IGR J19308+0530: Roche lobe overflow on to a compact object from a donor 1.8 times as massive

TL;DR

This study characterizes the binary system IGR J19308+0530, revealing a Roche lobe filling F-type donor star with a mass ratio suggesting potential instability in mass transfer, and identifies the accretor as likely a white dwarf.

Contribution

First detailed phase-resolved spectroscopy and photometry of IGR J19308+0530, determining system parameters and the nature of the accretor.

Findings

Donor star is Roche lobe filling with an orbital period of ~14.66 hours.

Mass ratio q = 1.78 indicates a donor more massive than the accretor.

Likely a white dwarf accretor, with possible neutron star scenario not excluded.

Abstract

We present phase-resolved spectroscopy and photometry of the optical counterpart to the X-ray binary IGR J19308+0530. Ellipsoidal modulations in the light curve show that the F-type companion star in the system is Roche lobe filling. The optical spectra are dominated by absorption features from the donor star, with ~10-20 per cent disc contribution to the optical continuum. We measure an orbital period of 14.662+-0.001 h, a radial velocity semiamplitude for the companion star of K2 = 91.4+-1.4 kms-1 and a rotational broadening of vsini = 108.9+-0.6 kms-1. From K2 and vsini, given that the donor star is filling its Roche lobe, we derive a mass ratio of q = M2/M1=1.78+-0.04, which is typically considered to be too large for stable Roche lobe overflow. Our observations support an inclination of ~50 degrees. The accretor in IGR J19308+0530 is most likely a white dwarf, although a neutron star cannot entirely be excluded.