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The spectral components of the prompt emission of gamma-ray bursts (GRBs)mainly consist of two possible origins: synchrotron (nonthermal) andphotosphere (thermal). The typical spectral properties of GRBs can be modeledby a dominant nonthermal component (a Band-like function or cutoff power law),while some of them have an additional thermal component (a Planck-likefunction). In this paper, we investigate the effects of thermal components onthe nonthermal spectral parameters. We focus on eight {\it Fermi} Gamma-rayBurst Monitor bursts of which the spectra deviate from a Band-only function,and the thermal components are significant. We sort them intothermal-subdominant Group I (e.g. GRB 110721A) and thermal-dominant Group II(e.g., GRB 090902B). Several interesting results are found assuming thespectral component is totally attributed to the nonthermal component: (i) thelow-energy photon index $\alpha$ becomes harder; (ii) the peak energy $E_{\rmc}$ is significantly smaller and lies between the peak temperature of blackbodycomponent 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 ratiobetween 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 thethermal component is important, and it significantly affects the spectralparameters and the consequential physical interpretations.

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