Swift XRT observations of the M 31 ULX XMMU J004243.6+412519

ATel #3959; M. Henze, W. Pietsch, F. Haberl (Max-Planck-Institut fuer extraterrestrische Physik)
on 9 Mar 2012; 15:00 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

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We report on recent target of opportunity (ToO) observations with the X-ray telescope (XRT) on board the Swift satellite of the transient ultralumious X-ray source (ULX) XMMU J004243.6+412519 in M 31 (see ATel's #3890,#3921,#3937). Five observations have been carried out in XRT photon counting mode: ObsIDs 00032286002 (started on 2012-02-19.03 UT, exposure time 3.8 ks), 00032286003 (2012-02-23.78 UT, 3.3 ks), 00032286006 (2012-03-02.20 UT, 6.3 ks), 00032286007 (2012-03-03.01 UT, 7.1 ks) and 00032286010 (2012-03-04.20 UT, 4.3 ks). We find that the XRT count rates of the ULX are in agreement for all observations with an average of (3.7±0.1) × 10^-1 ct s^-1, corrected for vignetting, PSF and dead time.

The X-ray spectrum of XMMU J004243.6+412519 did not change significantly over the course of the Swift monitoring. A combined powerlaw plus disk-black body fit as described for the XMM-Newton data in ATel #3921 does result in a best-fit (χ² dof^-1 of 582/570) N_H = (4.9^+0.7_-1.0) × 10²¹ cm^-2 and inner disk kT of (0.88±0.04) keV (similar to the results in ATel #3921) if the powerlaw photon index is fixed at Γ = 2.7 (see ATel #3921), otherwise the solution becomes degenerate. This degeneracy might be caused by a limited sensitivity of the XRT to the high-energy powerlaw tail of the spectrum. The resulting best-fit average luminosity is 1.6 × 10³⁹ erg s^-1 in the (0.2-10.0) keV band. This is only slightly lower than the luminosities reported in ATel #3921. The spectrum can also be fitted using an absorbed disk-black body model, resulting in a χ² dof^-1 of 593/575 and best-fit N_H = (3.4±0.3) × 10²¹ cm^-2 and inner disk kT of (0.90±0.03) keV. Here, the derived unabsorbed luminosity is 1.1 × 10³⁹ erg s^-1 in the (0.2-10.0) keV band. These model parameters and luminosity are in agreement with the results from a Chandra ACIS observation on 2012-02-19 (simultaneous with our first Swift observation) reported in ATel #3937.

More than 40 days after the first detection of XMMU J004243.6+412519 in an ULX state (on 2012-01-21.51 UT, see ATel #3921) the source is still bright at L ≥ 10³⁹ erg s^-1 and currently shows no indications of becoming fainter. Its spectrum and average luminosity were stable during the 2 weeks of Swift monitoring and likely over the entire 40 days. This behaviour contrasts strongly with that of CXOM31 J004253.1+411422, which was the first ULX found in M 31 (Kaur et al. 2012, A&A, 538A, 49K; Middleton et al. 2012, MNRAS, 420, 2969). Discovered in December 2009 (see also ATel #2356), the luminosity of this source was observed to decline exponentially with a time constant of about 32 days.

No optical/UV counterpart of the source was visible in corresponding, stacked Swift UVOT images in the filters UVW2 (112-264 nm) and Johnson U. We derived 3 sigma upper limits of 24.0 (UVW2, 16.1 ks exposure) and 22.7 (U, 13.4 ks) AB magnitudes, respectively. All magnitudes are on the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) and have not been corrected for extinction.

We wish to thank the Swift Team for making the ToO observations possible, in particular N. Gehrels, the duty scientists as well as the science planners.