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X-ray Transients in M31

ATel #76; A. Kong, M. Garcia, S. Murray, F. Primini, J. McClintock and R. Di Stefano (Harvard-Smithsonian Center for Astrophysics)
on 12 Sep 2001; 23:47 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Albert Kong (akong@cfa.harvard.edu)

Subjects: X-ray, Transient

A bright X-ray transient in M31 was detected in a 5000-sec Chandra ACIS-I observation on 2001 Aug 31.54 UT. The new transient (CXOU J004305.5+411703) is located at R.A. = 00h43m05s.55, Decl = +41o17'03''.3 (equinox 2000.0; based on the aspect solution, with uncertainty of ~1''). The energy spectrum can be fitted with a power-law model with a photon index of 1.5; the 0.5-10 keV luminosity is estimated to be 2.7 x 1038 erg/s (assuming a distance of 780 kpc). There is second transient (CXOU J004242.1+411424) located at R.A. = 00h42m42s.1, Decl = +41o14'24''.7 (equinox 2000.0; with uncertainty of ~1''). This source is not detected below 1 keV, with estimated 0.5-10 keV luminosity of 7 x 1036 erg/s (assuming a power-law spectrum with NH=1021 cm-2 and a photon index of 1.2). Both sources did not show up in the previous Chandra observation on 2001 June 10. Observations of the new X-ray transients at other wavelengths are encouraged.

The super-soft transient observed by XMM (XMMU J004308.5+411820, IAUC # 7659 ) in 2001 June is not detected in our observation. The other transient discovered in the June XMM observation (XMMU J004245.2+411721) is only present just above the 2-sigma level at 1036 erg/s (0.5-10 keV; assuming a power-law spectrum with photon index of 1.5), corresponding to an order of magnitude fainter than in 2001 June.