Standard GRB Spectral Models "Misused"?

TL;DR

This study reanalyzes GRB spectral models, revealing that using the misused model (COMP) instead of the statistically preferred BAND model leads to significant deviations in parameters, impacting physical interpretations.

Contribution

It demonstrates the importance of model selection in GRB spectral analysis and shows that BAND and COMP models may both have physical relevance.

Findings

Inconsistent parameter distributions between BAND- and COMP-preferred spectra.

Significant deviations in spectral parameters when using the misused model.

Both models potentially have physical origins, indicated by bimodal parameter distributions.

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 ($\alpha=-0.64\pm0.28$ and $\rm log_{10}(E_{\rm p})=\rm log_{10}(191)\pm0.41$) and the COMP-better spectra ($\alpha=-0.96\pm0.33$ and $\rm log_{10}(E_{\rm p})=\rm log_{10}(249)\pm0.40$). 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.