The Hardness-intensity Correlation of Photospheric Emission from a Structured Jet for Gamma-Ray Bursts

For many gamma-ray bursts (GRBs), hardness-intensity correlation (HIC) can be described by a power-law function, $E_{\rm p}\propto F^κ$, where $E_{\rm p}$ is the peak energy of $νF_ν$ spectrum, and $F$ is the instantaneous energy flux. In this paper, HIC of the non-dissipative photospheric emission from a structured jet is studied in different regimes. An intermediate photosphere, which contains both of unsaturated and saturated emissions is introduced, and we find positive $κ<1/4$ in this case. The same conclusion could be generalized to the photospheric emission from a hybrid jet without magnetic dissipations, or that with sub-photospheric magnetic dissipations and fully thermalized. This may imply that the contribution peaking at $\sim1/2$ in the distribution of observed $κ$ are mainly from the prompt emission of GRBs with synchrotron origin. Besides, emissions of the intermediate photosphere could give a smaller low-energy photon index $α$ than that in the unsaturated regime, and naturally reproduce anti-correlation in $α-E_{\rm p}$ in a GRB pulse.

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