Fermi Gamma-ray Space Telescope

Analysis of the duration--hardness ratio plane of gamma-ray bursts with skewed distributions

Mariusz Tarnopolski
(Mariusz Tarnopolski)


Two classes of gamma-ray bursts (GRBs) have been confidently identified thus far and are prescribed to different physical scenarios – neutron star-neutron star or neutron star-black hole mergers, and collapse of massive stars, for short and long GRBs, respectively. A third, intermediate in duration class, was suggested to be present in previous catalogues, such as BATSE and Swift, based on statistical tests regarding a mixture of two or three lognormal distributions of T90. However, this might possibly not be an adequate model. This work investigates whether the distributions of H32-T90 in a log-log plane of Fermi GRBs are described better by a mixture of skewed distributions rather than bivariate Gaussians. It is found that mixtures of two skewed distributions are more likely to describe the observed H32-T90 distribution than a mixture of three regular Gaussians. Based on statistical reasoning, the existence of a third (intermediate) class of GRBs in Fermi data is rejected.