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Holmberg IX X-1 is a well-known ultraluminous X-ray source with an X-rayluminosity of ~1e40 erg/s. The source has been monitored by the X-ray Telescopeof Swift regularly. Since 2009 April, the source has been in an extended lowluminosity state. We utilize the co-added spectra taken at different luminositystates to study the spectral behavior of the source. Simple power-law andmulti-color disk blackbody models can be ruled out. The best overall fits,however, are provided by a dual thermal model with a cool blackbody and a warmdisk blackbody. This suggests that Holmberg IX X-1 may be a 10 solar-mass blackhole accreting at 7 times above the Eddington limit or a 100 solar-massmaximally rotating black hole accreting at the Eddington limit, and we areobserving both the inner regions of the accretion disk and outflows from thecompact object.

Swift observations of the ultraluminous X-ray source Holmberg IX X-1