XMMU J004243.6+412519 becomes the second ultraluminous X-ray source in M 31
ATel #3921; M. Henze, W. Pietsch, F. Haberl (all MPE) and the XMM-Newton/Chandra M 31 nova monitoring collaboration
on 13 Feb 2012; 16:46 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)
Subjects: X-ray, Transient
We report a significant brightening of the new X-ray transient XMMU J004243.6+412519 (discovery reported by us in ATel #3890) which makes this object the second ultraluminous X-ray source (ULX) to be detected in M 31. The source had crossed the ULX threshold (L > 1e39 erg/s) first in a 17.3 ks XMM-Newton observation which started on 2012-01-21.51 UT (about six days after discovery) and was still detected at a similar brightness ten days later in a 26.0 ks XMM-Newton observation which started on 2012-01-31.10 UT. This constitutes a rise in luminosity by an order of magnitude with respect to ATel #3890.
The XMM-Newton EPIC spectrum of the transient did evolve significantly with respect to ATel #3890 and can now be described best using an absorbed powerlaw plus disk-black body model. The best fits for both new observations are in agreement within the errors and a combined fit produces the following parameters: absorption column density of (6.2+/-0.5)e21 cm-2, powerlaw index of 2.7+/-0.1 and inner disk kT of (0.79+/-0.05) eV. These parameters place XMMU J004243.6+412519 within the high-temperature group of ULXs which is believed to represent stellar mass black holes (see e.g. Kaur et al. 2012, A&A, 538A, 49K). The model results in unabsorbed luminosities of 2.0e39 and 2.5e39 erg/s, respectively, assuming a distance to M 31 of 780 kpc.
The short-term light curve of the ULX shows some evidence for irregular variability on time scales of 1000s in the first observation, but no periodicities have been found in either light curve. The long-term evolution of the source suggests that it was discovered during the rise to the maximum velocity. We found the ULX not to be present in a 17.3 ks XMM-Newton observation which started on 2012-01-07.12 UT, 8.5 days before discovery, with a 3 sigma luminosity upper limit of 1.6e37 erg/s. The currently relatively stable luminosity might indicate a slow decline.
Our on-going monitoring of the M 31 central region with XMM-Newton and Chandra therefore led to the discovery of already the second ULX transient, only two years after the first source of this kind was found by the same programme (CXOM31 J004253.1+411422; see ATel #2356 and Kaur et al. 2012, A&A, 538A, 49K). Nearby ULXs provide rare opportunities for detailed studies of those bright objects normally detected in more distant galaxies (see also Middleton et al. 2012, MNRAS accepted, arXiv:1111.1188). Within the monitoring, four up-coming Chandra HRC-I observations in February and March will cover the position of XMMU J004243.6+412519.
