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 ☉ (1 σ). 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 σ), 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 1 σ ranges for the photometric period (1.5430 days ≤ P ph ≤ 1.5440 days), K -velocity (350.2 ≤ K 2 ≤ 368.6 km s -1 ), inclination (67 0 ≤ i ≤ 77 4), mass ratio ( Q ≥ 12.0), and orbital separation (11.55 R ☉ ≤ a ≤ 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 1 σ range of 8.36 M ☉ ≤ M 1 ≤ 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 1 σ ranges of 1.5432 days ≤ P ph ≤ 1.5441 days for the photometric period, 352.2 ≤ K 2 ≤ 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 12.35 R ☉ ≤ a ≤ 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 1 σ range of 9.68 M ☉ ≤ M 1 ≤ 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.