Outliers to the Isotropic Energy - Peak Energy Relation in GRBs

The peak energy - isotropic energy (EpEi) relation is among the most intriguing recent discoveries concerning GRBs. It can have numerous implications on our understanding of the emission mechanism of the bursts and on the application of GRBs for cosmological studies. However, this relation was verified only for a small sample of bursts with measured redshifts. We propose here a test whether a burst with an unknown redshift can potentially satisfy the EpEi relation. Applying this test to a large sample of BATSE bursts we find that a significant fraction of those bursts cannot satisfy this relation. Our test is sensitive only to dim and hard bursts and therefore this relation might still hold as an inequality (i.e. there are no intrinsically bright and soft bursts). We conclude that the observed relation seen in the sample of bursts with a known redshift might be influenced by observational biases and from the inability to locate and well localize hard and weak bursts that have only a small number of photons. In particular we point out that the threshold for detection, localization and redshift measurement is essentially higher than the threshold for detection alone. We predict that Swift will detect some hard and weak bursts that would be outliers to the EpEi relation. However, we cannot quantify this prediction. We stress the importance of understanding the detection-localization-redshift threshold for the coming Swift detections

Early GRB afterglow from a reverse shock as a tracer of the prompt gamma-ray light curve

We discuss the optical and radio early afterglow emission of the reverse shock that crosses a baryonic ejecta as it interacts with the external interstellar medium (ISM). We show that the peak of the optical flash divides the light curve of the reverse shock into two distinctive phases. The emission after the peak depends weakly on the initial conditions of the ejecta and therefore it can be used as an identifiable signature of a reverse shock emission. On the other hand, the emission before the optical peak is highly sensitive to the initial conditions and therefore can be used to investigate the initial hydrodynamic profile of the ejecta. In particular, if the prompt $\gamma$-ray emission results from internal shocks, the early reverse shock emission should resemble a smoothed version of the prompt $\gamma$-ray light curve.Comment: Submitted to Il Nuovo Cimento, proceedings of the 4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 200

Cluster Merger Shock Constraints on Particle Acceleration and Nonthermal Pressure in the Intracluster Medium

X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well as the shock acceleration of particles and their escape, all affect shock jump conditions in distinct ways. Therefore, these processes can be constrained using X-ray surface brightness and temperature maps of merger shock fronts. Here we use these observations to place constraints on particle acceleration efficiency in intermediate Mach number (M ~ 2-3) shocks and explore the potential to constrain the contribution of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in merger shocks discovered in the galaxy clusters A520 (M ~ 2) and 1E 0657-56 (M ~ 3) using a multifluid model comprised of a thermal plasma, a nonthermal plasma, and a magnetic field. Based on the published X-ray spectroscopic data alone, we find that the fractional contribution of cosmic rays accelerated in these shocks is lower than about 10% of the shock downstream pressure. Current observations do not constrain the fractional contribution of nonthermal components to the pressure of the undisturbed shock upstream. Future X-ray observations, however, have the potential to either detect particle acceleration in these shocks through its effect on the shock dynamics, or to place a lower limit on the nonthermal pressure contributions in the undisturbed ICM. We briefly discuss implications for models of particle acceleration in collisionless shocks and the estimates of galaxy cluster masses derived from X-ray and Sunyaev-Zel'dovich effect observations.Comment: 10 pages, 4 figures, comments welcom