Fermi Gamma-ray Space Telescope

Search for evidence of high latitude emission in Gamma-ray burst broad pulses

Donggeun Tak
(Z.L. Uhm, J. Racusin. B. Zhang, S. Guiriec, and J. McEnery)


We perform a detailed analysis on broad pulses in bright Gamma-ray bursts (GRBs) and search for the evidence of high latitude emission (HLE). HLE originates from the curvature effect of a relativistic spherical jet, where higher latitude photons are delayed and softer than the observer's line-of-sight emission. Observationally, the signature of HLE has not yet been identified undisputedly during the prompt emission phase. HLE predicts a specific relation between the peak energy Ep in energy spectra and the spectral flux at peak energy nuFnu@Ep. We search for this relation in 2157 GRBs detected by Fermi Gamma-ray Burst Monitor (GBM) from the years 2008 to 2017. We select bright GRBs with fluence and peak flux and impose several criteria in order to minimize contaminations by background and overlaps of pulses, which results in a sample of 32 broad pulses from 32 GRBs. We perform a time-resolved spectral analysis on these 32 broad pulses and find that 18 pulses (56%) are clearly consistent with the HLE relation. For the 18 broad pulses, the exponent d in the relation between Ep and nuFnu@Ep is distributed as a Gaussian function with median and width of 2.04 and 0.42, respectively. We also discuss several implications of our findings.