Swift Observations of the Ultraluminous X-ray Source Holmberg IX X-1

Holmberg IX X-1 is a well-known ultraluminous X-ray source with an X-ray luminosity of ~10⁴⁰ erg s^-1. 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 M _sun black hole accreting at seven times above the Eddington limit or a 100 M _sun 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.