Abstract

Spectroscopic observations of the fast X-ray transient and superluminal jet source SAX J1819.3-2525 (V4641 Sgr) reveal a best-fitting period of P_spect=2.81678+/-0.00056 days and a semiamplitude of K₂=211.0+/-3.1 km s^-1. The optical mass function is f(M)=2.74+/-0.12 M_solar. We find a photometric period of P_photo=2.81730+/-0.00001 days using a light curve measured from photographic plates. The folded light curve resembles an ellipsoidal light curve with two maxima of roughly equal height and two minima of unequal depth per orbital cycle. The secondary star is a late B-type star that has evolved off the main sequence. Using a moderate resolution spectrum (R=7000) we measure T_eff=10500+/-200 K, logg=3.5+/-0.1, and V_rotsini=123+/-4 km s^-1 (1 σ errors). Assuming synchronous rotation, our measured value of the projected rotational velocity implies a mass ratio of Q≡M₁/M₂=1.50+/-0.08 (1 σ). The lack of X-ray eclipses implies an upper limit to the inclination of i<=70.7d. On the other hand, the large amplitude of the folded light curve (~0.5 mag) implies a large inclination (i>~60^deg). Using the above mass function, mass ratio, and inclination range, the mass of the compact object is in the range 8.73<=M₁<=11.70 M_solar and the mass of the secondary star is in the range 5.49<=M₂<=8.14 M_solar (90% confidence). The mass of the compact object is well above the maximum mass of a stable neutron star, and we conclude that V4641 Sgr contains a black hole. The B-star secondary is by far the most massive, the hottest, and the most luminous secondary of the dynamically confirmed black hole X-ray transients. We find that the α-process elements nitrogen, oxygen, calcium, magnesium, and titanium may be overabundant in the secondary star by factors of 2-10 times with respect to the Sun. Finally, assuming E(B-V)=0.32+/-0.10, we find a distance 7.40<=d<=12.31 kpc (90% confidence). This large distance and the high proper motions observed for the radio counterpart make V4641 Sgr possibly the most superluminal galactic source known, with an apparent expansion velocity of >~9.5c and a bulk Lorentz factor of Γ>~9.5, assuming that the jets were ejected during one of the bright X-ray flares observed with the Rossi X-ray Timing Explorer. Based on observations collected at the European Southern Observatory, Chile (program 65.H-0360) 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.