The hyperluminous X-ray source candidate in IC 4320: another HLX bites the dust

TL;DR

This study investigates a hyperluminous X-ray source candidate in IC 4320, revealing it is a background AGN, which impacts the understanding of true HLX candidates and their characteristics.

Contribution

The paper reports optical observations that identify a HLX candidate as a background AGN, refining the criteria for true hyperluminous X-ray sources.

Findings

The candidate is a background AGN, not a true HLX.

Excludes this object from the small HLX sample.

Highlights differences between confirmed HLXs and other sources.

Abstract

The known members of the class of hyperluminous X-ray sources (HLXs) are few in number, yet they are of great interest as they are regarded as the likeliest intermediate-mass black hole (IMBH) candidates amongst the wider population of ultraluminous X-ray sources (ULXs). Here we report optical photometry and spectroscopy of a HLX candidate associated with the galaxy IC 4320, that reveal it is a background AGN. We discuss the implications of the exclusion of this object from the small number of well-studied HLXs, that appears to accentuate the difference in characteristics between the good IMBH candidate ESO 243-49 HLX-1 and the small handful of other HLXs.