Thermal Components in Gamma-ray Bursts. I. How Do They Affect Non-Thermal Spectral Parameters?

Abstract

The spectral components of the prompt emission of gamma-ray bursts (GRBs) mainly consist of two possible origins: synchrotron (nonthermal) and photosphere (thermal). The typical spectral properties of GRBs can be modeled by a dominant nonthermal component (a Band-like function or cutoff power law), while some of them have an additional thermal component (a Planck-like function). In this paper, we investigate the effects of thermal components on the nonthermal spectral parameters. We focus on eight {\it Fermi} Gamma-ray Burst Monitor bursts of which the spectra deviate from a Band-only function, and the thermal components are significant. We sort them into thermal-subdominant Group I (e.g. GRB 110721A) and thermal-dominant Group II (e.g., GRB 090902B). Several interesting results are found assuming the spectral component is totally attributed to the nonthermal component: (i) the low-energy photon index $\alpha$ becomes harder; (ii) the peak energy $E_{\rm c}$ is significantly smaller and lies between the peak temperature of blackbody component and the peak energy of the cutoff power law + blackbody (CPL+BB) model; (iii) total flux $F$, is generally the same; (iv) the changes ($\Delta \alpha$ and $\Delta E_{\rm c}$) are positively correlated with the ratio between the thermal flux and total flux; and (v) parameter relations ($F-\alpha$, $F-E_{\rm c}$ and $E_{\rm c}$-$\alpha$) also changed prominently. The GRBs in both groups show the same results. Our analysis indicates that the thermal component is important, and it significantly affects the spectral parameters and the consequential physical interpretations.