The optical counterpart of the supersoft X-ray source r3-8 in M31

ATel #8204; M. Orio (University Wisconsin Madison, USA, and INAF-Padova, Italy), G. J.M. Luna (IAF, CONICET-University of Buenos Aires, Argentina), R. Kotulla and J. S.G. Gallagher (University of Wisconsin Madison, USA)
on 21 Oct 2015; 21:39 UT
Credential Certification: Marina Orio (orio@astro.wisc.edu)

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On behalf of a larger collaboration we announce that we have obtained spectra of the M31 supersoft X-ray source defined as r3-8 in the Chandra catalogs (see Chiosi et al. 2014, MNRAS 443, 1821, and references therein) using GMOS and the B600 grating at Gemini North, in the 4150-7100 Angstrom range, on 2015/9/9. The summed spectrum, with 9 exposures lasting 1000 s each, shows a moderately blue continuum with prominent emission lines, of which H alpha is the strongest. Other prominent lines are H beta and [Fe II]. The spectrum is consistent with a B[e] star classification (Lamers et al., 1998, A&A, 340, 117) except for a He II lambda4686 line, which has been observed intermittently only in two other peculiar B[e] stars (see Clark et al. 2013, A&A, 560, A10). We associate this line with a binary compact object component, most likely the source of the supersoft X-ray radiation. The emission wavelengths are consistent with the velocity expected for an intrinsic M31 object at the location of r3-8. The flux in the He II line is approximately 1.5 x10(-18) erg/cm(2)/s, and at M31 distance it would be produced in an accretion disk by ionizing radiation, either coming from a very massive hydrogen burning white dwarf, or from the disk itself if it surrounds a ~10 M(solar) black hole. Further analysis is under way. Monitoring with deep optical photometry this R~22 star, with the aim of determining its likely orbital variability, would be very useful. The X-ray source is variable over time scales of few months and it has shown X-ray luminosity above 10(37) erg/s at M31 distance, for the past 25 years.