Early multi-wavelength emission from gamma-ray bursts: from gamma-ray to x-ray
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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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
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
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
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
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
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TL;DR: In this paper, a series of gamma-ray burst spectroscopy catalogs from the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory, each covering a different aspect of burst phenomenology is presented.
Abstract: This is the first in a series of gamma-ray burst spectroscopy catalogs from the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory, each covering a different aspect of burst phenomenology. In this paper, we present time sequences of spectral fit parameters for 156 bursts selected for either their high peak flux or fluence. All bursts have at least eight spectra in excess of 45 σ above background and span burst durations from 1.66 to 278 s. Individual spectral accumulations are typically 128 ms long at the peak of the brightest events but can be as short as 16 ms, depending on the type of data selected. We have used mostly high energy resolution data from the Large Area Detectors, covering an energy range of typically 28-1800 keV. The spectral model chosen is from a small empirically determined set of functions, such as the well-known "GRB" function, that best fits the time-averaged burst spectra. Thus, there are generally three spectral shape parameters available for each of the 5500 total spectra: a low-energy power-law index, a characteristic break energy, and possibly a high-energy power-law index. We present the distributions of the observed sets of these parameters and comment on their implications. The complete set of data that accompanies this paper is necessarily large and thus is archived in the electronic edition of the Astrophysical Journal.
674 citations
TL;DR: In this paper, radio observations of the supernova SN1998bw, which exploded at about the same time and in the same direction as the γ-ray burst GRB980425, were reported.
Abstract: Data accumulated over the past year strongly favour the idea that γ-ray bursts lie at cosmological distances, although the nature of the power source remains unclear. Here we report radio observations of the supernova SN1998bw, which exploded at about the same time, and in about the same direction, as the γ-ray burst GRB980425. At its peak, the supernova was unusually luminous at radio wavelengths. A simple interpretation of the data requires that the source expanded with an apparent velocity of at least twice the speed of light, indicating that the supernova was accompanied by a shock wave moving at relativistic speeds (the ejects of supernovae are typically characterized by non-relativistic velocities). The energy of the shock is at least 10^(49)erg, with an inferred ejecta mass of 10^(-5) solar masses, and we suggest that the early phase of this shock wave produced the burst of γ-rays. Although in general the properties of supernovae are very different from those of γ-ray bursts, we argue that this unusual supernova establishes a second class of γ-ray burst, which is distinctly different from the cosmological kind.
641 citations
TL;DR: In this article, the authors derived the collimation-corrected energy of the GRB bursts, Eγ, from the achromatic break of their afterglow light curve, and found a surprisingly tight correlation between Eγ and the source frame Epeak.
Abstract: We consider all bursts with known redshift and νFν peak energy, E. For a good fraction of them an estimate of the jet opening angle is available from the achromatic break of their afterglow light curve. This allows the derivation of the collimation-corrected energy of the bursts, Eγ. The distribution of the values of Eγ is more spread out than in previous findings, covering about 2 orders of magnitude. We find a surprisingly tight correlation between Eγ and the source frame Epeak: E(1 + z) ∝ E. This correlation can shed light on the still uncertain radiation processes for the prompt GRB emission. More importantly, if the small scatter of this newly found correlation could be confirmed by forthcoming data, it would be possible to use it for cosmological purposes.
627 citations
INAF1
TL;DR: In this article, the authors considered all bursts with known redshift and peak energy, and they found a surprisingly tight correlation between the peak energy and the source frame of the burst, which can shed light on the uncertain radiation processes for the prompt GRB emission.
Abstract: We consider all bursts with known redshift and $
u F_
u$ peak energy, $E^{obs}_{peak}$. For a good fraction of them an estimate of the jet opening angle is available from the achromatic break of their afterglow light curve. This allows the derivation of the collimation--corrected energy of the bursts, $E_\gamma$. The distribution of the values of $E_\gamma$ is more spread with respect to previous findings, covering about two orders of magnitude. We find a surprisingly tight correlation between $E_\gamma$ and the source frame $E_{peak}$: $E^{obs}_{peak}(1+z) \propto E_\gamma^{0.7}$. This correlation can shed light on the still uncertain radiation processes for the prompt GRB emission. More importantly, if the small scatter of this newly found correlation will be confirmed by forthcoming data, it will be possible to use it for cosmological purposes.
593 citations
Goddard Space Flight Center1, University of Virginia2, University of Leicester3, University of Nevada, Las Vegas4, University College London5, Pennsylvania State University6, University of Maryland, Baltimore County7, National Research Council8, Universities Space Research Association9, University of Maryland, College Park10, Los Alamos National Laboratory11, Brera Astronomical Observatory12, INAF13, University of Milan14, Sonoma State University15, University of Copenhagen16, University of California, Berkeley17, Marshall Space Flight Center18, Stockholm University19, University of Amsterdam20, Saitama University21
TL;DR: In this article, the authors reported the detection of the X-ray afterglow from the short burst GRB 050509B and its location on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225.
Abstract: Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. The long-duration type leave a strong afterglow and have been extensively studied. So we have a good idea of what causes them: explosions of massive stars in distant star-forming galaxies. Short GRBs, with no strong afterglow, were harder to pin down. The Swift satellite, launched last November, is designed to study bursts as soon as they happen. Having shown its worth with long GRBs (reported in the 18 August issue of Nature), Swift has now bagged a short burst, GRB 050509B, precisely measured its location and detected the X-ray afterglow. Four papers this week report on this and another recent short burst. Now, over 20 years after they were first recognized, the likely origin of the short GRBs is revealed as a merger between neutron stars of a binary system and the instantaneous production of a black hole. Gamma-ray bursts (GRBs) come in two classes1: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars2. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect3,4 if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.
590 citations