Coronal Gamma Ray Bursts as the sources of Ultra High Energy Cosmic Rays?

I consider the possibility that Ultra High Energy Cosmic Rays are accelerated in Gamma Ray Bursts located in the Galactic corona, thus circumventing the problem raised by Greisen--Zatsepin--Kuz'min cutoff. The acceleration of UHECRs could occur in the pulsars which, in the coronal GRB model, produce them: the same parameters that permit fitting GRBs' observations in the model of Podsiadlowski, Rees and Ruderman (1995) lead to an estimate of the highest achievable energies corresponding to that of the Bird et al (1994) event, and to very low luminosities in cosmic rays. I show that, if the observations of Milgrom and Usov (1995a) are confirmed, the extragalactic GRBs' model for the acceleration of UHECRs is untenable, but the same constraint does not apply to the coronal model. Also, I show that the efficiency of particle acceleration needs be much smaller (and less demanding) than in cosmological models of GRBs. Uncertainties remain about the ensuing cosmic ray spectral distribution. I also briefly discuss observational strategies to distinguish between the two possibilities.Comment: In press in The Short Communications of the MNRAS, LATEX--mn.sty, no figure

What have we learned about gamma ray bursts from afterglows?

The discovery of GRBs' afterglows has allowed us to establish several facts: their distance and energy scales, the fact that they are due to explosions, that the explosions are relativistic, and that the afterglow emission mechanism is synchrotron radiation. On the other hand, recent data have shown that the fireball model is wrong when it comes to the emission mechanism of the true burst (which is unlikely to be synchrotron again) and that shocks are not external. Besides these relatively tame points, I will also discuss the less well established physics of the energy deposition mechanism, as well as the possible burst progenitors.Comment: Invited talk, to appear in Proceedings of the Conference X-ray Astronomy '999: Stellar Endpoints, AGNs and Diffuse Background, Astrophysical Letters and Communications, to appea

The afterglow of gamma ray bursts II: the cases of GRB 970228 and GRB 970508

Highly radiative expansion of a relativistic shell is shown to explain all observed features of the afterglows of the two bursts GRB 970228 and GRB 970508. In particular, in the first case the observed time-dependence t^-1.32 of the soft X--ray flux is easily reproduced. The same model, when the surrounding matter density scales as a r^-2, explains the afterglow of GRB 970508}, which may at first sight appear at odds with that of GRB 970228. In particular, it is shown that both the late peak in the optical luminosity and the flat time-dependence of the X--ray luminosity are simultaneously explained by nonuniformity of the surrounding matter, that the observed optical time-delay is correctly reproduced for standard parameter values, and that the time-delay and flux levels of the radio emission are also explained.Comment: 9 pages + 2 figures, AASTEX/LateX needed, submitted to Astrophysical Journal Letter

On the acceleration of Ultra High Energy Cosmic Rays in Gamma Ray Bursts

UHECRs are roughly isotropic and attain very large energies, $E \gtrsim 3\times 10^{20} \; eV$. Conventional models fail to explain both facts. I show here that acceleration of UHECRs in GRBs satisfies both observational constraints. Using M\'esz\'aros and Rees' (1994) model of GRBs as due to hyperrelativistic shocks, I show that the highest energies that can be attained thusly are $E\simeq 10^{20}\;\theta^{-5/3} n_1^{-5/6}\; eV$, explaining the energy of the Bird \etal\/ (1995) event even without beaming. The traditional photopion catastrophe affecting UHECR acceleration in AGNs is circumvented. An order of magnitude estimate shows that the total energy flux of UHECRs at the Earth is also correctly reproduced. A test of the model based upon the UHECRs' distribution upon the plane of the sky is briefly discussed.Comment: 20 pages, latex (AASTEX needed), no figures, to appear in ApJ, November 10 1995 issu