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 shaped spectrum with a mild cutoff at ∼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 of a cool (∼0.2 keV) multi-color disk blackbody emission model and 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 are observed directly. The 0.5-10 keV luminosity was 1.2 × 10⁴⁰ erg s^-1 in April, and ∼14% higher in October. This brightening was accompanied by spectral softening in ≥2 keV, with little change in the ≤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 × 10²¹ cm^-2. Together with the featureless spectra, these properties may not be reconciled easily with the super-critical accretion scenario of this source.