2002ApJ...568..845O

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 K₂=349±12 km.s^–1. The optical mass function is f(M)=6.86±0.71M_☉ (1 σ). We tentatively measure the rotational velocity of the companion star to be V_rotsini=90±10 km.s^–1, which when taken at face value implies a mass ratio of Q≡M₁/M₂=6.6^+2.5_–1.6 (1 σ), using the above value of K₂. We derive constraints on the binary parameters from simultaneous modeling of the ellipsoidal light and radial velocity curves. We find 1 σ ranges for the photometric period (1.5430days≤P_ph≤1.5440days), K-velocity (350.2≤K₂≤368.6 km.s^–1), inclination (67°.0≤i≤77°.4), mass ratio (Q≥12.0), and orbital separation (11.55R_☉≤a≤12.50R_☉). Given these geometrical constraints, we find that the most likely value of the mass of the compact object is 9.41 M_☉ with a 1 σ range of 8.36M_☉≤M₁≤10.76M_☉. If we apply our tentative value of V_rotsini=90±10 km.s^–1 as an additional constraint in the ellipsoidal modeling, we find 1 σ ranges of 1.5432days≤P_ph≤1.5441days for the photometric period, 352.2≤K₂≤370.1 km.s^–1 for the K-velocity, 70°.8≤i≤75°.4 for the inclination, 6.7≤Q≤11.0 for the mass ratio, and 12.35R_☉≤a≤13.22R_☉ 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 1 σ range of 9.68M_☉≤M₁≤11.58M_☉. 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.