HEAT mtg: Mike Moss, NASA GSFC
October 24
Correcting the Instrumental Bias of GRB Prompt Measurements
Mike Moss, NASA Goddard
Abstract: Gamma-Ray Bursts (GRBs) are the most luminous catastrophic events in our universe. They can be seen out to great distances (z~9.4), making them excellent cosmic probes. The average measured durations of GRBs should scale with cosmological time dilation, i.e., proportional to (1+z). However, the average measured duration of long GRBs (LGRBs; T90 > 2 sec) with redshift measurements remains constant at T90 ~ 100 sec out to redshifts >6 at which point the average duration begins to decrease for GRBs with the highest redshifts. In previous work, it has been shown that GRB prompt emission duration and fluence measurements are underestimations. These underestimations grow larger with increasing redshift as the signal to noise of the observations decreases, the so-called “tip-of-the-iceberg” effect. Due to the uniqueness of each GRB light curve, it is impossible to determine how much signal was lost into the background noise for any specific GRB. However, by simulating a sample of bursts across a redshift range of 0.1> z >10 and performing mock Swift/BAT observations of the prompt emission of the synthetic sample, we were able to calculate the average amount of signal lost across the entire sample as a function of redshift. Applying these correction factors to the observed average duration of LGRBs leads to a corrected average duration that directly scales with cosmological time dilation, T90,z = (1+z) T90,0. Furthermore, this provides an estimate on the average intrinsic duration of LGRBs, T90,0 ~ 120 sec, consistent with the average duration of GRBs at low redshifts. Our results suggest that the tip-of-the-iceberg effect alone can explain the behavior T90,z. Therefore, no additional evolution in the characteristics of GRB prompt emission (e.g., luminosity) is required. Finally, we discuss the implications of the lack of cosmic evolution in T90,z on the progenitors of long-duration GRBs.
Host: Jillian Chin Rastinejad