Discovery of a second outbursting hyperluminous X-ray source

TL;DR

This paper reports the discovery of a second outbursting hyperluminous X-ray source, showing significant X-ray and optical variability, supporting its classification as an outbursting HLX likely hosting an intermediate mass black hole.

Contribution

It provides the first evidence of an outbursting HLX beyond HLX-1, with multi-epoch observations confirming variability and supporting its classification as an HLX.

Findings

Detected a new outburst with >60 times X-ray variability.

Optical counterpart shows variability consistent with X-ray source.

Supports the existence of a second outbursting HLX, likely hosting an intermediate mass black hole.

Abstract

We report on six Chandra and one HST/WFC3 observation of CXO J122518.6+144545, discovered by Jonker et al. (2010) as a candidate hyperluminous X-ray source (HLX), X-ray bright supernova or recoiling supermassive black hole at $L_X = 2.2 \times 10^{41}$ erg/s (if associated with the galaxy at 182 Mpc). We detect a new outburst of the source in a Chandra image obtained on Nov 20, 2014 and show that the X-ray count rate varies by a factor $> 60$. New HST/WFC3 observations obtained in 2014 show that the optical counterpart is still visible at $g' = 27.1 \pm 0.1$, $1 \pm 0.1$ magnitude fainter than in the discovery HST/ACS observation from 2003. This optical variability strongly suggests that the optical and X-ray source are related. Furthermore, these properties strongly favour an HLX nature of the source over the alternative scenarios. We therefore conclude that CXO J122518.6+144545 is most likely an outbursting HLX. It is only the second such object to be discovered, after HLX-1 in ESO 243-49. Its high X-ray luminosity makes it a strong candidate to host an intermediate mass black hole.