IGR J17451--3022: constraints on the nature of the donor star

Abstract

We constrain the binary parameters of the eclipsing accreting X-ray binary IGR J17451--3022 and the nature of its donor star. The donor mass, its radius, and the system inclination angle are computed based on the system orbital period and eclipse duration recently reported by Bozzo et al.\ (2016). We find that the donor is most likely a main-sequence star with the mass comprised within the range $\sim$(0.5--$0.8){\rm M}_{\odot}$ and the radius of $\sim\! 0.7{\rm R}_{\odot}$. Assuming that the accreting compact object in IGR J17451--3022 is a neutron star, the duration of the nearly total rectangular eclipses yields the inclination angle of the system of $71^\circ\lesssim i\lesssim 76^\circ$, compatible with the presence of dips in this system. We rule out the presence of either brown or white dwarf. However, we find an alternative possibility that the donor star in IGR J17451--3022 could be a partially stripped giant with a very low mass, $\sim\! 0.2{\rm M}_{\odot}$. This case requires a substantial mass loss prior to the formation of the giant-star He core. According to that solution, the radius would be $\sim\! 0.4{\rm R}_{\odot}$, at $i\sim 80^\circ$. We additionally show that the well-known approximate dependence of the giant-star radius exclusively on its core mass breaks down below $\sim\!0.3{\rm M}_{\odot}$.