A Transient Black-Hole Low-Mass X-Ray Binary Candidate in Centaurus A

TL;DR

This paper reports the discovery of a transient X-ray source in Centaurus A, likely a stellar-mass black hole in a low-mass X-ray binary, showing significant luminosity increase and spectral state transition.

Contribution

It presents the first identification of a luminous transient X-ray source in Cen A as a black hole candidate with detailed timing and spectral analysis.

Findings

Source flux increased by over 770 times from 2003 to 2007.

The source exhibited a spectral state transition from steep-power law to thermal.

It is the most luminous X-ray source in an early-type galaxy with evidence of transience.

Abstract

We report the discovery of a bright transient X-ray source, CXOU J132518.2-430304, towards Centaurus A (Cen A) using six new Chandra X-Ray Observatory observations in 2007 March--May. Between 2003 and 2007, its flux has increased by a factor of >770. The source is likely a low-mass X-ray binary in Cen A with unabsorbed 0.3-10 keV band luminosities of (2-3) x 10^{39} erg s^-1 and a transition from the steep-power law state to the thermal state during our observations. CXOU J132518.2-430304 is the most luminous X-ray source in an early-type galaxy with extensive timing information that reveals transience and a spectral state transition. Combined with its luminosity, these properties make this source one of the strongest candidates to date for containing a stellar-mass black hole in an early-type galaxy. Unless this outburst lasts many years, the rate of luminous transients in Cen A is anomalously high compared to other early-type galaxies.