Dynamical evidence for a black hole in the microquasar XTE J1550-564

Optical spectroscopic observations of the companion star ( type G8 IV to K4 III) in the microquasar system XTE J1550-564 reveal a radial velocity curve with a best-fitting spectroscopic period of P-sp = 1.552 +/- 0.010 days and a semiamplitude of K-2 = 349 +/- 12 km s(-1). The optical mass function is f(M) = 6.86 +/- 0.71 M-. (1sigma). We tentatively measure the rotational velocity of the companion star to be V-rot sin i = 90 +/- 10 km s(-1), which when taken at face value implies a mass ratio of Q = M-1/M-2 = 6.6(-1.6)(+2.5) (1 sigma),using the above value of K-2. We derive constraints on the binary parameters from simultaneous modeling of the ellipsoidal light and radial velocity curves. We find 1sigma ranges for the photometric period (1.5430 days less than or equal to P-ph less than or equal to 1.5440 days), K-velocity (350.2 less than or equal to K-2 less than or equal to 368.6 km s(-1)), inclination (67.degrees0 less than or equal to i less than or equal to 77.degrees4), mass ratio (Q greater than or equal to 12.0), and orbital separation (11.55 R-. less than or equal to a less than or equal to 12.50 R-.). Given these geometrical constraints, we find that the most likely value of the mass of the compact object is 9.41 M-. with a 1sigma range of 8.36 M-. less than or equal to M-1 less than or equal to 10.76 M-.. If we apply our tentative value of V-rot sin i = 90 +/- 10 km s(-1) as an additional constraint in the ellipsoidal modeling,we find 1sigma ranges of 1.5432 days less than or equal to P-ph less than or equal to 1.5441 days for the photometric period, 352.2 less than or equal to K-2 less than or equal to 370.1 km s(-1) for the K-velocity, 70.degrees8 less than or equal to i less than or equal to 75.degrees4 for the inclination, 6.7 less than or equal to Q less than or equal to 11.0 for the mass ratio, and 12.35 R-. less than or equal to a less than or equal to 13.22 R-. for the orbital separation. These geometrical constraints imply the most likely value of the mass of the compact object of 10.56 M-. with a 1sigma range of 9.68 M-. less than or equal to M-1 less than or equal to 11.58 M-.. In either case the mass of the compact object is well above the maximum mass of a stable neutron star, and we therefore conclude that XTE J1550-564 contains a black hole.