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X-ray flares of gamma-ray bursts (GRBs) are usually observed in the softX-ray range and the spectral coverage is limited. In this paper, we present ananalysis of 32 GRB X-ray flares that are simultaneously observed by both BATand XRT on board the Swift mission, so a joint spectral analysis with a widerspectral coverage is possible. Our results show that the joint spectra of 19flares are fitted with the absorbed single power-law or the Band functionmodels. More interestingly, the joint spectra of the other 13 X-ray flares arefitted with the absorbed single power-law model plus a black body (BB)component. Phenomenally, the observed spectra of these 13 flares are analogousto several GRBs with a thermal component, but only with a much lowertemperature of $kT=1\sim 3$ keV. Assuming that the thermal emission is thephotosphere emission of the GRB fireball, we derive the fireball properties ofthe 13 flares that have redshift measurements, such as the bulk Lorentz factor$\Gamma_{\rm ph}$ of the outflow. The derived $\Gamma_{\rm ph}$ range from $50$to $150$ and a relation of $\Gamma_{\rm ph}$ to the thermal emission luminosityis found. It is consistent with the $\Gamma_0-L_{\rm iso}$ relation that arederived for the prompt gamma-ray emission. We discuss the physical implicationsof these results within the content of jet composition and radiation mechanismof GRBs and X-ray flares.

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