A&A 411, 553-557 (2003)
DOI: 10.1051/0004-6361:20031513

Tracing a Z-track in the M 31 X-ray binary RX J0042.6+4115

R. Barnard¹, U. Kolb¹ and J. P. Osborne<sup>2

1</sup>  The Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6BT, UK
²  The Department of Physics and Astronomy, The University of Leicester, Leicester, LE1 7RH, UK

(Received 27 May 2003 / Accepted 16 September 2003)

Abstract
Four XMM-Newton observations of the core of M 31, spaced at 6 month intervals, show that the brightest point X-ray source, RX J0042.6+4115, has a 0.4-10 keV luminosity of ~ 10 ³⁸ erg s ^-1, and exhibits significant variability in intensity and X-ray spectrum over a time scale of ~100 s including hard flares; such behaviour is only observed in Z-sources and transient black hole binaries in our Galaxy. The lightcurves, X-ray spectra and hardness-intensity data from the four XMM-Newton observations all strongly suggest that it is a Z-source, bringing the total number of known Z-sources to nine.

Key words: X-rays: general -- galaxies: individual: M 31 -- X-rays: binaries

Offprint request: R. Barnard, r.barnard@open.ac.uk

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