma-ray burst (GRB) prompt emission is believed to be from highly relativistic electrons accelerated in relativistic shocks. From the GRB high-energy power-law spectral indices β observed by the Burst and Transient Source Experiment (BATSE) Large Area Detectors (LAD), we determine the spectral index, p, of the electrons' energy distribution. Both the theoretical calculations and numerical simulations of the particle acceleration in relativistic shocks show that p has a universal value ≈2.2-2.3. We show that the observed distribution of p during GRBs is not consistent with a δ-function distribution or a universal p value, with the width of the distribution ≥0.54. The distributions of p during X-ray afterglows are also investigated and found to be inconsistent with a δ-function distribution. The p distributions in blazars and pulsar wind nebulae are also broad, inconsistent with a δ-function distribution.

GRB 060313: A New Paradigm for Short-Hard Bursts?

Abstract: We report the simultaneous observations of the prompt emission in the gamma-ray and hard X-ray bands by the Swift-BAT and the KONUS-Wind instruments of the short-hard burst, GRB 060313. The observations reveal multiple peaks in both the gamma-ray and hard X-ray bands suggesting a highly variable outflow from the central explosion. We also describe the early-time observations of the X-ray and UV/Optical afterglows by the Swift XRT and UVOT instruments. The combination of the X-ray and UV/Optical observations provide the most comprehensive lightcurves to date of a short-hard burst at such an early epoch. The afterglows exhibit complex structure with different decay indices and flaring. This behavior can be explained by the combination of a structured jet, radiative loss of energy, and decreasing microphysics parameters occurring in a circum-burst medium with densities varying by a factor of approximately two on a length scale of 10^17 cm. These density variations are normally associated with the environment of a massive star and inhomogeneities in its windy medium. However, the mean density of the observed medium (n approximately 10^−4 cm^3) is much less than that expected for a massive star. Although the collapse of a massive star as the origin of GRB 060313 is unlikely, the merger of a compact binary also poses problems for explaining the behavior of this burst. Two possible suggestions for explaining this scenario are: some short bursts may arise from a mechanism that does not invoke the conventional compact binary model, or soft late-time central engine activity is producing UV/optical but no X-ray flaring.

A Spectacular Radio Flare from XRF 050416a at 40 days and Implications for the Nature of X-ray Flashes

Abstract: We present detailed optical, near-infrared, and radio observations of the X-ray flash 050416a obtained with Palomar and Siding Springs Observatories as well as HST and the VLA, placing this event among the best-studied X-ray flashes to date. In addition, we present an optical spectrum from Keck LRIS from which we measure the redshift of the burst, z=0.6528. At this redshift the isotropic-equivalent prompt energy release was about 10^51 erg, and using a standard afterglow synchrotron model we find that the blastwave kinetic energy is a factor of 10 larger, E_K,iso ~ 10^52 erg. The lack of an observed jet break to t ~ 20 days indicates that the opening angle is larger than 7 deg and the total beaming-corrected relativistic energy is larger than 10^50 erg. We further show that the burst produced a strong radio flare at t ~ 40 days accompanied by an observed flattening in the X-ray band which we attribute to an abrupt circumburst density jump or an episode of energy injection (either from a refreshed shock or off-axis ejecta). Late-time observations with HST show evidence for an associated supernova with peak optical luminosity roughly comparable to that of SN 1998bw. Next, we show that the host galaxy of XRF 050416a is actively forming stars at a rate of at least 2 M_solar per year with a luminosity of L_B ~ 0.5L* and metallicity of Z ~ 0.2-0.8 Z_solar. Finally, we discuss the nature of XRF 050416a in the context of short-hard gamma-ray bursts and under the framework of off-axis and dirty fireball models for X-ray flashes.

Reverse shock emission as a probe of gamma-ray burst ejecta

Abstract: We calculate the reverse shock (RS) synchrotron emission in the optical and the radio wavelength bands from electron-positron pair-enriched gamma-ray burst ejecta with the goal of determining the pair content of gamma-ray bursts (GRBs) using early-time observations. We take into account an extensive number of physical effects that influence radiation from the RS-heated GRB ejecta. We find that optical/infrared flux depends very weakly on the number of pairs in the ejecta, and there is no unique signature of ejecta pair enrichment if observations are confined to a single wavelength band. It may be possible to determine if the number of pairs per proton in the ejecta is ≥100 by using observations in optical and radio bands; the ratio of flux in the optical and radio at the peak of each respective RS light curve is dependent on the number of pairs per proton. We also find that over a large parameter space, RS emission is expected to be very weak: GRB 990123 seems to have been an exceptional burst in that only a very small fraction of the parameter space produces optical flashes this bright. Also. it is often the case that the optical flux from the forward shock is brighter than the RS flux at deceleration. This could be another possible reason for the paucity of prompt optical flashes with a rapidly declining light curve at early times as was seen in GRBs 990123 and 021211. Some of these results are a generalization of similar results reported in Nakar & Piran.

Swift observations of the prompt X-ray emission and afterglow from GRB050126 and GRB050219A

Abstract: We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Bursts GRB050126 and GRB050219A as observed by Swift. The X-ray light-curves of these 2 bursts both show remarkably steep early decays (F(t) oc t -3 ), breaking to flatter slopes on timescales of a few hundred seconds. For GRB050126 the burst shows no evidence of spectral evolution in the 20-150 keV band, and the spectral index of the γ-ray and X-ray afterglows are significantly different suggesting a separate origin. By contrast the BAT spectrum of GRB050219A displays significant spectral evolution, becoming softer at later times, with Γ evolving toward the XRT photon index seen in the early X-ray afterglow phase. For both bursts, the 0.2-10 keV spectral index pre- and post-break in the X-ray decay light-curve are consistent with no spectral evolution. We suggest that the steep early decline in the X-ray decay light-curve is either the curvature tail of the prompt emission; X-ray flaring activity; or external forward shock emission from a jet with high density regions of small angular size (>Γ -1 ). The late slope we associate with the forward external shock.

A unified picture for gamma-ray burst prompt and X-ray afterglow emissions

Abstract: Also available at http://www.blackwell-synergy.com/doi/abs/10.1111/j.1745-3933.2006.00138.x. The definitive version is available at www.blackwell-synergy.com.

A unified picture for the gamma-ray and prompt optical emissions of GRB 990123

Abstract: The prompt optical emission of GRB 990123 was uncorrelated to the gamma-ray light-curve and exhibited temporal properties similar to those of the steeply-decaying, early X-ray emission observed by Swift at the end of many bursts. These facts suggest that the optical counterpart of GRB 990123 was the large-angle emission released during (the second pulse of) the burst. If the optical and gamma-ray emissions of GRB 990123 have, indeed, the same origin then their properties require that (i) the optical counterpart was synchrotron emission and the gamma-rays arose from inverse-Compton scatterings ("synchrotron self-Compton model"), (ii) the peak-energy of the optical-synchrotron component was at ~20 eV, and (iii) the burst emission was produced by a relativistic outflow moving at Lorentz factor > 450 and at a radius > 10^{15} cm, which is comparable to the outflow deceleration radius. Because the spectrum of GRB 990123 was optically thin above 2 keV, the magnetic field behind the shock must have decayed on a length-scale of <1% of the thickness of the shocked gas, which corresponds to 10^6-10^7 plasma skin-depths. Consistency of the optical counterpart decay rate and its spectral slope (or that of the burst, if they represent different spectral components) with the expectations for the large-angle burst emission represents the most direct test of the unifying picture proposed here for GRB 990123.