Discovery of a second outbursting hyperluminous X-ray source

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 × 10⁴¹ erg s^-1 (if associated with the galaxy at 182 Mpc). We detect a new outburst of the source in a Chandra image obtained on 2014 November 20 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 ± 0.1, 1 ± 0.1 mag 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.