The Donor of the Black-Hole X-Ray Binary MAXI J1820+070
Abstract
We estimate the parameters of the donor of the accreting black-hole binary MAXI J1820+070. The measured values of the binary period, rotational and radial velocities and constraints on the orbital inclination imply the donor is a subgiant with the mass of M2≈ 0.49+0.10-0.10M and the radius of R2≈ 1.19+0.08-0.08R. We re-analyze the previously obtained optical spectrum from the Gran Telescopio Canarias, and found it yields a strict lower limit on the effective temperature of T>4200 K. We compile optical and infrared fluxes observed during the quiescence of this system. From the minima r and i-band fluxes found in Pan-STARSS1 Data Release 2 pre-discovery imaging and for a distance of D≈3 kpc, reddening of E(B--V)=0.23 and R2≈1.11R, we find T4230 K, very close to the above lower limit. For a larger distance, the temperature can be higher, up to about 4500 K (corresponding to a K5 spectral type, preferred by previous studies) at D=3.5 kpc, allowed by the Gaia parallax. We perform evolutionary calculations for the binary system and compare them to the observational constraints. Our model fitting the above temperature and radius constraints at D≈ 3 kpc has the mass of 0.4M, T≈4200 K and solar metallicity. Two alternative models require D 3.3--3.4 kpc at 0.4 M, T≈4500 K and half solar metallicity, and 0.5M, T≈4300 K and solar metallicity. These models yield mass transfer rates of \!\!10-10M/yr, compatible with those based on the estimated accreted mass of ≈\!2× 1025 g and the time between the 2018 discovery and the 1934 historical outburst.