On the $\alpha$-Intensity Correlation in Gamma-Ray Bursts: Subphotospheric Heating with Varying Entropy

Abstract

The emission mechanism during the prompt phase in gamma-ray bursts (GRBs) can be investigated through correlations between spectral properties. Here, we revisit the correlation relating the instantaneous flux, $F$, and the photon index below the spectral break, $\alpha$, in individual emission pulses, by studying the 38 most prominent pulses in the Fermi/GBM GRB catalogue. First, we search for signatures of the bias in the determination of $\alpha$ due to the limited spectral coverage (window effect) expected in the synchrotron case. The absence of such a characteristic signature argues against the simplest synchrotron models. We instead find that the observed correlation between $F$ and $\alpha$ can, in general, be described by the relation $F(t)\propto {\mathrm{e}}^{k\alpha (t)}$, for which the median $k=3$. We suggest that this correlation is a manifestation of subphotospheric heating in a flow with a varying entropy. Around the peak of the light curve, a large entropy causes the photosphere to approach the saturation radius, leading to an intense emission with a narrow spectrum. As the entropy decreases the photosphere secedes from the saturation radius, and weaker emission with a broader spectrum is expected. This simple scenario naturally leads to a correlated variation of the intensity and spectral shape, covering the observed range.

15 pages, 10 figures, 1 table, accepted for publication in MNRAS

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• Non-dissipative photospheres in GRBs: Spectral appearance in the Fermi/GBM catalogue
• GRB 140102A: Insight into Prompt Spectral Evolution and Early Optical Afterglow Emission
• Time-resolved Spectral Properties of Fermi-GBM Bright Long Gamma-Ray Bursts