Suzaku observations of spectral variations of the ultra-luminous X-ray source Holmberg IX X-1

Abstract

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 ∼ 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 (∼ 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 × 10 40 erg s −1 in April, and ∼ 14% higher in October. This brightening was accompanied by spectral softening in ≥ 2 keV, with little changes 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 × 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.

A new possible accretion scenario for ultra-luminous X-ray sources

“…Our final issue is how to interpret the HPL state, wherein the Comptonization is further enhanced from the SPL state (Kobayashi et al 2017). From the above two assignments, it is natural to consider that the HPL state is a new spectral state (possibly with 0.4 < η < 1), which would fall, in BHBs, between the VHS and SDS.…”