A BLACK HOLE IN THE X-RAY NOVA GS 2000+25

摘要

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 Ha emission line (Δv/2 ≈ 700 km s~(-1)). The derived mass function is 4.97 ± 0.10 solar mass, the second highest known for any low-mass X-ray binary. Being considerably above the conventional limiting mass for a neutron star (～3.2 solar mass), it strongly suggests that the compact primary is a black hole. Plausible mass estimates based on M_2 = 0.4-0.7 solar mass and i = 67.5°-80° are M_1 = 5.9-7.5 solar mass. Under the assumption that the radial velocity curve of the centroid of the Hα emission line represents the true motion of the black hole, we formally find q = M_2/M_1 = 0.050 ± 0.026.