The supergiant optical counterpart of ULX P13 in NGC7793

TL;DR

This paper identifies the optical counterpart of ULX P13 in NGC 7793 as a supergiant star and estimates the black hole mass and orbital period based on spectroscopic observations and radial velocity variations.

Contribution

It provides the first detailed optical characterization of ULX P13, revealing the nature of its companion star and constraining the black hole mass and orbital parameters.

Findings

Optical counterpart is a late B supergiant star.

Black hole mass estimated between 3 and 100 solar masses.

Orbital period likely between 20 and 40 days.

Abstract

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, HeI and MgII absorption lines indicating a late B type supergiant companion star with mass between 10 and 20 Msol. Stellar H beta and HeII 4686 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 while the H beta and HeI 4686 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 Msol with a most probable value of ~ 10 to 20 Msol. 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.