175 research outputs found
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
-ray emission results from internal shocks, the early reverse shock
emission should resemble a smoothed version of the prompt -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
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