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Photosphere emission in the X-Ray Flares of Swift Gamma-Ray Bursts and Implications for the Fireball Properties

X-ray flares of gamma-ray bursts (GRBs) are usually observed in the soft X-ray range and the spectral coverage is limited. In this paper, we present an analysis of 32 GRB X-ray flares that are simultaneously observed by both BAT and XRT on board the Swift mission, so a joint spectral analysis with a wider spectral coverage is possible. Our results show that the joint spectra of 19 flares are fitted with the absorbed single power-law or the Band function models. More interestingly, the joint spectra of the other 13 X-ray flares are fitted with the absorbed single power-law model plus a black body (BB) component. Phenomenally, the observed spectra of these 13 flares are analogous to several GRBs with a thermal component, but only with a much lower temperature of $kT=1\sim 3$ keV. Assuming that the thermal emission is the photosphere emission of the GRB fireball, we derive the fireball properties of the 13 flares that have redshift measurements, such as the bulk Lorentz factor $Γ_{\rm ph}$ of the outflow. The derived $Γ_{\rm ph}$ range from $50$ to $150$ and a relation of $Γ_{\rm ph}$ to the thermal emission luminosity is found. It is consistent with the $Γ_0-L_{\rm iso}$ relation that are derived for the prompt gamma-ray emission. We discuss the physical implications of these results within the content of jet composition and radiation mechanism of GRBs and X-ray flares.