The Donor of the Black Hole XRay 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 ${M}_{2}\approx {0.49}_{0.10}^{+0.10}{M}_{\odot }$ and the radius of ${R}_{2}\approx {1.19}_{0.08}^{+0.08}{R}_{\odot }$ . We reanalyze 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 iband fluxes found in PanSTARSS1 Data Release 2 prediscovery imaging and for a distance of D ≈ 3 kpc, reddening of E(B  V) = 0.23, and R _{2} ≈ 1.11R _{⊙}, we find T ≲ 4230 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 a mass of 0.4M _{⊙}, T ≈ 4200 K, and solar metallicity. Two alternative models require D ≳ 3.33.4 kpc at 0.4M _{⊙}, T ≈ 4500 K and halfsolar metallicity, and 0.5M _{⊙}, T ≈ 4300 K, and solar metallicity. These models yield masstransfer rates of ~10^{10} M _{⊙} yr^{1}, compatible with those based on the estimated accreted mass of ≈2 × 10^{25} g and the time between the 2018 discovery and the 1934 historical outburst.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 2022
 DOI:
 10.3847/15384357/ac6099
 arXiv:
 arXiv:2201.13201
 Bibcode:
 2022ApJ...930....9M
 Keywords:

 Stellar mass black holes;
 Lowmass xray binary stars;
 Xray binary stars;
 Astrophysical black holes;
 1611;
 939;
 1811;
 98;
 Astrophysics  Solar and Stellar Astrophysics;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 ApJ, in press