GeV emission from gamma-ray burst afterglows

We calculate the GeV afterglow emission expected from a few mechanisms related to gamma-ray bursts (GRBs) and their afterglows. Given the brightness of the early X-ray afterglow emission measured by Swift /X-Ray Telescope, Gamma-ray Large Area Space Telescope (GLAST)/Large Area Telescope (LAT) should detect the self-Compton emission from the forward shock driven by the GRB ejecta into the circumburst medium. Novel features discovered by Swift in X-ray afterglows (plateaus and chromatic light-curve breaks) indicate the existence of a pair-enriched, relativistic outflow located behind the forward shock. Bulk and inverse-Compton upscattering of the prompt GRB emission by such outflows provide another source of GeV afterglow emission detectable by LAT. The large-angle burst emission and synchrotron forward-shock emission are, most likely, too dim at high photon energy to be observed by LAT. The spectral slope of the high-energy afterglow emission and its decay rate (if it can be measured) allow the identification of the mechanism producing the GeV transient emission following GRBs.     

Impulsive and Varying Injection in Gamma-Ray Burst Afterglows

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.     

A unified treatment of the gamma-ray burst 021211 and its afterglow

The gamma-ray burst (GRB) 021211 had a simple light curve, containing only one peak and the expected Poisson fluctuations. Such a burst may be attributed to an external shock, offering the best chance for a unified understanding of the gamma-ray burst and afterglow emissions. We analyse the properties of the prompt (burst) and delayed (afterglow) emissions of GRB 021211 within the fireball model. Consistency between the optical emission during the first 11 min (which, presumably, comes from the reverse shock heating of the ejecta) and the later afterglow emission (arising from the forward shock) requires that, at the onset of deceleration (∼2 s), the energy density in the magnetic field in the ejecta, expressed as a fraction of the equipartition value  (ɛ _B )  , is larger than in the forward shock at 11 min by a factor of approximately 10³. We find that synchrotron radiation from the forward shock can account for the gamma-ray emission of GRB 021211; to explain the observed GRB peak flux requires that, at 2 s,  ɛ _B   in the forward shock is larger by a factor 100 than at 11 min. These results suggest that the magnetic field in the reverse shock and early forward shock is a frozen-in field originating in the explosion and that most of the energy in the explosion was initially stored in the magnetic field. We can rule out the possibility that the ejecta from the burst for GRB 021211 contained more than 10 electron–positron pairs per proton.     

Very Early Optical Afterglows for Geometric Models of X-ray Flashes and X-ray Rich GRBs

If X-ray flashes (XRFs) and X-ray rich Gamma-ray Bursts (XRRGs) have the same origin as the Gamma-ray bursts (GRBs) but are viewed off-center from structured jets,their early afterglows may differ from those of GRBs,and when the ultra-relativistic outflow inter-acts with the surrounding medium,there are two shocks formed,a forward shock (FS),and a reverse shock (RS). We calculate numerically the early afterglow powered by uniform jets,Gaussian jets and power-law jets in the forward-reverse shock scenario. A set of differen-tial equations govern the dynamical evolution. The synchrotron self-Compton effect has been taken into account in the calculation. In the uniform jets,the very early afterglows of XRRGs and XRFs are significantly lower than the GRBs and the observed peak times of RS emission are later in the interstellar medium environment. The RS components in XRRGs and XRFs are difficult to detect,but in the stellar wind environment,the reduction of the very early flux and the delay of the RS peak time are not so remarkable. In nonuniform jets (Gaussian and power-law jets),where there are emission materials on the line of sight,the very early light curve resembles equivalent isotropic ejecta in general although the RS flux decay index shows notable deviations if the RS is relativistic (in stellar wind).     

GRB 990123: reverse and internal shock flashes and late afterglow behaviour

The prompt ( t ≲0.16 d) light curve and initial 9th-magnitude optical flash from GRB 990123 can be attributed to a reverse external shock, or possibly to internal shocks. We discuss the time decay laws and spectral slopes expected under various dynamical regimes, and the constraints imposed on the model by the observations, arguing that they provide strongly suggestive evidence for features beyond those in the simple standard model. The longer term afterglow behaviour is discussed in the context of the forward shock, and it is argued that, if the steepening after 3 d is due to a jet geometry, this is likely to be a result of jet-edge effects, rather than sideways expansion.     

Synchrotron emission in the fast cooling regime: which spectra can be explained?

We consider the synchrotron emission from relativistic shocks assuming that the radiating electrons cool rapidly (either through synchrotron or any other radiation mechanism). It is shown that the theory of synchrotron emission in the fast cooling regime can account for a wide range of spectral shapes. In particular, the magnetic field, which decays behind the shock front, brings enough flexibility to the theory to explain the majority of gamma-ray burst spectra even in the parameter-free fast cooling regime. Also, we discuss whether location of the peak in observed spectral energy distributions of gamma-ray bursts and active galactic nuclei can be made consistent with predictions of diffusive shock acceleration theory, and find that the answer is negative. This result is a strong indication that a particle injection mechanism, other than the standard shock acceleration, works in relativistic shocks.     

A systematic description of shocks in gamma-ray bursts – II. Simulation

In Paper I, we presented a detailed formulation of the relativistic shocks and synchrotron emission in the context of gamma-ray burst (GRB) physics. To see how well this model reproduces the observed characteristics of the GRBs and their afterglows, here we present the results of some simulations based on this model. They are meant to reproduce the prompt and afterglow emissions in some intervals of time during a burst. We show that this goal is achieved for both short and long GRBs and their afterglows, at least for part of the parameter space. Moreover, these results are evidence of the physical relevance of the two phenomenological models we have suggested in Paper I for the evolution of the active region – synchrotron emitting region in a shock. The dynamical active region model seems to reproduce the observed characteristics of prompt emissions and late afterglow better than the quasi-steady model which is more suitable for the onset of afterglows. Therefore, these simulations confirm the arguments presented in Paper I about the behaviour of these models based on their physical properties.     

Relativistic blast waves and synchrotron emission

Relativistic shocks can accelerate particles by the first-order Fermi mechanism; the particles then emit synchrotron emission in the post-shock gas. This process is of particular interest in the models used for the afterglow of gamma-ray bursts. In this paper we use recent results in the theory of particle acceleration at highly relativistic shocks to model the synchrotron emission in an evolving, inhomogeneous and highly relativistic flow. We have developed a numerical code that integrates the relativistic Euler equations for fluid dynamics with a general equation of state, together with a simple transport equation for the accelerated particles. We present tests of this code and, in addition, we use it to study the gamma-ray burst afterglow predicted by the fireball model, along with the hydrodynamics of a spherically-symmetric relativistic blast wave.
We find that, while broadly speaking the behaviour of the emission is similar to that already predicted with semi-analytic approaches, the detailed behaviour is somewhat different. The 'breaks' in the synchrotron spectrum behave differently with time, and the spectrum above the final break is harder than had previously been expected. These effects are due to the incorporation of the geometry of the (spherical) blast wave, along with relativistic beaming and adiabatic cooling of the energetic particles leading to a mix, in the observed spectrum, between recently injected 'uncooled' particles and the older 'cooled' population in different parts of the evolving, inhomogeneous flow.     

Radiative losses and cut-offs of energetic particles at relativistic shocks

We investigate the acceleration and simultaneous radiative losses of electrons in the vicinity of relativistic shocks. Particles undergo pitch angle diffusion, gaining energy as they cross the shock by the Fermi mechanism and also emitting synchrotron radiation in the ambient magnetic field. A semi-analytic approach is developed which allows us to consider the behaviour of the shape of the spectral cut-off and the variation of that cut-off with the particle pitch angle. The implications for the synchrotron emission of relativistic jets, such as those in gamma-ray burst sources and blazars, are discussed.     

The Large-Scale, Decelerating X-ray Jets from the Microquasar Xte J1550−564: Evidence for External Shocks Caused by the Jet-Ism Interaction?

Large-scale, decelerating, relativistic X-ray jets from microquasar XTE J1550−564 has been recently discovered with Chandra by Corbel et al. (2002). We find that the dynamical evolution of the approaching jet at the late time is consistent with the well-known Sedov evolutionary phase R∝ t ^2/5. A trans-relativistic external shock dynamic model by analogy with the evolution of gamma-ray burst remnants, is shown to be able to fit the proper-motion data of the approaching jet reasonably well. The inferred interstellar medium density around the source is well below the canonical value n _ISM∼1 cm⁻³. The rapidly fading X-ray emission can be interpreted as synchrotron radiation from the non-thermal electrons in the adiabatically expanding ejecta. These electrons were accelerated by the reverse shock (moving back into the ejecta) which becomes important when the inertia of the swept external matter leads to an appreciable slowing down of the original ejecta.     

Gamma-ray burst afterglow scaling coefficients for general density profiles

Gamma-ray burst (GRB) afterglows are well described by synchrotron emission originating from the interaction between a relativistic blast wave and the external medium surrounding the GRB progenitor. We introduce a code to reconstruct spectra and light curves from arbitrary fluid configurations, making it especially suited to study the effects of fluid flows beyond those that can be described using analytical approximations. As a check and first application of our code, we use it to fit the scaling coefficients of theoretical models of afterglow spectra. We extend earlier results of other authors to general circumburst density profiles. We rederive the physical parameters of GRB 970508 and compare with other authors.     

Early afterglows of short gamma-ray bursts

In the relativistic fireball model, the afterglow of a gamma-ray burst (GRB) is produced by synchrotron radiation of the electrons accelerated in the external shock that emerges as the relativistic flow moves. According to this model, the afterglow peaks on a time scale of ～10 s when observed in the soft gamma-ray band. The peak flux can be high enough to be detected by modern all-sky monitors. We investigate the emission from short (ΔT<1 s) GRBs on a time scale t≈10 s using BATSE/CGRO data. A significant flux is recorded for ～20% of the events. In most cases, the observed persistent emission can be explained in terms of the model as an early burst afterglow. No early afterglows of most short GRBs are observed. The model parameters for these bursts are constrained.     

Quiescent times in gamma-ray bursts – II. Dormant periods in the central engine?

Within the framework of the internal–external shocks model for γ -ray bursts, we study the various mechanisms that can give rise to quiescent times in the observed γ -ray light curves. In particular, we look for the signatures that can provide us with evidence as to whether or not the central engine goes dormant for a period of time comparable to the duration of the gaps. We show that the properties of the prompt γ -ray and X-ray emission can, in principle, determine whether the quiescent episodes are caused by a modulated relativistic wind or a switching off of the central engine. We suggest that detailed observations of the prompt afterglow emission from the reverse shock will strongly constrain the possible mechanisms for the production of quiescent times in γ -ray bursts.     

Solar radio bursts of spectral type II,coronal shocks,and optical coronal transients

The association of solar radio bursts of spectral type II and coronal shocks with solar flare ejecta observed in H, the green coronal line, and white-light coronagraphs is examined. Rather than identifying fast-moving optical coronal transients with outward-travelling shock waves that generate type II radio bursts, as has been suggested in some earlier papers, we suggest that, for the most part, such transients should probably be identified with piston-type phenomena well behind the shock. We then discuss a general model, consisting of three main velocity regimes, in which we relate type II radio bursts and coronal shocks to optically-observed ejecta.     

Hard electron energy distribution in the relativistic shocks of gamma-ray burst afterglows

Particle acceleration in relativistic shocks is not a very well understood subject. Owing to that difficulty, radiation spectra from relativistic shocks, such as those in gamma-ray burst (GRB) afterglows, have been often modelled by making assumptions about the underlying electron distribution. One such assumption is a relatively soft distribution of the particle energy, which need not be true always, as is obvious from observations of several GRB afterglows. In this paper, we describe modifications to the afterglow standard model to accommodate energy spectra which are 'hard'. We calculate the overall evolution of the synchrotron and Compton flux arising from such a distribution. We also model two afterglows, GRB010222 and GRB020813, under this assumption and estimate the physical parameters.     

伽玛射线暴及其余辉研究进展

伽玛射线暴是一种来自宇宙空间随机方向的短时间内伽玛射线突然增亮的现象。伽玛射线暴虽然早在1967年就由Vela卫星观测到,但直到1997年人们才通过余辉观测确定其寄主星系,并通过寄主星系的红移最终确定了伽玛射线暴的宇宙学起源。对伽玛射线暴研究概况进行了评述:详细介绍了伽玛射线暴及其余辉的观测进展,特别是近期Swift卫星和Fermi卫星带来的新发现;系统描述了伽玛射线暴标准火球模型、伽玛射线暴余辉物理(相对论性外流与暴周环境介质的相互作用过程、辐射产生机制等)及伽玛射线暴的前身星等。也对伽玛射线暴的未来研究进行了展望。     

The possible high-energy emission from GRB 080319B and origins of the GeV emission of GRBs 080514B, 080916C and 081024B

We calculate the high-energy (sub-GeV to TeV) prompt and afterglow emission of GRB 080319B that was distinguished by a naked-eye optical flash and by an unusual strong early X-ray afterglow. There are three possible sources for high-energy emission: the prompt optical and γ-ray photons IC scattered by the accelerated electrons, the prompt photons IC scattered by the early external reverse-forward shock electrons, and the higher band of the synchrotron and the synchrotron self-Compton emission of the external shock. There should have been in total hundreds of high-energy photons detectable for the Large Area Telescope onboard the Fermi satellite, and tens of photons of those with energy >10 GeV. The >10 GeV emission had a duration about twice that of the soft γ-rays. Astro-rivelatore Gamma a Immagini Leggero (AGILE) could have observed these energetic signals if it was not occulted by the Earth at that moment. The physical origins of the high-energy emission detected in GRB 080514B, GRB 080916C and GRB 081024B are also discussed. These observations seem to be consistent with the current high-energy emission models.     

Radio signature of cosmological structure formation shocks

In the course of the formation of cosmological structures, large shock waves are generated in the intracluster medium (ICM). In analogy to processes in supernova remnants, these shock waves may generate a significant population of relativistic electrons which, in turn, produce observable synchrotron emission. The extended radio relics found at the periphery of several clusters and possibly also a fraction of radio halo emission may have this origin. Here, we derive an analytic expression for (i) the total radio power in the downstream region of a cosmological shock wave, and (ii) the width of the radio-emitting region. These expressions predict a spectral slope close to −1 for strong shocks. Moderate shocks, such as those produced in mergers between clusters of galaxies, lead to a somewhat steeper spectrum. Moreover, we predict an upper limit for the radio power of cosmological shocks. Comparing our results to the radio relics in Abell 115, 2256 and 3667, we conclude that the magnetic field in these relics is typically at a level of 0.1 μG. Magnetic fields in the ICM are presumably generated by the shocks themselves; this allows us to calculate the radio emission as a function of the cluster temperature. The resulting emissions agree very well with the radio power–temperature relation found for cluster haloes. Finally, we show that cosmic accretion shocks generate less radio emission than merger shock waves. The latter may, however, be detected with upcoming radio telescopes.     

3-D Rpic Simulations of Relativistic Jets: Particle Acceleration, Magnetic Field Generation, and Emission

We have applied numerical simulations and modeling to the particle acceleration, magnetic field generation, and emission from relativistic shocks. We investigate the nonlinear stage of theWeibel instability and compare our simulations with the observed gamma-ray burst emission. In collisionless shocks, plasma waves and their associated instabilities (e.g., the Weibel, Buneman and other two-stream instabilities) are responsible for particle (electron, positron, and ion) acceleration and magnetic field generation. 3-D relativistic electromagnetic particle (REMP) simulations with three different electron-positron jet velocity distributions and also with an electron-ion plasma have been performed and show shock processes including spatial and temporal evolution of shocks in unmagnetized ambient plasmas. The growth time and nonlinear saturation levels depend on the initial jet parallel velocity distributions. Simulations show that the Weibel instability created in the collisionless shocks accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The nonlinear fluctuation amplitude of densities, currents, electric, and magnetic fields in the electron-positron shocks are larger for smaller jet Lorentz factor. This comes from the fact that the growth time of the Weibel instability is proportional to the square of the jet Lorentz factor. We have performed simulations with broad Lorentz factor distribution of jet electrons and positrons, which is assumed to be created by photon annihilation. Simulation results with this broad distribution show that the Weibel instability is excited continuously by the wide-range of jet Lorentz factor from lower to higher values. In all simulations the Weibel instability is responsible for generating and amplifying magnetic fields perpendicular to the jet propagation direction, and contributes to the electron’s (positron’s) transverse deflection behind the jet head. This small scale magnetic field structure contributes to the generation of “jitter” radiation from deflected electrons (positrons), which is different from synchrotron radiation in uniform magnetic fields. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks. The detailed studies of shock microscopic process evolution may provide some insights into early and later GRB afterglows.