Possible Supercritical Accretion on the ULX in the Metal-poor Galaxy IZw18
Marina Yoshimoto, Tomokage Yoneyama, Hirofumi Noda, Hirokazu Odaka, Hironori Matsumoto
arXiv:2405.13107v1 Announce Type: new
Abstract: 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 In this study, we investigate the spectral evolution of the ULX up to 2014, combining the previously-unpublished Suzaku data with those from the other two satellites.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 $Lpropto T_{rm in}^{2.1pm0.4}$ between the bolometric luminosity $L$ and inner-disk temperature $T_{rm in}$ 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 $Lpropto T_{rm in}^{4}$ for the standard disk is rejected at a significance level of 1.5 %. These results suggest that the ULX was in the slim-disk state 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 $T (r)propto r^{-p}$ with $parXiv:2405.13107v1 Announce Type: new
Abstract: 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 In this study, we investigate the spectral evolution of the ULX up to 2014, combining the previously-unpublished Suzaku data with those from the other two satellites.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 $Lpropto T_{rm in}^{2.1pm0.4}$ between the bolometric luminosity $L$ and inner-disk temperature $T_{rm in}$ 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 $Lpropto T_{rm in}^{4}$ for the standard disk is rejected at a significance level of 1.5 %. These results suggest that the ULX was in the slim-disk state 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 $T (r)propto r^{-p}$ with $p