  Holmberg IX X-1 is a well-known ultraluminous X-ray source with an X-ray
luminosity of ~1e40 erg/s. The source has been monitored by the X-ray Telescope
of Swift regularly. Since 2009 April, the source has been in an extended low
luminosity state. We utilize the co-added spectra taken at different luminosity
states to study the spectral behavior of the source. Simple power-law and
multi-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 warm
disk blackbody. This suggests that Holmberg IX X-1 may be a 10 solar-mass black
hole accreting at 7 times above the Eddington limit or a 100 solar-mass
maximally rotating black hole accreting at the Eddington limit, and we are
observing both the inner regions of the accretion disk and outflows from the
compact object.
