Observations of the recurrent M 31 transient XMMU J004215.8+411924 with Swift, Chandra, HST, and Einstein

¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
e-mail: rbarnard@head.cfa.harvard.edu
² Dublin Institute for Advanced Studies, Dublin, Republic of Ireland
³ Max-Planck-Institut für extraterrestrische Physik, Garching, Germany

Received: 23 July 2010
Accepted: 9 November 2010

Abstract

Context. The transient X-ray source XMMU J004215.8+411924 within M 31 was found to be in outburst again in the 2010 May 27 Chandra observation. We present results from our four Chandra and seven Swift observations that covered this outburst.

Aims. X-ray transient behaviour is generally caused by one of two things: mass accretion from a high mass companion during some restricted phase range in the orbital cycle, or disc instability in a low mass system. We aim to exploit Einstein, HST, Chandra and Swift observations to determine the nature of XMMU J004215.8+411924.

Methods. We model the 2010 May spectrum, and use the results to convert from intensity to counts in the fainter Chandra observations, as well as the Swift observations; these data are used to create a lightcurve. We also estimate the flux in the 1979 January 13 Einstein observation. Additionally, we search for an optical counterpart in HST data.

Results. Our best X-ray positions from the 2006 and 2010 outbursts are 0.3′′ apart, and 1.6′′ from the Einstein source; these outbursts are likely to come from the same star system. We see no evidence for an optical counterpart with m_B ≲ 25.5; this new limit is 3.5 mag fainter than the existing one. Furthermore, we see no V band counterpart with m_V ≲ 26. The local absorption is  ~7 times higher than the Galactic line-of-sight, and provides  ~2 mag of extinction in the V band. Hence M_V ≳ −0.5. Fits to the X-ray emission spectrum suggest a black hole primary.

Conclusions. We find that XMMU J004215.8+411924 is most likely to be a transient LMXB, rather than a HMXB as originaly proposed. The nature of the primary is unclear, although we argue that a black hole is likely.