Standard GRB Spectral Models "Misused"?

Abstract

The standard model characterizing the gamma-ray burst (GRB) spectrum invokes a four-parameter empirical function, the so-called the BAND model. An alternative model named cutoff power law (COMP) implements a power law with an exponential cutoff. These functions achieve almost equally good fits on observed spectra, and are adopted in nearly all of the GRB literature. Here, we reanalyze the sample defined in Li.et al.,2021,ApJS,254,35 (39 bursts including 944 spectra). We classify the spectra by two methods: (1) checking their corner-corner plots of the posteriors to determine well-constrained $\beta$ (BAND-better) and unconstrained $\beta$ (COMP-better) categories; and (2) defining the four groups by difference of the deviance information criterion (DIC). We find inconsistent peaks of the parameter distributions between the BAND-better spectra ($\alpha =-0.64\pm 0.28$ and $\mathrm{lo}{\mathrm{g}}_{10}({\mathrm{E}}_{\mathrm{p}})=\mathrm{lo}{\mathrm{g}}_{10}(191)\pm 0.41$) and the COMP-better spectra ($\alpha =-0.96\pm 0.33$ and $\mathrm{lo}{\mathrm{g}}_{10}({\mathrm{E}}_{\mathrm{p}})=\mathrm{lo}{\mathrm{g}}_{10}(249)\pm 0.40$). With the statistically preferred model and vice versa the misused model defined based on DIC statistics, we also find that the fitted parameters obtained by the misused model (COMP) significantly deviate from those obtained by the statistically preferred model (BAND). This means that if a spectrum is statistically preferred, described as the BAND, applying COMP to derive the spectral parameters will prominently deviate from their intrinsic shape, therefore affecting the physical interpretation. Our analysis indicates that the better or statistically preferred model should be duly examined during GRB spectral analysis. In addition, the $\beta$ distribution exhibits a bimodal structure containing the BAND-better and COMP-better spectra, respectively, implying that BAND and COMP both may have physical origin.

38 pages, 12 figures (including 263 panels),2 tables, ApJ in press

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• Redshift Dependence of the Low-energy Spectral Index of Gamma-Ray Bursts Revisited