Optical Observations of GRO J1655-40 in Quiescence I: A Precise Mass for the Black Hole Primary

We report photometric and spectroscopic observations of the black hole binary GRO J1655-40 in complete quiescence. In contrast to the 1995 photometry, the light curves from 1996 are almost completely dominated by ellipsoidal modulations from the secondary star. Model fits to the light curves, which take into account the temperature profile of the accretion disk and eclipse effects, yield an inclination of i = 69.50±0.08 deg and a mass ratio of Q = M1/M2 = 2.99 ± 0.08. The precision of our determinations of i and Q allow us to determine the black hole mass to an accuracy of ≈ 4% (M1 = 7.02± 0.22M⊙). The secondary star’s mass is M2 = 2.34 ± 0.12M⊙. The position of the secondary on the Hertzsprung-Russell diagram is consistent with that of a ≈ 2.3M⊙ star which has evolved off the main sequence and is halfway to the start of the giant branch. Using the new spectra we present an improved value of the spectroscopic period (P = 2. 62157±0. 00015), radial velocity semiamplitude (K = 228.2±2.2 km s), and mass function (f(M) = 3.24 ± 0.09M⊙). Based on the new spectra of the source and spectra of several MK spectral type standards, we classify the secondary star as F3IV to F6IV. Evolutionary models suggest an average mass transfer rate for such a system of Ṁ2 = 3.4× 10 −9 M⊙ yr −1 = 2.16× 10 g s, which is much larger than the average mass transfer rates implied in the other six transient black hole systems, but still barely below the critical mass transfer rate required for stability. Subject headings: binaries: spectroscopic — black hole physics — X-rays: stars — stars: individual (GRO J1655-40) Visiting Astronomer at Cerro Tololo Inter-American Observatory (CTIO), which is operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. Present Address: Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802 National Young Investigator