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We report on the results of detailed spectral studies of the ultra-luminousX-ray source (ULX), M81 X-9 (Holmberg IX X-1), made with XMM-Newton on 2001April 22 and with ASCA on 1999 April 6. On both occasions, the source showed anunabsorbed 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 aninnermost disk temperature of 1.3-1.5 keV. Adding a power-law model furtherimproved the fit. However, as previously reported, the high luminosity cannotbe reconciled with the high disk temperature within a framework of the standardaccretion disk radiating at a sub-Eddington luminosity. Therefore, we modifiedthe multi-color disk model, and allowed the local disk temperature to scale asr^(-p) on the distance r from the black hole, with p being a free parameter. Wethen found that the XMM-Newton and the ASCA spectra can be both reproducedsuccessfully with p~0.6 and the innermost disk temperature of 1.4-1.8 keV.These flatter temperature profiles suggest deviation from the standardShakura-Sunyaev disk, and are consistent with predictions of a slim disk model.Comment: 9 pages, 9 figures, to appear in PASJ vol 58 No.

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