Suzaku Observations of Spectral Variations of the Ultra Luminous X-ray Source Holmberg IX X-1

Observations of the Ultra Luminous X-ray source (ULX) Holmberg IX X-1 were carried out with Suzaku twice, once on 2012 April 13 and then on 2012 October 24, with exposures of 180 ks and 217 ks, respectively. The source showed a hard power-law shape spectrum with a mild cutoff at $\sim 8$ keV, which is typical of ULXs when they are relatively dim. On both occasions, the 0.6-11 keV spectrum was explained successfully in terms a cool ($\sim 0.2$ keV) multi-color disk blackbody emission model and a thermal Comptonization emission produced by an electron cloud with a relatively low temperature and high optical depth, assuming that a large fraction of the disk-blackbody photons are Comptonized whereas the rest is observed directly. The 0.5-10 keV luminosity was $1.2\times10^{40}$ erg s$^{-1}$ in April, and $\sim 14\%$ higher in October. This brightening was accompanied by spectral softening in $\ge 2$ keV, with little changes in the $\le 2$ keV spectral shape. This behavior can be understood if the accretion disk remains unchanged, while the electron cloud covers a variable fraction of the disk. The absorbing column density was consistent with the galactic line-of sight value, and did not vary by more than $1.6\times 10^{21}$ cm$^{-2}$. Together with the featureless spectra, these properties may not be reconciled easily with the super-critical accretion scenario of this source.