{ "id": "2002.06408", "version": "v1", "published": "2020-02-15T16:24:30.000Z", "updated": "2020-02-15T16:24:30.000Z", "title": "Classification of Photospheric Emission in Short GRBs", "authors": [ "Hüsne Dereli-Bégué", "Asaf Pe'er", "Felix Ryde" ], "comment": "Submitted for publication in Ap.J", "categories": [ "astro-ph.HE" ], "abstract": "In order to better understand the physical origin of short duration gamma-ray bursts (GRBs), we perform time-resolved spectral analysis on a sample of 70 pulses in 68 short GRBs with burst duration $T_{90}\\lesssim2$~s detected by the \\textit{Fermi}/GBM during the first 11 years of its mission. We apply Bayesian analysis to all spectra that have statistical significance $S\\ge15$ within each pulse and apply a cut-off power law (CPL) model. For our analysis, we identify the maximal values of the low energy spectral index, $\\alpha_{\\rm max}$. Under the assumption that the main emission mechanism is the same throughout each pulse, such an analysis is indicative of pulse emission. We find that $\\sim$1/3 of short GRBs are consistent with a pure, non-dissipative photospheric model, at least, around the peak of the pulse. This fraction is larger compare to the corresponding one (1/4) obtained for long GRBs. For those bursts which are compatible with a pure photospheric origin, we find (i) a bi-modal distribution in the values of the Lorentz factors and the hardness ratios; (ii) an anti-correlation between $T_{90}$ and the peak energy, $E_{\\rm pk}$: $T_{90} \\propto E_{\\rm pk}^{-0.35\\pm0.01}$. This correlation disappears when we consider the entire short GRB sample. Our results thus imply that short GRB population may in fact be composed of two separate populations: a continuation of the long GRB population to shorter duration, and a separate population with distinct physical properties. Furthermore, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron), making time-resolved spectroscopy absolutely necessary in analyzing GRB pulses.", "revisions": [ { "version": "v1", "updated": "2020-02-15T16:24:30.000Z" } ], "analyses": { "keywords": [ "photospheric emission", "time-resolved spectroscopy absolutely necessary", "separate population", "long grb", "low energy spectral index" ], "note": { "typesetting": "TeX", "pages": 0, "language": "en", "license": "arXiv", "status": "editable" } } }