The ultraluminous X-ray source NGC 5643 ULX1: a large stellar mass black hole accreting at super-Eddington rates?

TL;DR

This study analyzes XMM-Newton data of ULX NGC 5643 ULX1, suggesting it hosts a stellar-mass black hole accreting at super-Eddington rates, challenging the IMBH hypothesis for such luminous sources.

Contribution

It provides a detailed spectral and temporal analysis of NGC 5643 ULX1, favoring super-Eddington accretion onto a stellar-mass black hole over the IMBH scenario.

Findings

Spectra are better described by a thermal component, such as an advection dominated disc or Comptonising corona.

No clear correlation between variability and luminosity was observed.

The source belongs to the hard/ultraluminous ULX class.

Abstract

A sub-set of the brightest ultraluminous X-ray sources (ULXs), with X-ray luminosities well above $10^{40}$ erg s$^{-1}$, typically have energy spectra which can be well described as hard power-laws, and short-term variability in excess of $\sim10\%$. This combination of properties suggests that these ULXs may be some of the best candidates to host intermediate mass black holes (IMBHs), which would be accreting at sub-Eddington rates in the hard state seen in Galactic X-ray binaries. In this work, we present a temporal and spectral analysis of all of the available XMM-Newton data from one such ULX, the previously poorly studied 2XMM J143242.1$-$440939, located in NGC 5643. We report that its high quality EPIC spectra can be better described by a broad, thermal component, such as an advection dominated disc or an optically thick Comptonising corona. In addition, we find a hint of a marginal change in the short-term variability which does not appear to be clearly related to the source unabsorbed luminosity. We discuss the implications of these results, excluding the possibility that the source may be host an IMBH in a low state, and favouring an interpretation in terms of super-Eddington accretion on to a black hole of stellar origin. The properties of NGC 5643 ULX1 allow us to associate this source to the population of the hard/ultraluminous ULX class.