Detailed Spectral Study of an Ultra-Luminous Compact X-Ray Source, M81 X-9, in the Disk-Dominated State

We report on the results of detailed spectral studies of the ultra-luminous X-ray source (ULX), M81 X-9 (Holmberg IX X-1), made with XMM-Newton on 2001 April 22 and with ASCA on 1999 April 6. On both occasions, the source showed an unabsorbed 0.5-10 keV luminosity of ~2E40 erg/s (assuming a distance of 3.4Mpc) and a soft spectrum apparently represented by a multi-color disk model with an innermost disk temperature of 1.3-1.5 keV. Adding a power-law model further improved the fit. However, as previously reported, the high luminosity cannot be reconciled with the high disk temperature within a framework of the standard accretion disk radiating at a sub-Eddington luminosity. Therefore, we modified the multi-color disk model, and allowed the local disk temperature to scale as r^(-p) on the distance r from the black hole, with p being a free parameter. We then found that the XMM-Newton and the ASCA spectra can be both reproduced successfully with p~0.6 and the innermost disk temperature of 1.4-1.8 keV. These flatter temperature profiles suggest deviation from the standard Shakura-Sunyaev disk, and are consistent with predictions of a slim disk model.