Swift XRT finds the M 31 ULX XMMU J004243.6+412519 still active after Sun block
ATel #4125; M. Henze, W. Pietsch, F. Haberl, J. Greiner (Max-Planck-Institut fuer extraterrestrische Physik)
on 25 May 2012; 14:51 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)
Subjects: X-ray, Transient
We report a detection of the transient ultraluminous X-ray source (ULX) XMMU J004243.6+412519 in M 31 (see ATel's #3890,#3921,#3937,#3959) with the X-ray telescope (XRT) on board the Swift satellite. Soon after the last Swift/XRT observations of the source in early March this year (see ATel #3959) M 31 became unobservable for X-ray telescopes due to Sun constraints. Recently, the visibility window opened again. In a 4.2 ks Swift/XRT PC mode observation, which started on 2012-05-24.62 UT, XMMU J004243.6+412519 is clearly seen to be still active.
The luminosity of the source has declined notably with respect to ATel #3959. We derived an XRT count rate of (1.6±0.1) × 10-1 ct s-1 (corrected for vignetting, PSF and dead time) which is more than a factor of 2 lower than before the Sun block. In our analysis we discarded the last pointing of the observation (about 1 ks) because the source position was projected onto a group of bad detector columns. The source spectrum contains 370 source counts and suggests significant spectral evolution along with the fading. The best fit (χ² dof-1 of 34/31) is provided by an absorbed disk-black body model with NH = (4.0+1.4-1.1) × 1021 cm-2 and inner disk kT of (0.66+0.07-0.06) keV. While in ATel #3959 for an absorbed disk-black body model the best-fit column density was comparable, the disk temperature was higher (0.90±0.03 keV). The unabsorbed luminosity of the source based on the spectral model dropped by more than a factor of 2 to 4.6 × 1038 erg s-1 in the (0.2-10.0) keV band. This is in agreement with the count rate estimate. The model shows that XMMU J004243.6+412519, although still bright 124 days after reaching the ULX state (ATel #3921), is not in the ultraluminous X-ray regime anymore.
We thank the Swift Team for the scheduling of the observations.
