Abstract
In 2021–2022, photometric observations in the \(Ic\) filter of the faint X-ray nova in a quiescent state, GS 2000+25 (QZ Vul), have been carried out. Compared to 1995–1999, no significant long-term changes in the orbital period have been detected. There are also no any changes in the average brightness of the system, and the amplitude and the shape of the orbital light curve. The average \(Ic\) -light curve of the system (the total amplitude is \(A \sim \;{{0.3}^{m}}\) ) has been constructed. The shape of the curve corresponds to the effect of ellipticity with a negligible effect of X-ray heating of the star and with a small difference in the heights of the maxima. The main contribution to the total flux from the system in the \(Ic\) filter is made by the secondary component K3 V–K6 V (81–87%). The contribution of the radiation from the disk with a hot spot is \( \sim {\kern 1pt} 10{-} 14\% \) . An interpretation of the \(Ic\) mean light curve of QZ Vul has been performed within the framework of a model of a close binary system consisting of an optical K4.5 V star that completely fills its Roche lobe and a compact (relativistic) object surrounded by a weakly elliptical accretion disk of complex shape with thin near the boundary layer and with a thick outer edge. The presence of a gas flow (hot line) and a hot spot on the lateral surface of the disk has been taken into account. Based on the dependences of the residuals \({{\chi }^{2}}\) on the mass ratio \(q\) and on the inclination of the orbit \(i\) at a significance level of 5%, confidence intervals have been obtained for \(q\) ( \(q = 26.2{-} 30.4\) , \({{q}_{{\min }}} = {{M}_{x}}{\text{/}}{{M}_{v}} = 28\) ) and \(i\) ( \(i = 61{\kern 1pt} ^\circ {-} 66^\circ \) , \({{i}_{{\min }}} = 64^\circ \) ). The masses of the stars in the system have been estimated based on the mass function of the optical star. The mass of the black hole is \({{M}_{x}} = (6.8{-} 8.2){\kern 1pt} {{M}_{ \odot }}\) with the optimal value of \(7.34{\kern 1pt} {{M}_{ \odot }}\) . The mass of the optical star lies in the range of \((0.21{-} 0.34){\kern 1pt} {{M}_{ \odot }}\) with the optimal value of \(0.265{\kern 1pt} {{M}_{ \odot }}\) . The radius of the optical star K4.5 V is \( \sim {\kern 1pt} 0.64{\kern 1pt} {{R}_{ \odot }}\) . Its mass, radius, and spectral type are inconsistent with the data for main-sequence stars and correspond to an evolved star that has lost part of its mass during a long-term mass exchange in the system.
