A Black Hole in the X-Ray Nova GS 2000+25

Abstract

We have obtained 13 moderate-resolution (~2.5 A) spectra of the Galactic X-ray nova GS 2000+25 in quiescence with the W. M. Keck 10 m telescope. Cross-correlation with the spectra of late-type dwarfs (especially K2--K7) yields excellent radial velocities for the secondary star. The orbital period is consistent with that previously obtained from photometry (~8.3 hr). A semiamplitude of 518.4 +/- 3.5 km s-1 is found, ~25% smaller than the outer disk velocity implied by the double-peaked H alpha emission line ( Delta v/2 ~ 700 km s-1). The derived mass function is 4.97 +/- 0.10 Mȯ, the second highest known for any low-mass X-ray binary. Being considerably above the conventional limiting mass for a neutron star (~3.2 Mȯ), it strongly suggests that the compact primary is a black hole. Plausible mass estimates based on M2 = 0.4--0.7 Mȯ and i = 67.dg5--80 deg are M1 = 5.9--7.5 Mȯ. Under the assumption that the radial velocity curve of the centroid of the H alpha emission line represents the true motion of the black hole, we formally find q = M2/M1 = 0.050 +/- 0.026.