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

The prompt optical emission of GRB 990123 was uncorrelated to the γ-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 γ-ray emissions of GRB 990123 have, indeed, the same origin then their properties require that (i) the optical counterpart was synchrotron emission and γ-rays arose from inverse-Compton scatterings (the '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  ≳10¹⁵  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  per cent  of the thickness of the shocked gas, which corresponds to  10⁶–10⁷  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.     

Possible effects of pair echoes on gamma-ray burst afterglow emission

High-energy emission from gamma-ray bursts (GRBs) is widely expected but had been sparsely observed until recently when the Fermi satellite was launched. If >TeV gamma-rays are produced in GRBs and can escape from the emission region, they are attenuated by the cosmic infrared background photons, leading to regeneration of ∼GeV–TeV secondary photons via inverse-Compton scattering. This secondary emission can last for a longer time than the duration of GRBs, and it is called a pair echo. We investigate how this pair echo emission affects spectra and light curves of high-energy afterglows, considering not only prompt emission but also afterglow as the primary emission. Detection of pair echoes is possible as long as the intergalactic magnetic field (IGMF) in voids is weak. We find (1) that the pair echo from the primary afterglow emission can affect the observed high-energy emission in the afterglow phase after the jet break and (2) that the pair echo from the primary prompt emission can also be relevant, but only when significant energy is emitted in the TeV range, typically     . Even non-detections of the pair echoes could place interesting constraints on the strength of IGMF. The more favourable targets to detect pair echoes may be the 'naked' GRBs without conventional afterglow emission, although energetic naked GRBs would be rare. If the IGMF is weak enough, it is predicted that the GeV emission extends to >30–300 s.     

A possible energy mechanism for cosmological γ-ray bursts

We suggest that an extreme Kerr black hole with a mass ∼10⁶ M_⊙, a dimensionless angular momentum     and a marginally stable orbital radius     located in a normal galaxy may produce a γ -ray burst (GRB) by capturing and disrupting a star. During the capture period, a transient accretion disc is formed and a strong transient magnetic field ∼     lasting for     may be produced at the inner boundary of the accretion disc. A large amount of rotational energy of the black hole is extracted and released in an ultrarelativistic jet with a bulk Lorentz factor Γ larger than 10³ via the Blandford–Znajek process. The relativistic jet energy can be converted into γ -radiation via an internal shock mechanism. The GRB duration should be the same as the lifetime of the strong transient magnetic field. The maximum number of sub-bursts is estimated to be     because the disc material is likely to break into pieces with a size about the thickness of the disc h at the cusp     The shortest risetime of the burst estimated from this model is ∼     The model GRB density rate is also estimated.     

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

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.     

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.     

On the role of extinction in failed gamma-ray burst optical/infrared afterglows

While all but one of the gamma-ray bursts observed in the X-ray band showed an X-ray afterglow, about 60 per cent of them have not been detected in the optical band. We demonstrate that in many cases this is not as a result of adverse observing conditions, or delay in performing the observations. We also show that the optically non-detected afterglows are not affected by particularly large Galactic absorbing columns, since its distribution is similar for both the detected and non-detected burst subclasses. We then investigate the hypothesis that the failure of detecting the optical afterglow is due to absorption at the source location. We find that this is a marginally viable interpretation, but only if the X-ray burst and afterglow emission and the possible optical/UV flash do not destroy the dust responsible for absorption in the optical band. If dust is efficiently destroyed, we are led to conclude that bursts with no detected optical afterglow are intrinsically different. Prompt infrared observations are the key to solving this issue.     

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.     

Taxonomy of gamma-ray burst optical light curves: identification of a salient class of early afterglows

The temporal behaviour of the early optical emission from gamma-ray burst afterglows can be divided into four classes: fast-rising with an early peak, slow-rising with a late peak, flat plateaus and rapid decays since first measurement. The fast-rising optical afterglows display correlations among peak flux, peak epoch and post-peak power-law decay index that can be explained with a structured outflow seen off-axis, but the shock origin (reverse or forward) of the optical emission cannot be determined. The afterglows with plateaus and slow rises may be accommodated by the same model, if observer location offsets are larger than for the fast-rising afterglows, or could be due to a long-lived injection of energy and/or ejecta in the blast wave. If better calibrated with more afterglows, the peak flux–peak epoch relation exhibited by the fast- and slow-rising optical light curves could provide a way to use this type of afterglows as standard candles.     

GeV emission from neutron-rich internal shocks of some long γ-ray bursts

In the neutron-rich internal shocks model for γ-ray bursts (GRBs), the Lorentz factors (LFs) of ion shells are variable, and so are the LFs of accompanying neutron shells. For slow neutron shells with a typical LF of approximate tens, the typical β-decay radius is  ∼10¹⁴–10¹⁵ cm  . As GRBs last long enough  [ T ₉₀ > 14(1 + z ) s]  , one earlier but slower ejected neutron shell will be swept successively by later ejected ion shells in the range  ∼10¹³–10¹⁵ cm  , where slow neutrons have decayed significantly. Part of the thermal energy released in the interaction will be given to the electrons. These accelerated electrons will mainly be cooled by the prompt soft γ-rays and give rise to GeV emission. This kind of GeV emission is particularly important for some very long GRBs and is detectable for the upcoming satellite Gamma-Ray Large Area Space Telescope (GLAST).     

Gamma-ray burst afterglows: effects of radiative corrections and non-uniformity of the surrounding medium

The afterglow of a gamma-ray burst (GRB) is commonly thought to be the result of continuous deceleration of a relativistically expanding fireball in the surrounding medium. Assuming that the expansion of the fireball is adiabatic and that the density of the medium is a power-law function of shock radius, i.e. n _ext ∝  R ^{− k} , we study the effects of the first-order radiative correction and the non-uniformity of the medium on a GRB afterglow analytically. We first derive a new relation among the observed time, the shock radius and the Lorentz factor of the fireball: t _⊕ =  R /4(4− k ) γ²c, and also derive a new relation among the comoving time, the shock radius and the Lorentz factor of the fireball: t _co = 2 R /(5− k ) γc. We next study the evolution of the fireball by using the analytic solution of Blandford &38; McKee. The radiation losses may not significantly influence this evolution. We further derive new scaling laws both between the X-ray flux and observed time and between the optical flux and observed time. We use these scaling laws to discuss the afterglows of GRB 970228 and GRB 970616, and find that if the spectral index of the electron distribution is p  = 2.5, implied from the spectra of GRBs, the X-ray afterglow of GRB 970616 is well fitted by assuming k  = 2.     

Gamma-ray burst efficiency and possible physical processes shaping the early afterglow

The discovery by Swift that a good fraction of gamma-ray bursts (GRBs) have a slowly decaying X-ray afterglow phase led to the suggestion that energy injection into the blast wave takes place several hundred seconds after the burst. This implies that right after the burst the kinetic energy of the blast wave was very low and in turn the efficiency of production of γ-rays during the burst was extremely high, rendering the internal shocks model unlikely. We re-examine the estimates of kinetic energy in GRB afterglows and show that the efficiency of converting the kinetic energy into γ-rays is moderate and does not challenge the standard internal shock model. We also examine several models, including in particular energy injection, suggested to interpret this slow decay phase. We show that with proper parameters, all these models give rise to a slow decline lasting several hours. However, even those models that fit all X-ray observations, and in particular the energy injection model, cannot account self-consistently for both the X-ray and the optical afterglows of well-monitored GRBs such as GRB 050319 and GRB 050401. We speculate about a possible alternative resolution of this puzzle.