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 = 69.50d +/- 0.08d 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 +/- 0.22 M_⊙). The secondary star's mass is M₂ = 2.34 +/- 0.12 M_⊙. The position of the secondary on the Hertzsprung-Russell diagram is consistent with that of a ~2.3 M_⊙ star that 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^-1), and mass function [f(M) = 3.24 +/- 0.09 M_⊙]. Based on the new spectra of the source and spectra of several MK spectral type standards, we classify the secondary star as F3 IV-F6 IV. Evolutionary models suggest an average mass transfer rate for such a system of \Mdot₂=3.4×10^-9 M_⊙ yr^-1 = 2.16 × 10¹⁷ g s^-1, which is much larger than the average mass transfer rates implied in the other six transient black hole systems but is still barely below the critical mass transfer rate required for stability.