Dynamical Evidence for a Black Hole in the Microquasar XTE J1550−564

Abstract

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+/-0.71M solar (1 σ). We tentatively measure the rotational velocity of the companion star to be V rot sini=90+/-10 km s -1, which when taken at face value implies a mass ratio of Q≡M 1 /M 2 =6.6 +2.5 -1.6 (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 (1.5430days ph 2 -1), inclination (67.0d =12.0), and orbital separation (11.55R solar solar). Given these geometrical constraints, we find that the most likely value of the mass of the compact object is 9.41 M solar with a 1 σ range of 8.36M solar 1 solar.If we apply our tentative value of V rot sini=90+/-10 km s -1 as an additional constraint in the ellipsoidal modeling, we find 1 σ ranges of 1.5432days ph 2 -1 for the K-velocity, 70.8d solar solar for the orbital separation. These geometrical constraints imply the most likely value of the mass of the compact object of 10.56 M solar with a 1 σ range of 9.68M solar 1 solar. 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. Based on observations collected at the European Southern Observatory, Chile (program 67.D-0229), at the Magellan Walter Baade Telescope at the Las Campanas Observatory, Chile, and the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.