Astrophysical parameters and orbital solution of the peculiar X-ray transient IGR J00370+6122

Context. BD +60 degrees 73 is the optical counterpart of the X-ray source IGR J00370+6122, a probable accretion-powered X-ray pulsar. The X-ray light curve of this binary system shows clear periodicity at 15.7 d, which has been interpreted as repeated outbursts around the periastron of an eccentric orbit. Aims. We aim to characterise the binary system IGR J00370+6122 by deriving its orbital and physical parameters. Methods. We obtained high-resolution spectra of BD + 60 degrees 73 at different epochs. We used the FASTWIND code to generate a stellar atmosphere model to fit the observed spectrum and obtain physical magnitudes. The synthetic spectrum was used as a template for cross-correlation with the observed spectra to measure radial velocities. The radial velocity curve provided an orbital solution for the system. We also analysed the RXTE/ASM and Swift/BAT light curves to confirm the stability of the periodicity. Results. BD +60 degrees 73 is a BN0.7 Ib low-luminosity supergiant located at a distance similar to 3.1 kpc, in the Cas OB4 association. We derive T-eff = 24 000 K and log g(c) = 3.0, and chemical abundances consistent with a moderately high level of evolution. The spectroscopic and evolutionary masses are consistent at the 1-sigma level with a mass M-* approximate to 15 M-circle dot. The recurrence time of the X-ray flares is the orbital period of the system. The neutron star is in a high-eccentricity (e = 0.56 +/- 0.07) orbit, and the X-ray emission is strongly peaked around orbital phase phi = 0.2, though the observations are consistent with some level of X-ray activity happening at all orbital phases. Conclusions. The X-ray behaviour of IGR J00370+6122 is reminiscent of intermediate supergiant X-ray transients, though its peak luminosity is rather low. The orbit is somewhat wider than those of classical persistent supergiant X-ray binaries, which when combined with the low luminosity of the mass donor, explains the low X-ray luminosity. IGR J00370+6122 will very likely evolve towards a persistent supergiant system, highlighting the evolutionary connection between different classes of wind-accreting X-ray sources.