A spectral study of the black hole candidate XTE J1752-223 in the high/soft state with MAXI, Suzaku, and Swift

We report on an X-ray spectral analysis of the black hole candidate XTE J1752-223 in the 2009-2010 outburst, utilizing data obtained with the MAXI/Gas Slit Camera (GSC), the Swift/XRT, and Suzaku, which work comple-mentarily. As already reported by Nakahira et al. (2010, PASJ, 62, L27), MAXI monitored the source continuously throughout the entire outburst for about eight months. All of the MAXI/GSC energy spectra in the high/soft state, lasting for 2 months, are well represented by a multi-color disk plus power-law model. The innermost disk temperature changed from ∼0.7 keV to ∼0.4 keV and the disk flux decreased by an order of magnitude. Nevertheless, the innermost radius is constant at ∼41 D_3.5(cos i)^-1/2km, where D _3.5 is the source distance in units of 3.5kpc and i the inclination. The multi-color disk parameters obtained with the MAXI/GSC are consistent with those with the Swift/XRT and Suzaku. The Suzaku data also suggest a possibility that the disk emission is slightly Comptonized, which could account for broad iron-K features reported previously. Assuming that the obtained innermost radius represents the innermost stable circular orbit for a non-rotating black hole, we estimate the mass of the black hole to be 5.51 ± 0.28 M _⊙ D_3.5(cos ^-1/2, where the correction for the stress-free inner boundary condition and color hardening factor of 1.7 are taken into account. If the inclination is less than 49°, as suggested from radio monitoring of transient jets, and the soft-to-hard transition in 2010 April occurred at 1%-4% of Eddignton luminosity, the fitting of the Suzaku spectra with a relativistic accretion-disk model derives constraints on the mass and the distance to be 3.1-55M_⊙ and 2.3-22 kpc, respectively. This confirms that the compact object in XTE J1752-223 is a black hole.