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Recent X-ray transients in the M31 disk found with XMM-Newton

ATel #8227; M. Henze (IEEC/CSIC), M. Sasaki (IAAT), F. Haberl (MPE), D. Hatzidimitriou (U. Athens), for a larger collaboration
on 30 Oct 2015; 13:21 UT
Credential Certification: Martin Henze (henze@ice.cat)

Subjects: X-ray, Nova, Transient

Referred to by ATel #: 8228, 8825, 8826, 8827

We report the detection of three new X-ray transients and one recurrent X-ray source in recent XMM-Newton observations of the M31 disk. Two 100-ks observations were carried out on 2015-06-28 (ObsID 0763120101; June) and 2015-08-11 (ObsID 0763120301; August). The observations had only a small overlap and consequently only a few sources are visible in both. One of the new X-ray transients is likely to be a nova counterpart and we encourage optical observers to search their archives for a missed eruption.

SWIFT J004420.1+413702 is detected as a bright X-ray source in both observations. It was previously seen in 2008 by Swift and in 1992 by ROSAT (ATel #1671). The EPIC count rates are consistent at about 2 × 10-1 ct s-1. The X-ray spectrum can be fitted by an absorbed powerlaw with Γ = 1.8±0.1 and NH = (1.1±0.2) × 1021 cm-2 (all uncertainties are 1σ unless otherwise noted). The resulting unabsorbed flux (0.5-5 keV) of about 4.4 × 10-13 erg cm-2 s-1 is consistent with the 2008 measurements.

XMMU J004458.3+414207 is a potentially new X-ray transient in M31 at RA = 00:44:58.31, Dec = +41:42:07.5 (J2000; 3σ uncertainty of 2.5 arcsec). No source was found at this position in the catalogues of Pietsch et al. 2005 or Stiele et al. 2011. However, there is a possible faint counterpart in the Chandra Source Catalogue (Evans et al. 2010): CXO J004458.2+414206 is only 1.4 arcsec away, which is easily within the combined error circles. Our source has an EPIC count rate of (5.3±0.5) × 10-3 ct s-1. Its X-ray spectrum can be fitted using an absorbed powerlaw with Γ = 2.1±0.2 and NH = (0.2±0.2) × 1021 cm-2, implying an unabsorbed flux of about 1 × 10-14 erg cm-2 s-1 (0.5 -5 keV).

XMMU J004423.2+415536 is a new X-ray transient in the August pointing. The coordinates are RA = 00:44:23.26, Dec = +41:55:36.2 (J2000; 3σ uncertainty of 3 arcsec). Nothing is present within 10 arsec around this position in CDS/VizieR or in the catalogues of Pietsch et al. (2005) or Stiele et al. (2011). The source has an EPIC count rate of (3.8±0.4) × 10-3 ct s-1. The spectrum can be modelled using an absorbed powerlaw with Γ = 2.0±0.2 and NH = (0.3±0.2) × 1021 cm-2, indicating an unabsorbed flux of about 1 × 10-14 erg cm-2 s-1 (0.5 -5 keV).

XMMU J004347.8+413310 is a new bright supersoft X-ray source in the June pointing. Its coordinates are RA = 00:43:47.89, Dec = +41:33:10.6 (J2000). No object is present in CDS/VizieR within the 1.5 arcsec 3σ positional error circle. The closest object is a hard X-ray source from the catalogue of Pietsch et al. (2005; source 494) at a distance of about 10 arcsec. The EPIC count rate was (7.7±0.1) × 10-2 ct s-1. The X-ray spectrum was very soft, with the majority of counts below 0.6 keV. We fitted an absorbed blackbody model with kT = 34±1 eV and NH = (1.4±0.1) × 1021 cm-2. The spectral parameters are similar to those of nova counterparts (cf. Henze et al. 2014). However, no optical nova is known at this position. The low effective temperature would suggest a long SSS turn-on time. We encourage optical observers to examine their archives for a possible nova eruption during the last years.