The supergiant optical counterpart of ULX P13 in NGC 7793

Motch, C.; Pakull, M. W.; Grisé, F.; Soria, R.
Referencia bibliográfica

Astronomische Nachrichten, Vol.332, Issue 4, p.367

Fecha de publicación:
5
2011
Número de autores
4
Número de autores del IAC
0
Número de citas
50
Número de citas referidas
44
Descripción
We have identified the optical counterpart of the ULX source P13 in the nearby spiral galaxy NGC 7793. The object is a V ˜ 20.5 mag star, ten times brighter than any other established counterpart of a ULX in nearby galaxies. Medium resolution optical spectroscopy carried out in 2008 and 2009 with the ESO-VLT reveals the presence of narrow high order Balmer, He I and Mg II absorption lines indicating a late B-type supergiant companion star with mass between 10 and 20 M&sun;. Stellar Heta and He II lambda4686 Å emission lines are also seen superposed on the photospheric spectrum. We detect different patterns of radial velocity variations from the emission and absorption lines over a time interval of one month. The velocity of the high order Balmer absorption lines changes by ˜100 km s-1 while the Heta and He II lambda4686 Å emission components vary by about the same amount but with a different phasing. Assuming that the observed velocity changes trace the motion of the mass-donor star and of the X-ray source implies a mass of the accreting black hole in the range of 3 to 100 M&sun; with a most probable value of ˜10 to 20 M&sun;. We expect an orbital period in the range of 20 to 40 days based on the low density of the supergiant star. P13 is likely in a short-lived, and thus rare high X-ray luminosity evolutionary state associated with the ascension of the donor star onto the supergiant stage. Based on optical observations obtained at ESO under programme 084.D.0881(A).