The mass of the black hole in GS 2000+25

We measured the I-band orbital light curve of the soft X-ray transient GS 2000+25 in 1992 August, and the R-and I-band light curves between 1998 September and 2000 August. The light curves are dominated by the ellipsoidal variations of the secondary star in the system. The light curves also showed transient distortions that persisted through individual observing runs, but were diluted and nearly absent in the mean light curve. A small, residual distortion in the mean light curve is consistent with extra flux from a bright spot where the stream of material from the secondary star interacts with the accretion disk around the compact star. The light curve does not show eclipses. The best fits to the mean light curve were achieved with a model that included ellipsoidal variations of the secondary star plus a bright spot on the edge of the accretion disk. The flux from the accretion disk is not well constrained but must be less than 32% of the total R-band flux. Combining our data with previously published spectroscopy of GS 2000+25, we find that the mass of the compact star lies in the range 5.5≲M ₁/M.≲8.8, and therefore it is a black hole. The derived mass of the secondary star depends sensitively on the mass ratio, which is measured from the rotational broadening of the absorption lines in the secondary's spectrum. We find 0.16≲M ₂/M.≲0.47, but we deem this result less secure than the mass for M ₁. The radius and spectral type (K5) of the secondary are not consistent with those of main-sequence stars with masses in this range but are consistent with an evolved star that has been stripped of mass.