http://arxiv.org/abs/2201.13201
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 radius of $1.08\lesssim R_2/R_\odot\lesssim 1.32$. 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 $D\approx 3$ kpc, reddening of $E(B$–$V)=0.23$ and $R_2\approx 1.08 R_\odot$, we find $T\lesssim 4300$ K, close to the above lower limit. For a larger distance, the temperature can be higher, up to 4600 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 $D\approx 3$ kpc has the mass of $0.4 M_\odot$, $T\approx 4200$ K and solar metallicity. Two alternative models require $D\gtrsim 3.3$–3.4 kpc at $0.4 M_\odot$, $T\approx 4500$ K and half solar metallicity, and $0.5 M_\odot$, $T\approx 4300$ K and solar metallicity. These models yield mass transfer rates of $\sim!! 10^{-10}M_\odot$/yr, compatible with those based on the estimated accreted mass of $\approx! 2\times 10^{25}$ g and the time between the 2018 discovery and the 1934 historical outburst.
J. Mikolajewska, A. Zdziarski, J. Ziolkowski, et. al.
Tue, 1 Feb 22
68/73
Comments: submitted to ApJ, comments are welcome
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