Accretion Power in GRBs

I discuss the implication of the temporal structure of GRBs to the nature of their inner engine. I argue that the temporal strucutre shows that GRBs must involve internal shocks (or another kind of internal interaction within a relativistic outflow). To produce these internal shocks GRB inner engines must vary on a time scale of a fraction of a second and, on the other hand, they should be active for the whole duration of the burst, namely for several dozen of seconds. This implies that from the point of view of the central engine GRBs are a "quasi steady state" phenomenon. Accretion onto a newly formed black hole is the most likely mechanism that can satisfy these conditions and can power GRBs. I discuss the implication of accretion models of massive disks around black holes to GRB modelling

Gamma-Ray Bursts - a Primer For Relativists

Gamma-Ray Bursts (GRBs) - short bursts of 100-1MeV photons arriving from random directions in the sky are probably the most relativistic objects discovered so far. Still, somehow they did not attract the attention of the relativistic community. In this short review I discuss briefly GRB observations and show that they lead us to the fireball model - GRBs involve macroscopic relativistic motion with Lorentz factors of a few hundred or more. I show that GRB sources involve, most likely, new born black holes, and their progenitors are Supernovae or neutron star mergers. I show that both GRB progenitors and the process of GRB itself produce gravitational radiation and I consider the possibility of detecting this emission. Finally I show that GRBs could serve as cosmological indicators that could teach us about the high redshift ($z \approx 5-15$) dark ages of the universe.Comment: Review talk given at GR1

Gamma-Ray Bursts from Neutron Star Mergers

Binary neutron stars merger (NS$^2$M) at cosmological distances is probably the only $\gamma$-ray bursts model based on an independently observed phenomenon which is known to be taking place at a comparable rate. We describe this model, its predictions and some open questions.Comment: 4 pages, 1 Figure can be obtained on request by e-mail from the Autho

Gamma-Ray Bursts and Neutron Star Mergers - Possibly the Strongest Explosions in the Universe

$\gamma$-ray bursts have baffled theorists ever since their accidental discovery at the sixties. We suggest that these bursts originate in merger of neutron star binaries, taking place at cosmological distances. These mergers release $\approx 10^{54}ergs$, in what are possibly the strongest explosions in the Universe. If even a small fraction of this energy is channeled to an electromagnetic signal it will be detected as a grbs. We examine the virtues and limitations of this model and compare it with the recent Compton \g-ray observatory results.Comment: 8 pages, to appear in the XXVI INTERNATIONAL HIGH ENERGY PHYSICS CONFERENC

Gamma-Ray Bursts and Binary Neutron Star Mergers

Neutron star binaries, such as the one observed in the famous binary pulsar PSR 1916+13, end their life in a catastrophic merge event (denoted here NS$^2$M). The merger releases $\approx 5 \cdot 10^{53}$ergs, mostly as neutrinos and gravitational radiation. A small fraction of this energy suffices to power $\gamma$-ray bursts (GRBs) at cosmological distances. Cosmological GRBs must pass, however, an optically thick fireball phase and the observed $\gamma$-rays emerge only at the end of this phase. Hence, it is difficult to determine the nature of the source from present observations (the agreement between the rates of GRBs and NS$^2$Ms being only an indirect evidence for this model). In the future a coinciding detection of a GRB and a gravitational radiation signal could confirm this model.Comment: 13 pages, uuencoded ps files to apprear in IAU SYMPOSIUM 165 `COMPACT STARS IN BINARIES' 15-19 August 1994, The Hague, Netherland

Gamma-Ray Bursts - When Theory Meets Observations

Gamma-Ray Bursts (GRBs) are the brightest objects observed. They are also the most relativistic objects known so far. GRBs occur when an ultrarelativisitic ejecta is slowed down by internal shocks within the flow. Relativistic particles accelerated within these shocks emit the observed gamma-rays by a combination of synchrotron and inverse Compton emission. External shocks with the circumstellar matter slow down further the ejecta and produce the afterglow, which lasts for months. Comparison of the predictions of this fireball model with observations confirm a relativistic macroscopic motion with a Lorentz factor of $\Gamma \ge 100$. Breaks in the light curves of the afterglow indicate that GRBs are beamed with typical opening angles of a few degrees. The temporal variability of the gamma-rays signal provide us with the best indirect evidence on the nature of the ``internal engine'' that powers the GRBs and accelerates the relativistic ejecta, suggesting accretion of a massive disk onto a newborn black hole: GRBs are the birth cries of these black holes. Two of the most promising models: Neutron Star Mergers and Collapars lead naturally to this scenario.Comment: Invited talk Texas Symposium, 12 page