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.
Citations
More filters
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
References
More filters
Brera Astronomical Observatory1, University of Leicester2, University of Milan3, Pennsylvania State University4, Goddard Space Flight Center5, Universities Space Research Association6, National Research Council7, University of Texas at Austin8, University of Nevada, Las Vegas9, Johns Hopkins University10, INAF11
TL;DR: In this article, the early phases of the X-ray emission of five long-lived γ-ray bursts were studied using the Swift satellite, and it was shown that violent shock interactions take place in the early jet outflows, leading to a rapid falloff for the first few hundred seconds followed by a less rapid decline lasting several hours.
Abstract: The Swift satellite, launched in November last year, is designed to study γ-ray bursts (GRBs) as soon as they happen. GRBs are the most powerful explosions known in the Universe, and Swift's ability to study the early phases of the X-ray afterglow was expected to yield exciting results. Swift has now bagged its first two long GRBs: in both, the X-ray afterglow emission declined rapidly in the first few hundred seconds, then flattened out. The steep decline was unexpected, and neither it nor the spectral properties of the afterglow can be explained by current models. ‘Long’ γ-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the γ-rays we see1,2. As the jet travels further outward into the surrounding circumstellar medium, ‘external’ shocks create the afterglow emission seen in the X-ray, optical and radio bands1,2. Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.
278 citations
TL;DR: In this article, the authors reanalyze the refreshed shock scenario, in which slower material catches up with the decelerating ejecta and reenergizes it, and find that during this time the overall spectral peak is found at the characteristic frequency of the reverse shock.
Abstract: The standard model of gamma-ray burst afterglows is based on synchrotron radiation from a blast wave produced when the relativistic ejecta encounters the surrounding medium. We reanalyze the refreshed shock scenario, in which slower material catches up with the decelerating ejecta and reenergizes it. This energization can be done either continuously or in discrete episodes. We show that such a scenario has two important implications. First, there is an additional component coming from the reverse shock that goes into the energizing ejecta. This persists for as long as the reenergization itself, which could extend for up to days or longer. We find that during this time the overall spectral peak is found at the characteristic frequency of the reverse shock. Second, if the injection is continuous, the dynamics will be different from that in constant energy evolution and will cause a slower decline of the observed fluxes. A simple test of the continuously refreshed scenario is that it predicts a spectral maximum in the far-infrared or millimeter range after a few days.
266 citations
TL;DR: In particular, the presence of flaring activity on a wide range of timescales probably requires late-time energy production within the GRB engine as discussed by the authors, which is in common with the two likely progenitors of the two classes of gamma-ray bursts.
Abstract: Early-time X-ray observations of gamma-ray bursts (GRBs) with the Swift satellite have revealed a more complicated phenomenology than was known before. In particular, the presence of flaring activity on a wide range of timescales probably requires late-time energy production within the GRB engine. Since the flaring activity is observed in both long and short GRBs, its origin must be within what is in common for the two likely progenitors of the two classes of bursts: a hyperaccreting accretion disk around a black hole of a few solar masses. Here we show that some of the observational properties of the flares, such as the duration-timescale correlation, and the duration-peak luminosity anticorrelation displayed by most flares within a given burst, are qualitatively consistent with viscous disk evolution, provided that the disk at large radii either fragments or otherwise suffers large-amplitude variability. We discuss the physical conditions in the outer parts of the disk and conclude that gravitational instability, possibly followed by fragmentation, is the most likely candidate for this variability.
266 citations
TL;DR: In this article, the authors reanalyze the refreshed shock scenario, in which slower material catches up with the decelerating ejecta and reenergizes it, and find that during this time the overall spectral peak is found at the characteristic frequency of the reverse shock.
Abstract: The standard model of Gamma-Ray Bursts afterglows is based on synchrotron radiation from a blast wave produced when the relativistic ejecta encounters the surrounding medium. We reanalyze the refreshed shock scenario, in which slower material catches up with the decelerating ejecta and reenergizes it. This energization can be done either continuously or in discrete episodes. We show that such scenario has two important implications. First there is an additional component coming from the reverse shock that goes into the energizing ejecta. This persists for as long as the re-energization itself, which could extend for up to days or longer. We find that during this time the overall spectral peak is found at the characteristic frequency of the reverse shock. Second, if the injection is continuous, the dynamics will be different from that in constant energy evolution, and will cause a slower decline of the observed fluxes. A simple test of the continuously refreshed scenario is that it predicts a spectral maximum in the far IR or mm range after a few days.
253 citations
TL;DR: In this paper, the authors investigated the spectral evolution of gamma-ray bursts observed with the BATSE Spectroscopy Detectors and found that the peak energy either rises with or slightly precedes major intensity increases and softens for the remainder of the pulse.
Abstract: We investigate spectral evolution in 37 bright, long gamma-ray bursts observed with the BATSE Spectroscopy Detectors. High resolution spectra are characterized by the energy of the peak of
fn~and the evolution of this quantity is examined relative to the emission intensity. In most cases it is found that this peak energy either rises with or slightly precedes major intensity increases and softens for the remainder of the pulse. Inter-pulse emission is generally harder early in the burst. For bursts with multiple intensity pulses, later spikes tend to be softer than earlier ones indicating that the energy of the peak of
fn~is bounded by an envelope which decays with time. Evidence is found that bursts in which the bulk of the flux comes well after the event which triggers the instrument tend to show less peak energy variability and are not as hard as several bursts in which the emission occurs promptly after the trigger. Several recently proposed burst models are examined in light of these results and no qualitative conflicts with the observations presented here are found.
227 citations