Spectral State Transitions of the Ultraluminous X-ray Source IC 342 X-1

TL;DR

This study uses simultaneous X-ray and radio observations of ULX IC 342 X-1 to analyze its spectral state, finding a thermal spectrum with no significant radio emission, and estimates the black hole mass to be between 30 and 200 solar masses.

Contribution

First simultaneous X-ray and radio observations of IC 342 X-1 revealing its thermal state and providing black hole mass estimates based on spectral modeling.

Findings

X-ray spectrum is softer than previous observations.

No significant radio emission detected.

Black hole mass estimated between 30 and 200 solar masses.

Abstract

We observed the Ultraluminous X-ray Source IC 342 X-1 simultaneously in X-ray and radio with Chandra and the JVLA to investigate previously reported unresolved radio emission coincident with the ULX. The Chandra data reveal a spectrum that is much softer than observed previously and is well modelled by a thermal accretion disc spectrum. No significant radio emission above the rms noise level was observed within the region of the ULX, consistent with the interpretation as a thermal state though other states cannot be entirely ruled out with the current data. We estimate the mass of the black hole using the modelled inner disc temperature to be $30~\mathrm{M_{\odot}} \lesssim M\sqrt{\mathrm{cos}i}\lesssim200~\mathrm{M_{\odot}}$ based on a Shakura-Sunyaev disc model. Through a study of the hardness and high-energy curvature of available X-ray observations, we find that the accretion state of X-1 is not determined by luminosity alone.