XMM-Newton observation of two X-ray transients in M 31
ATel #1191; H. Stiele (Max-Planck-Institut fuer extraterrestrische Physik, MPE), W. Pietsch (MPE), F. Haberl (MPE), M. Freyberg (MPE), M. Diaz Trigo (ESA Science Operations Centre)
on 24 Aug 2007; 10:37 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)
Subjects: Optical, X-ray, Binary, Black Hole, Transient
The field of the X-ray transient CXOM31 J004059.2+411551 (ATel #1147) was covered with XMM-Newton for 22 ks in a target of opportunity observation (TOO) starting on 2007-07-25.45 UT. About 20 days after the Chandra detection the transient was still bright. The EPIC pn position of the source is RA(J2000) 00:40:59.2, Dec(J2000) +41:15:50.5 (1 sigma position error 1.5" determined by the assumed systematic error) consistent with the position given in ATel #1147. The 0.2-7.0 keV EPIC pn spectrum can be fitted by an absorbed disk blackbody model with best fit parameters
nH = 2.1e21 ± 0.2e21 atoms/cm2, Tin = 0.51 ± 0.02 keV and Rin<\sub>sqrt(cos(i)) = 82 ± 10 km, where i is the inclination angle of the disk. The unabsorbed luminosity (0.5-10 keV) is Lx ~ 9.1e37 erg/s. We assume a M 31 distance
of 780 kpc. Spectral parameters and luminosity did not change significantly compared to the Chandra values reported in ATel #1147. A FFT period search did not reveal any significant periodicities in
the 0.3 to 2000 s range.
From Chandra observations on 2007-07-31, three new X-ray transients in M 31 were reported (ATel #1171). Two of the source positions are covered by the field of view of the XMM-Newton TOO observation 6 days earlier.
The first transient, CXOM31 J004100+411531.5 (note error in naming convention) has an angular distance of about 20" to CXOM31 J004059.2+411551 and shows very similar spectral parameters. It is positioned at an offset of more than 19 arcmin from the pointing direction. As we do not find any additional source near CXOM31 J004100+411531.5 in the XMM-Newton observation, it is likely that CXOM31 J004100+411531.5 is the same source as CXOM31 J004059.2+411551 with the wrong position caused by the detector only partly covering the point spread function of the source.
The second transient, CXOM31 J004145+411108.8 coincides within the positional errors with the
transient XMMU J004144.7+411110 detected by Trudolyubov et al. (2006, ApJ 645, 277) in 2004 July XMM-Newton observations. We also detect the source in the XMM-Newton TOO observation at RA(J2000) 00:41:44.6, DEC(J2000) 41:11:12.9 (1 sigma position error 1.5"). The 0.2-7.0 keV EPIC pn spectrum can be fitted by an absorbed disk blackbody model with best fit parameters nH = 2.7e21 ± 1.2e21 atoms/cm2, Tin = 0.54 ± 0.11 keV and Rin<\sub>sqrt(cos(i)) = 31 ±
20 km. The unabsorbed luminosity (0.3-7 keV) is Lx ~ 1.9e37 erg/s. The position, best fit parameters and luminosity are in agreement with the values of Trudolyubov et al. (2006). A FFT period search did not reveal any significant periodicities in the 0.3 to 2000 s range. However, the
period search was not as sensitive for this significantly fainter source.
We checked the XMM-Newton optical monitor UVW1, UVM2 images taken during the TOO observation and also the Local Group Survey M 31 images (Massey et al. 2006, AJ 131, 2478). No counterparts are evident for both of the transients. The lack of bright optical counterparts and the X-ray parameters (X-ray spectrum, lack of periodicity, transient nature, luminosity) are consistent with these sources being black hole X-ray transients as already suggested in ATel #1147 and ATel #1171.
