Possible Supercritical Accretion on the ULX in the Metal-poor Galaxy IZw18

We present an analysis of X-ray observations of the Ultraluminous X-ray source (ULX) in IZw18 based on archival data taken with Chandra, XMM-Newton, and Suzaku. This ULX is considered to be an intermediate-mass black hole candidate simply because it is in the lowest metallicity environment among ULXs, where formation of heavy black holes is facilitated. However, actual study of the ULX based on observations spanning for a long period has been too limited to determine its nature. In this study, we investigate the spectral evolution of the ULX up to 2014, combining the previously-unpublished Suzaku data with those from the other two satellites. We derive a positive correlation of L∝ T_ in^2.1±0.4 between the bolometric luminosity L and inner-disk temperature T_ in on the basis of the multi-color disk-blackbody model, where we exclude the Chandra data, which has the lowest luminosity and systematic residuals in the fitting. The nominal relation L∝ T_ in^4 for the standard disk is rejected at a significance level of 1.5 during these observations except at the time of the Chandra observation, in which the ULX was likely to be in a different state. The apparent inner-disk radius appears negatively correlated with the inner-disk temperature. Moreover, we find a radial dependence of the disk temperature of T (r)∝ r^-p with p<0.75, which also supports the hypothesis that the ULX has a slim disk. In consequence, the IZw18 ULX is most likely to be powered by a stellar-mass compact object in supercritical accretion.