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

TL;DR

This study analyzes X-ray data of the ULX in IZw18, revealing it likely operates in a slim-disk state powered by supercritical accretion onto a stellar-mass black hole, challenging the intermediate-mass black hole hypothesis.

Contribution

It provides the first detailed spectral evolution analysis of the ULX in IZw18 over multiple years, demonstrating evidence for supercritical accretion in a low-metallicity environment.

Findings

ULX shows a $L\propto T_{\rm in}^{2.1}$ relation, deviating from the standard $L\propto T_{\rm in}^{4}$.

The ULX was in a slim-disk state during most observations, except during the Chandra data.

Inner-disk radius and temperature show negative correlation, supporting slim-disk presence.

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 In this study, we investigate the spectral evolution of the ULX up to 2014, combining the previously-unpublished Suzaku data with those from the other two satellites.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 $L\propto T_{\rm in}^{2.1\pm0.4}$ 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 $L\propto T_{\rm in}^{4}$ 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 $p<0.75$, 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.