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ULX candidate in M31

ATel #5743; R. Sturm, W. Pietsch, J. Greiner, F. Haberl (MPE, Germany)
on 9 Jan 2014; 11:21 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 5752, 5753, 5771

In recent monitoring observations of the M31 centre with the Swift satellite, we found an X-ray transient at RA=00:43:27.7 Dec=+41:04:53 (J2000, ePOS=3.6", 90% confidence), named Swift J004327.6+410452.

The source was detected first in a 195 s XRT exposure (ObsID 00032702045) starting on 2013-12-28.6 with a count rate of 0.20±0.03 s-1. In a 3474 s exposure (ObsID 00032702046) starting on 2014-01-05.2, the rate increased to 0.34±0.01 s-1.

The X-ray spectrum is described well (reduced χ2=0.97, 51 dof) by an absorbed power law with NH=(2.5±0.4)×1021cm-2 and Γ=2.4±0.2 and a flux of 1.40±0.05 ×10-11 erg cm-2 s-1 in the 0.3-7.0 keV band. For a distance of 780 kpc, this corresponds to an unabsorbed luminosity of 2.1×1039 erg s-1. Alternatively, using a disc black-body model (reduced χ2=0.82, 51 dof), we obtain NH=(3.7±2.5)×1020cm-2, kT=(1.02±0.08) keV, F=(1.40±0.08)×10-11 erg cm-2 s-1 and L=1.1 ×1039 erg s-1. In a previous observation (ObsID 00032702044, 3433 s exposure, 2013-12-20.0) no source was detected, resulting in a 3 sigma upper-limit flux of 5.5×10-14 erg cm-2 s-1 for the power-law model, a factor of 250 below the maximum flux.

There is a nearby red star 2MASS J00432709+4104521 (R=14.4 mag) which is unlikely to be associated with the X-ray source (angular separation 6.8"). Further arguments against an association are that a thermal plasma model results in an inferior fit (reduced χ2=1.44) and that the duration (>8 days) of the X-ray emission is atypical for an X-ray outburst of a flare star.

While LGGS J004327.49+410450.7 (Massey et al. 2006, AJ, 131, 2478) is within the X-ray error circle (3.3" separation), it cannot be the counterpart for a standard HMXB (V=22.5 mag, B-V=0.8 mag) or LMXB interpretation.

The X-ray spectrum and X-ray luminosity suggest a black-hole X-ray transient in soft state at ULX level with low-mass counterpart, if located in M31.

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