1,215 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
The Afterglows of Swift-era Gamma-ray Bursts. I. Comparing pre-Swift and Swift-era Long/Soft (Type II) GRB Optical Afterglows
We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overal
Time Scales in Long GRBs
We analyze a sample of bright long bursts and find that the pulses duration
have a lognormal distribution while the intervals between pulses have an excess
of long intervals (relative to lognormal distribution). This excess can be
explained by the existence of quiescent times, long periods with no signal
above the background. The lognormal distribution of the intervals (excluding
the quiescent times) is similar to the distribution of the pulses width. This
result suggests that the quiescent times are made by a different mechanism than
the rest of the intervals. It also suggests that the intervals (excluding the
quiescent times) and the pulse width are connected to the same parameters of
the source. We find that there is a correlation between a pulse width and the
duration of the interval preceding it. There is a weaker, but still a
significant, correlation between a pulse width and the interval following it.
The significance of the correlation drops substantially when the intervals
considered are not adjacent to the pulse.Comment: 5 pages, 8 figures, accepted for publication by MNRA
- …