New X-ray Transients in M31: XMMU J004215.8+411924 and SWIFT J004217.4+411532

ATel #881; F. Haberl (MPE), W. Pietsch (MPE), J. Greiner (MPE), M. Ajello (MPE) & the collaboration to monitor the supersoft state of optical novae in M31
on 8 Sep 2006; 21:40 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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A 20 ksec XMM-Newton observation on 2006, August 9 as part of the monitoring program of the X-ray supersoft state of optical novae in the core of M31 (see http://www.mpe.mpg.de/xray/research/normal_galaxies/m31/novae.php ) revealed a new bright transient at R.A.(J2000) = 00:42:15.88, Dec.(J2000) = +41:19:24.4 with a position uncertainty of ~2 arcseconds. The source designated XMMU J004215.8+411924 shows a hard X-ray spectrum with an absorption column density of 4.2x10^21 cm^-2 and a photon index of 1.57 when modeled with an absorbed power-law. The observed (0.5-10.0 keV) flux of 1.16x10^-12 erg s^-1 cm^-2 corresponds to an intrinsic source luminosity of 9.1x10^37 erg s^-1 at a distance of 780 kpc. A follow-up target of opportunity observation with SWIFT on 2006, September 1 revealed a decay of the source intensity by a factor of ~4. The UVOT images show a source within the X-ray error circle, at R.A.(J2000) = 00:42:16.06, Dec.(J2000) = +41:19:22.6 (error of 1 arcsec) with magnitudes of UVM2 = 20.7 +/- 0.4 and UVW1 = 19.8 +/- 0.6 (not extinction corrected). The X-ray spectral and optical properties are consistent with a high mass X-ray binary, most likely a Be/X-ray transient in type II outburst. The 6 ksec SWIFT observation revealed another new bright transient at R.A.(J2000) = 00:42:17.4, Dec.(J2000) = +41:15:32 (uncertainty ~6 arcseconds). This source, designated SWIFT J004217.4+411532 shows a softer X-ray spectrum with an absorption column density of 3.3x10^21 cm^-2 and a photon index of 2.7. The 0.5-5.0 keV observed flux was 3.5x10^-12 erg s^-1 cm^-2 and the intrinsic source luminosity 4.5x10^38 erg s^-1. These properties suggest a black hole candidate observed during a high/soft outburst state. No source is visible in the UVOT images to limiting magnitudes of UVM2 = 20.0 and UVW1 = 19.1 (worse than the above detections due to higher intrinsic background light). Images can be obtained from: http://www.mpe.mpg.de/xray/research/normal_galaxies/m31/transients.php