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New and recurrent X-ray transients in M31 observed with XMM-Newton in January 2016 - part 1

ATel #8826; M. Henze (IEEC/CSIC), M. Sasaki (IAAT), F. Haberl (MPE), B. F. Williams (U. Washington), D. Hatzidimitriou (U. Athens / NOA), for a larger collaboration
on 16 Mar 2016; 12:45 UT
Credential Certification: Martin Henze (henze@ice.cat)

Subjects: X-ray, Nova, Transient

Referred to by ATel #: 8827

Here we report the first set of new and recurrent X-ray transients found in two XMM-Newton 100-ks pointings of the M31 northern disk on Jan 1 and 21 (see ATel #8825).

SWIFT J004420.1+413702 is still clearly detected on Jan 21. This source is a recurring hard X-ray transient in M31 (ATels #1671, #8227). With respect to ATel #8227, its EPIC count rate did decrease by a factor of 5 to (4.3±0.1) × 10-2 ct s-1 (see also ATel #1671). All count rates are given in the 0.2-4.5 keV band. The X-ray spectrum appeared to be unchanged, with absorbed powerlaw best-fit parameters of Γ = 1.8±0.1 and NH = (0.7±0.2) × 1021 cm-2 (all given uncertainties indicate 1σ confidence ranges unless otherwise noted). The source seems to be returning to its quiescence state.

XMMU J004458.3+414207 was reported by us as a possible new X-ray transient in M31 in August 2015 (ATel #8227). A potential literature counterpart is source CXO J004458.2+414206 in the Chandra Source Catalogue (Evans et al. 2010). With respect to the August observation, on Jan 21 the EPIC count rate of the source had decreased by a factor of about two to (2.2±0.4) × 10-3 ct s-1. Assuming an absorbed powerlaw model the X-ray spectrum of the source is broadly consistent with the one described in ATel #8227. However, the number of counts was insufficient to determine useful constraints on the spectral parameters.

XMMU J004423.2+415536 had been reported as a new X-ray transient in ATel #8227. In the Jan 1 observation the source is not detected anymore.

XMMU J004347.8+413310 was reported as a new supersoft X-ray transient in ATel #8227. It is still clearly detected as a bright source in the Jan 21 observation with a consistent X-ray spectrum (assuming an absorbed blackbody parametrisation): kT = 30±2 eV and NH = (1.8±0.2) × 1021 cm-2. This confirms the supersoft X-ray source (SSS) nature of the transient. The EPIC count rate appeared to have dropped slightly from (7.7±0.1) × 10-2 ct s-1 in Jun 2015 (ATel #8227) to (4.7±0.1) × 10-2 ct s-1. The majority of SSS in M31 were found to be optical novae (Pietsch et al. 2005), however there is no nova counterpart known for this SSS transient. If the source is a nova, then based on the M31 population correlations of Henze et al. 2014 we would expect an SSS phase duration of several years for a blackbody temperature as low as 30 eV. We encourage optical observers to check their archives for a nova eruption that might have been missed at this position.