Combined spectral and timing analysis of the black hole candidate MAXI J1659-152, discovered by MAXI and Swift

We report on X-ray spectral and timing results of a new black hole candidate (BHC), MAXI J1659-152, with an orbital period of 2.41 hr (shortest among BHCs) in a 2010 outburst from 65 Rossi X-ray Timing Explorer (RXTE) observations and 8 simultaneous Swift and RXTE observations. According to the definitions of the spectral states in Remillard and McClintock (2006, ARA&A, 44, 49), most of the observations have been classified into the intermediate state. All of the X-ray broadband spectra can be modeled by a multi-color disk plus a power-law with an exponential cutoff or a multi-color disk plus a Comptonization component. During the initial phase of the outburst, a high-energy cutoff was visible at 30-40 keV. The innermost radius of the disk gradually decreased by a factor of more than 3 from the onset of the outburst, and reached a constant value of 35 d₁₀cos i ^-1/2 km, where d₁₀ is the distance in units of 10 kpc and i is the inclination. The type-C quasi-periodic oscillation (QPO) frequency varied from 1.6Hz to 7.3Hz in association with a change of the innermost radius, while the innermost radius remained constant during the type-B QPO detections at 1.6-4.1Hz. Hence, we suggest that the origin of the type-B QPOs is different from that of type-C QPOs, the latter of which would originate from the disk truncation radius. Assuming the constant innermost radius in the latter phase of the outburst as the innermost stable circular orbit, the black hole mass in MAXI J1659-152 is estimated to be 3.6-8.0M ⊙ for a distance of 5.3-8.6 kpc and an inclination angle of 60°-75°.