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Journal ArticleDOI

Early multi-wavelength emission from gamma-ray bursts: from gamma-ray to x-ray

31 Jul 2006-New Journal of Physics (IOP Publishing)-Vol. 8, Iss: 7, pp 121-121
TL;DR: The early high-energy emission from both long and short gamma-ray bursts (GRBs) has been revolutionized by the Swift mission as discussed by the authors, which showed that the non-thermal x-ray emission transitions smoothly from the prompt phase into a decaying phase regardless of the details of the light curve.
Abstract: The study of the early high-energy emission from both long and short gamma-ray bursts (GRBs) has been revolutionized by the Swift mission. The rapid response ofSwiftshows that the non-thermal x-ray emission transitions smoothly from the prompt phase into a decaying phase whatever the details of the light curve. The decay is often categorized by a steep-to-shallow transition suggesting that the prompt emission and the afterglow are two distinct emission components.InthoseGRBswithaninitiallysteeplydecayingx-raylightcurve,we are probably seeing off-axis emission due to termination of intense central engine activity. This phase is usually followed, within the first hour, by a shallow decay, giving the appearance of a late-emission hump. The late-emission hump can last for up to a day, and hence, although faint, is energetically very significant. The energy emitted during the late-emission hump is very likely due to the forward shock being constantly refreshed by either late central engine activity or less relativistic material emitted during the prompt phase. In other GRBs, the early x-ray emission decays gradually following the prompt emission with no evidence for early temporal breaks, and in these bursts the emission may be dominated by classical afterglow emission from the external shock as the relativistic jet is slowed by interaction with the surrounding circum-burst medium. At least half of the GRBs observed by Swift also show erratic x-ray flaring behaviour, usually within the first few hours. The properties of the x-ray flares suggest that they are due to central engine activity. Overall, the observed wide variety of early high-energy phenomena pose a major challenge to GRB models.

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Citations
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Journal ArticleDOI
TL;DR: In this article, the authors review the rapid observational and theoretical progress in this dynamical research field during the first two-year of the Swift mission, focusing on how observational breakthroughs have revolutionized our understanding of the physical origins of GRBs.
Abstract: Since the successful launch of NASA's dedicated gamma-ray burst (GRB) mission, Swift, the study of cosmological GRBs has entered a new era. Here I review the rapid observational and theoretical progress in this dynamical research field during the first two-year of the Swift mission, focusing on how observational breakthroughs have revolutionized our understanding of the physical origins of GRBs. Besides summarizing how Swift helps to solve some pre-Swift mysteries, I also list some outstanding problems raised by the Swift observations. An outlook of GRB science in the future, especially in the GLAST era, is briefly discussed.

514 citations

Journal ArticleDOI
TL;DR: In this article, the authors analyzed the properties of the shallow decay segment in Swift XRT light curves and concluded that the observed shallow decay phase likely has diverse physical origins, likely a refreshed external shock.
Abstract: The origin of the shallow decay segment in Swift XRT light curves remains a puzzle. We analyze the properties of this segment with a sample of 53 long Swift GRBs detected before 2007 February. We show that the distributions of the sample's characteristics are lognormal or normal, and its isotropic X-ray energy (E(iso),X) is linearly correlated with the prompt gamma-ray energy but with a steeper photon spectrum, aside from some X-ray flashes. No significant spectral evolution is observed from this phase to the following phase, and the latter is usually consistent with external shock models, implying that the shallow decay is also of external-shock origin, likely a refreshed external shock. Within the refreshed-shock model, the data are generally consistent with a roughly constant injection luminosity up to the end of this phase, t(b). A positive correlation between Eiso; X and tb also favors this scenario. Among the 13 bursts that have well-sampled optical light curves, six have an optical break around tb and the breaks are consistent with being achromatic. However, the other seven either do not show an optical break or have a break at an epoch different from tb. This raises a concern for the energy injection scenario, suggesting that the optical and X-ray emission may not be the same component, at least for some bursts. There are four significant outliers in the sample, GRBs 060413, 060522, 060607A, and 070110. The shallow decay phase in these bursts is immediately followed by a very steep decay after tb, which is inconsistent with any external-shock model. The optical data for these bursts evolve independently from the X-ray data. These X-ray plateaus likely have an internal origin and demand continuous operation of a long-term central engine. We conclude that the observed shallow decay phase likely has diverse physical origins.

296 citations

Journal ArticleDOI
TL;DR: In this article, the authors present the Swift observations of GRB 090515 and compare it to other gamma-ray bursts (GRBs) in the Swift sample, and suggest it might be energy injection from an unstable millisecond pulsar contributing to their emission.
Abstract: The majority of short gamma-ray bursts (SGRBs) are thought to originate from the merger of compact binary systems collapsing directly to form a black hole. However, it has been proposed that both SGRBs and long gamma-ray bursts (LGRBs) may, on rare occasions, form an unstable millisecond pulsar (magnetar) prior to final collapse. GRB 090515, detected by the Swift satellite was extremely short, with a T90 of 0.036 ± 0.016 s, and had a very low fluence of 2 × 10−8 erg cm−2 and faint optical afterglow. Despite this, the 0.3–10 keV flux in the first 200 s was the highest observed for an SGRB by the Swift X-ray Telescope (XRT). The X-ray light curve showed an unusual plateau and steep decay, becoming undetectable after ∼500 s. This behaviour is similar to that observed in some long bursts proposed to have magnetars contributing to their emission. In this paper, we present the Swift observations of GRB 090515 and compare it to other gamma-ray bursts (GRBs) in the Swift sample. Additionally, we present optical observations from Gemini, which detected an afterglow of magnitude 26.4 ± 0.1 at T+ 1.7 h after the burst. We discuss potential causes of the unusual 0.3–10 keV emission and suggest it might be energy injection from an unstable millisecond pulsar. Using the duration and flux of the plateau of GRB 090515, we place constraints on the millisecond pulsar spin period and magnetic field.

246 citations

Journal ArticleDOI
TL;DR: In this article, the optical afterglow data for 57 pre- and post-Swift GRBs were analyzed to explore whether the observed breaks in the afterglog light curves can be interpreted as jet breaks, as well as their implications for jet energetics.
Abstract: The Swift XRT data for 179 GRBs (050124 to 070129) and the optical afterglow data for 57 pre- and post-Swift GRBs are analyzed to explore whether the observed breaks in the afterglow light curves can be interpreted as jet breaks, as well as their implications for jet energetics. We find that no burst is included in our "Platinum" sample, in which the data fully satisfy the jet break criteria. By relaxing one or more of the requirements for a jet break, candidates to various degrees are identified. In the X-ray band, 42 of 103 well-sampled X-ray light curves have a decay slope greater than or similar to 1.5 in the postbreak segment (the "Bronze" sample), and 27 of these also satisfy the closure relations of the forward-shock models ("Silver" sample). The numbers of "Bronze" and "Silver" candidates in the optical light curves are 27 and 23, respectively. The X-ray break time is earlier than that in the optical bands. Among 13 bursts having both optical and X-ray light curves, only seven have an achromatic break, and even in these cases, only in one band do the data satisfy the closure relations ("Gold" sample). These results raise concerns about interpreting the breaks as jet breaks and further inferring GRB energetics. Assuming that the "Silver" and "Gold" breaks are jet breaks, we derive jet opening angles (theta(j)) and kinetic energies (E(K)) or lower limits on them and find that the EK distribution is much more scattered than the pre-Swift sample, but a tentative anticorrelation between theta(j) and E(K,iso) is found, indicating that the E(K) could still be quasi-universal.

234 citations

Journal ArticleDOI
TL;DR: In this article, the authors consider the long-term evolution of debris following the tidal disruption of compact stars in the context of short gamma ray bursts and find that powerful winds are launched from the surface of the disk, driven by the recombination of free nucleons into α-particles.
Abstract: We consider the long-term evolution of debris following the tidal disruption of compact stars in the context of short gamma ray bursts. The initial encounter impulsively creates a hot, dense, neutrino-cooled disk capable of powering the prompt emission. After a long delay, we find that powerful winds are launched from the surface of the disk, driven by the recombination of free nucleons into α-particles. The associated energy release depletes the mass supply and eventually shuts off activity of the central engine. As a result, the luminosity and mass accretion rate deviate from the earlier self-similar behavior expected for an isolated ring with efficient cooling. This then enables a secondary episode of delayed activity to become prominent as an observable signature, when material in the tidal tails produced by the initial encounter returns to the vicinity of the central object. The timescale of the new accretion event can reach tens of seconds to minutes, depending on the details of the system. The associated energies and timescales are consistent with those occurring in X-ray flares.

155 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors considered the problem of a gamma-ray burst (GRB) fireball with an additional energy injection, either in the form of a Poynting-flux-dominated outflow or a kinetic-energy-dominated matter shell injected after the burst.
Abstract: We consider generically the problem of a gamma-ray burst (GRB) fireball with an additional energy injection, either in the form of a Poynting-flux-dominated outflow or a kinetic-energy-dominated matter shell injected after the burst. Generally, a total injection energy comparable to that of the impulsive energy in the initial fireball is required to make a detectable signature in the afterglow light curves. When this criterion is met in the case of Poynting-flux-dominated injection, this leads to a gradual achromatic bump appearing in the otherwise power-law afterglow light curve. Alternatively, in the case when the injection is kinetic-energy-dominated, the results depend on whether the collision between the rear (injected) and the forward shell is mild or violent. If the relative velocity between the colliding shells does not exceed a critical value defined by their energy ratio, the collision is mild, and the injection may be analogous to the Poynting-flux injection case. Otherwise, the injection is violent, and an additional pair of strong shocks will form at the discontinuity between two colliding shells, so that there are altogether three shock-heated regions from which the emission contributes to the final light curves. We describe the shell-merging process in detail, including collision and relaxation, by taking into account the dynamical evolution and the emission from the various shocks involved. Assuming synchrotron emission, we calculate afterglow light curves in the X-ray, optical, and radio bands for the various cases. The injection signatures due to violent matter-dominated collisions are abrupt and complicated, because of the emission from any of the three emitting regions and depending on the injection parameters and the observed energy bands. This differs from the gradual bump signature found in the Poynting-flux injection case. In both the Poynting-flux-dominated and the kinetic-energy-dominated cases, the energetics of the fireball as well as the absolute afterglow flux level after the injection are boosted with respect to the one without postburst injection. Identifying the different injection signatures from future early afterglow observations may provide diagnostics about the nature of the fireball and of the central engine.

172 citations

Journal ArticleDOI
TL;DR: In this paper, the structure and time dependence of relativistic jets depend on the stellar envelope and central engine properties, assuming a steady jet injection, and it takes a few seconds for the jet to bore its way through the stellar core; most of the energy output during that period goes into a cocoon of magnetized plasma surrounding the jet.
Abstract: In the collapsar scenario, gamma-ray bursts are caused by relativistic jets expelled along the rotation axis of a collapsing stellar core. We discuss how the structure and time dependence of such jets depend on the stellar envelope and central engine properties, assuming a steady jet injection. It takes a few seconds for the jet to bore its way through the stellar core; most of the energy output during that period goes into a cocoon of relativistic plasma surrounding the jet. This material subsequently forms a bubble of magnetized plasma that takes several hours to expand, subrelativistically, through the envelope of a high-mass supergiant. Jet breakthrough and a conventional burst would be expected not only in He stars but possibly also in blue supergiants. Shock waves and magnetic dissipation in the escaping bubble can contribute a nonthermal UV/X-ray afterglow, and also excite Fe line emission from thermal gas, in addition to the standard jet deceleration power-law afterglow.

159 citations

Journal ArticleDOI
20 Feb 2006
TL;DR: Swift observations of the � -ray burst GRB 050315 from 80 s to 10 days after the onset of the burst suggest that the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt prompt emission; this strongsimilarity to that of the prompt hard X-rays/� -ray emission is suggested.
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

152 citations

Journal ArticleDOI
TL;DR: In this paper, the authors suggest that fragmentation and subsequent accretion during the collapse of a rapidly rotating stellar core offers a natural mechanism for this phenomenon, and they also suggest that the central engine is active or restarted at late times.
Abstract: Recent gamma-ray burst observations have revealed late-time, highly energetic events that deviate from the simplest expectations of the standard fireball picture. Instead, they may indicate that the central engine is active or restarted at late times. We suggest that fragmentation and subsequent accretion during the collapse of a rapidly rotating stellar core offers a natural mechanism for this.

141 citations

Journal ArticleDOI
TL;DR: In this paper, the authors consider three possible models for the geometry of relativistic blast-waves (spherical outflows, non-spreading jets and spreading jets), two possible dynamical regimes for the forward shock (adiabatic and fully radiative), and take into account a possible angular structure of the outflow and delayed energy injection in the blast-wave to identify the models which reconcile the X-ray light-curve decay with the slope of the Xray continuum for each of the above three afterglow phases.
Abstract: The X-ray light curves of nine Swift XRT afterglows (050126, 050128, 050219A, 050315, 050318, 050319, 050401, 050408 and 050505) display a complex behaviour: a steep t −3.0±0.3 decay until ∼400 s, followed by a significantly slower t −0.65±0.20 fall-off, which at 0.2‐ 2 day after the burst evolves into a t −1.7±0.5 decay. We consider three possible models for the geometry of relativistic blast-waves (spherical outflows, non-spreading jets and spreading jets), two possible dynamical regimes for the forward shock (adiabatic and fully radiative), and we take into account a possible angular structure of the outflow and delayed energy injection in the blast-wave to identify the models which reconcile the X-ray light-curve decay with the slope of the X-ray continuum for each of the above three afterglow phases. By piecing together the various models for each phase in a way that makes physical sense, we identify possible models for the entire X-ray afterglow. The major conclusion of this work is that a long-lived episode of energy injection in the blast-wave, during which the shock energy increases at t 1.0±0.5 , is required for five afterglows and could be at work in the other four as well. For some afterglows, there may be other mechanisms that can explain the t < 400 s fast falling-off X-ray light curve (e.g. the large-angle gamma-ray burst emission), the 400 s to 5 h slow decay (e.g. a structured outflow), or the steepening at 0.2‐2 day (e.g. a jet-break, a collimated outflow transiting from a wind with a r −3 radial density profile to a homogeneous or outward-increasing density region). Optical observations in conjunction with the X-ray can distinguish among these various models. Our simple tests allow the determination of the location of the cooling frequency relative to the X-ray domain and, thus, of the index of the electron power-law distribution with energy in the blast-wave. The resulting indices are clearly inconsistent with a universal value.

131 citations

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