Tsvi Piran

Spectra and light curves of gamma-ray burst afterglows

The recently discovered gamma-ray burst afterglow is believed to be described reasonably well by synchrotron emission from a decelerating relativistic shell that collides with an external medium. To compare theoretical models with afterglow observations, we calculate here the broadband spectrum and corresponding light curve of synchrotron radiation from a power-law distribution of electrons in an expanding relativistic shock. Both the spectrum and the light curve consist of several power-law segments with related indices. The light curve is constructed under two limiting models for the hydrodynamic evolution of the shock: fully adiabatic and fully radiative. We give explicit relations between the spectral index and the temporal power-law index. Future observations should be able to distinguish between the possible behaviors and determine the type of solution.

The physics of gamma-ray bursts

Gamma-ray bursts (GRBs), short and intense pulses of low-energy

Gamma-ray bursts as the death throes of massive binary stars

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JETS IN GAMMA-RAY BURSTS

rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions\char22{}BATSE (Burst and Transient Source Experiment) on the Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer)\char22{}together with ground-based optical, infrared, and radio observatories have revolutionized our understanding of GRBs, showing that they are cosmological, that they are accompanied by long-lasting afterglows, and that they are associated with core-collapse supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRBs are produced when the kinetic energy of an ultrarelativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circumburst matter. This model has had numerous successful predictions, like the predictions of the afterglow itself, of jet breaks in the afterglow light curve, and of the optical flash that accompanies the GRBs. This review focuses on the current theoretical understanding of the physical processes believed to take place in GRBs.

Can Internal Shocks Produce the Variability in Gamma-Ray Bursts?

We propose that gamma-ray bursts are created in the mergers of double neutron star binaries and black hole neutron star binaries at cosmological distances. Two different processes provide the electromagnetic energy for the bursts: neutrino-antineutrino annihilation into electron-position pairs during the merger, and magnetic flares generated by the Parker instability in a postmerger differentially rotating disk. In both cases, an optically thick fireball of size less than or approximately equal to 100 km is initially created, which expands ultrarelativistically to large radii before radiating. The scenario is only qualitative at this time, but it eliminates many previous objections to the cosmological merger model. The strongest bursts should be found close to, but not at the centers of, galaxies at redshifts of order 0.1, and should be accompanied by bursts of gravitational radiation from the spiraling-in binary which could be detected by LIGO.

Predictions for the very early afterglow and the optical flash

In several GRBs afterglows, rapid temporal decay is observed which is inconsistent with spherical (isotropic) blast-wave models. In particular, GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, with t^{-2.05\pm 0.04} in optical as well as in X-rays. We show that such temporal decay is more consistent with the evolution of a jet after it slows down and spreads laterally, for which t^{-p} decay is expected (where p is the index of the electron energy distribution). Such a beaming model would relax the energy requirements on some of the more extreme GRBs by a factor of several hundreds. It is likely that a large fraction of the weak (or no) afterglow observations are also due to the common occurrence of beaming in GRBs, and that their jets have already transitioned to the spreading phase before the first afterglow observations were made. With this interpretation, a universal value of p~2.5 is consistent with all data.In the afterglows of several gamma-ray bursts (GRBs), rapid temporal decay, which is inconsistent with spherical (isotropic) blast-wave models, is observed. In particular, GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, with t-2.05±0.04 in optical as well as in X-rays. We show that such temporal decay is more consistent with the evolution of a jet after it slows down and spreads laterally, for which t-p decay is expected (where p is the index of the electron energy distribution). Such a beaming model would relax the energy requirements on some of the more extreme GRBs by a factor of several hundred. It is likely that a large fraction of the weak- (or no-) afterglow observations are also due to the common occurrence of beaming in GRBs and that their jets have already transitioned to the spreading phase before the first afterglow observations were made. With this interpretation, a universal value of p 2.4 is consistent with all data.

The afterglow of GRB 050709 and the nature of the short-hard gamma-ray bursts.

We discuss the possibility that gamma-ray bursts result from internal shocks in ultrarelativistic matter. Using a simple model, we calculate the temporal structure and estimate the efficiency of this process. In this model the flow of ultrarelativistic matter is represented by a succession of shells with random values of the Lorentz factor. We calculate the shocks that take place between those shells, and we estimate the resulting emission. Internal shocks can produce the highly variable temporal structure observed in most of the bursts, provided that the source emitting the relativistic flow is highly variable. The observed peaks are in almost one-to-one correlation with the activity of the emitting source. A large fraction of the kinetic energy is converted to radiation. The most efficient case is when an inner engine produces shells with comparable energy but very different Lorentz factors. It also gives the most desirable temporal structure.

Relativistic ejecta from X-ray flash XRF 060218 and the rate of cosmic explosions

According to the internal-external shocks model for gamma-ray bursts (GRBs), the GRB is produced by internal shocks within a relativistic flow while the afterglow is produced by external shocks with the interstellar medium. We explore the early afterglow emission. For short GRBs the peak of the afterglow will be delayed, typically by few dozens of seconds after the burst. For long GRBs the early afterglow emission will overlap the GRB signal. We calculate the expected spectrum and the light curves of the early afterglow in the optical, X-ray, and gamma-ray bands. These characteristics provide a way to discriminate between late internal shocks emission (part of the GRB) and the early afterglow signal. If such a delayed emission, with the characteristics of the early afterglow, is detected, it can be used to prove the internal shock scenario as producing the GRB, as well as to measure the initial Lorentz factor of the relativistic flow. The reverse shock, at its peak, contains energy which is comparable to that of the GRB itself but has a much lower temperature than that of the forward shock so it radiates at considerably lower frequencies. The reverse shock dominates the early optical emission, and an optical flash brighter than 15th magnitude is expected together with the forward shock peak at X-rays or gamma-rays. If this optical flash is not observed, strong limitations can be put on the baryonic contents of the relativistic shell deriving the GRBs, leading to a magnetically dominated energy density.

The afterglow, redshift and extreme energetics of the gamma-ray burst of 23 January 1999

The final chapter in the long-standing mystery of the γ-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

The afterglow, the redshift, and the extreme energetics of the gamma-ray burst 990123

Over the past decade, long-duration γ-ray bursts (GRBs)—including the subclass of X-ray flashes (XRFs)—have been revealed to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 1048 erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.