Detailed spectral study of an ultra-luminous compact X-ray source, M 81 X-9, in the disk-dominated state

We report on the results of detailed spectral studies of an ultra-luminous X-ray source (ULX), M 81 X-9 (Holmberg IX X-1), conducted with XMM-Newton on 2001 April 22 and with ASCA on 1999 April 6. On both occasions, the source showed an unabsorbed 0.5-10keV luminosity of ∼2 × 10⁴⁰ erg s^-1 (assuming a distance of 3.4 Mpc) 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 the 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 successfully reproduced with p ∼ 0.6 and the innermost disk temperature of 1.4-1.8 keV. These flatter temperature profiles suggest a deviation from the standard Shakura-Sunyaev disk, and are consistent with predictions of a slim-disk model.