Prompt optical emission from gamma-ray bursts with multiple timescale variability of central engine activities

Complete high-resolution light curves of GRB 080319B observed by Swift present an opportunity for detailed temporal analysis of prompt optical emission. With a two-component distribution of initial Lorentz factors, we simulate the dynamical process of shells being ejected from the central engine in the framework of the in- ternal shock model. The emitted radiations are decomposed into different frequency ranges for a temporal correlation analysis between the light curves in different energy bands. The resulting prompt optical and gamma-ray emissions show similar tempo- ral profiles, with both showing a superposition of a component with slow variability and a component with fast variability, except that the gamma-ray light curve is much more variable than its optical counterpart. The variability in the simulated light curves and the strong correlation with a time lag between the optical and gamma-ray emis- sions are in good agreement with observations of GRB 080319B. Our simulations suggest that the variations seen in the light curves stem from the temporal structure of the shells injected from the central engine of gamma-ray bursts. Future observations with high temporal resolution of prompt optical emission from GRBs, e.g., by UFFO- Pathfinder and SVOM-GWAC, will provide a useful tool for investigating the central engine activity.     

Search for optical counterparts of gamma-ray burst sources

The examination of nearly 9600 archival photographic plates covering six gamma-ray burst error boxes has unveiled several star-like images. Unfortunately, the reality of none of these images can be proved by duplicate plates.A laboratory test was performed to get a more detailed estimation of the number of plate faults. The result emphasizes the necessity of duplicate synchronous plates to exclude plate defects.     

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.     

The first hours of the optical afterglow from the cosmic gamma-ray burst 030329

We describe the first results of our observations of the exceptionally bright optical afterglow from the cosmic gamma-ray burst (GRB) of March 29, 2003 (030329), with the 1.5-m Russian-Turkish telescope (RTT150) installed at the TUBITAK National Observatory (Turkey) at Mount Bakyrlytepe. RTT150 was one of the first medium-class telescopes pointed at the afterglow. The observations began as early as about six hours after the GRB. During the first five hours of our observations, the BV RI flux fell off exactly as a power law with the same slope ?1.19±0.01. Subsequently, in all of the BV RI bands, we observed the same increase in the power-law slope of the light curve to a value that was later recorded during the observations at observatories in the western hemisphere. The break in the power-law light curve occurs at t ? t ₀ ≈ 0.57 days (13.5 h) and lasts for about 0.2 days. Apart from this smooth decrease in the flux, the afterglow exhibited no flux variability. The upper limits on the variability are 10–1% on time scales of 0.1–1000 s, respectively. The BV RI spectral flux distribution during the first night of our observations closely corresponds to a power-law spectrum with a spectral index α=0.66±0.01. The change in the power-law slope of the light curve at the end of our observations is probably attributable to the deceleration of the ultrarelativistic jet to a gamma factor when its structural features begin to show up in the light curve. The radio, optical, and X-ray broadband spectrum is consistent with the assumption about the synchrotron radiation of the ultrarelativistic jet. This unique object continues to be observed with RTT150.     

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.     

Testing the gamma-ray burst variability/peak luminosity correlation on a Swift homogeneous sample

We test the gamma-ray burst (GRB) correlation between temporal variability and peak luminosity of the γ-ray profile on a homogeneous sample of 36 Swift /Burst Alert Telescope (BAT) GRBs with firm redshift determination. This is the first time that this correlation can be tested on a homogeneous data sample. The correlation is confirmed, as long as the six GRBs with low luminosity (  <5 × 10⁵⁰   erg s⁻¹  in the rest-frame 100–1000 keV energy band) are ignored. We confirm that the considerable scatter of the correlation already known is not due to the combination of data from different instruments with different energy bands, but it is intrinsic to the correlation itself. Thanks to the unprecedented sensitivity of Swift /BAT, the variability/peak luminosity correlation is tested on low-luminosity GRBs. Our results show that these GRBs are definite outliers.     

Theories of gamma-ray burst sources

A review of recent theoretical work on gamma-ray bursts is given. The emphasis is put on the localization of sources. It is concluded that sources of gamma-ray bursts must be either old Population I or Population II objects with a mechanism implying that the sources are not too far from the galactic plane. According to this conclusion the more relevant models are probably flare stars or accretion on old neutron stars, radiation of the gravitational energy of the accretion, or thermonuclear explosions.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts, held at Toulouse, France, 26–29 November, 1979.     

On the pair-instability supernovae and gamma-ray burst phenomenon

We present a model of a gamma-ray burst (GRB) related to “very massive” stars. In the framework of our model, the GRB phenomenon is a result of helium burning in degenerate conditions in a massive star (≳130 M _⊙), in which the thermal nuclear burning occurs in the deflagration regime and has a pulsating temporal pattern. The shock runs away from the burning (reaction) zone, which leads to the development of a coronal outflow (jet-like) structure. In our scenario the GRB observable prompt fast rise and decay part can be a result of photon propagation through the hot corona (Comptonization photosphere) of the star. On the other hand, the GRB afterglow is a cooling phase of the expanding and outflowing envelope. Presumably, the X-ray part of the GRB emergent spectrum is formed due to upscattering of soft photons of outer layers of the star off hot coronal electrons, and thus it should have a specific shape of the Comptonization spectrum.     

X-ray observations of gamma-ray burst sources

We report on preliminary results of EXOSAT observations of three gamma-ray burst error boxes. No source was detected down to a limit of 10^–10 erg cm^–2s^–1, assuming a black-body spectrum for the burst counterpart. Results are interpreted in the framework of current theoretical models.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

On the optical light curves of afterglows from jetted gamma-ray burst ejecta: effects of parameters

Owing to some refinements in the dynamics, we can follow the overall evolution of a realistic jet numerically until its bulk velocity is as small as c 10³ c . We find no obvious break in the optical light curve during the relativistic phase itself . However, an obvious break does exist at the transition from the relativistic phase to the non-relativistic phase, which typically occurs at time t 10⁶10^6.5 s (i.e. 1030 d). The break is affected by many parameters, such as the electron energy fraction _e, the magnetic energy fraction the initial half-opening angle ₀ and the medium number density n . Increasing any of them to a large enough value will make the break disappear. Although the break itself is parameter-dependent, afterglows from jetted GRB remnants are uniformly characterized by a quick decay during the non-relativistic phase, with power-law timing index 2.1. This is quite different from that of isotropic fireballs, and may be of fundamental importance for determining the degree of beaming in -ray bursts observationally.     

Analysis of COMPTEL gamma-ray burst locations and spectra

In its first three years of operation, the COMPTEL instrument on theCompton Gamma-Ray Observatory has measured the locations (mean accuracy 1°) and spectra (0.75-30 MeV) of 18 gamma-ray bursts and continues to observe new events at a rate of 1/month. With good angular resolution and sensitivity at MeV energies, the growing COMPTEL burst catalog is an important new piece of evidence in the on-going GRB mystery. The COMPTEL burst locations are consistent with an isotropic distribution of sources, yet the spatial coincidence of two of the bursts indicates the possibility of repetition. The COMPTEL burst spectra are in most cases consistent with a single power law model with spectral index in the range 2–3. However, two bursts show evidence of a spectral break in the MeV range. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed. We present an overview of analysis results obtained from the COMPTEL burst catalog concentrating on the search for burst repetition and the implications of highly variable MeV emission.