A Black Hole in the Superluminal Source Sax J1819.3-2525 (v4641 Sgr)1

Spectroscopic observations of the fast X-ray transient and superluminal jet source SAX J1819.3-2525 (V4641 Sgr) reveal a best fitting period of Pspect = 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 . We find a photometric period of Pphoto = 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 which has evolved off the main sequence. Using a moderate resolution spectrum (R = 7000) we measure Teff = 10500 200 K, logg = 3:5 0:1, and Vrot sin i = 123 4 km s−1 (1 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 ). The lack of X-ray eclipses implies an upper limit to the inclination of i 70:7. On the other hand, the large amplitude of the folded light curve ( 0:5 mag) implies a large inclination (i & 60). 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 and the mass of the secondary star is in the range 5:49 M2 8:14 M (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 -process elements nitrogen, oxygen, calcium, magnesium, and titanium may be overabundant in the secondary star by factors of two to 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 & 9:5, assuming the jets were ejected during during one of the bright X-ray flares observed with the Rossi X-ray Timing Explorer. Subject headings: binaries: spectroscopic — black hole physics — stars: individual (V4641 Sgr) — X-rays: stars 1 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 2 Also at Astronomical Institute, Utrecht University 1