A Black Hole in the Superluminal Source SAX J1819.3−2525 (V4641 Sgr)

(shortened) Spectroscopic observations of the fast X-ray transient andsuperluminal jet source SAX J1819.3-2525 (V4641 Sgr) reveal a best fittingperiod of P_spect=2.81678 +/- 0.00056 days and a semiamplitude of K_2=211.0 +/-3.1 km/sec. The optical mass function is f(M)=2.74 +/- 0.12 solar masses. Wefind a photometric period of P_photo=2.81730 +/- 0.00001 days using a lightcurve measured from photographic plates. The folded light curve resembles anellipsoidal light curve with two maxima of roughly equal height and two minimaof unequal depth per orbital cycle. The secondary star is a late B-type starwhich has evolved off the main sequence. Using a moderate resolution spectrum(R=7000) we measure T_eff=10500 +/- 200K, log(g)=3.5 +/- 0.1, andV_rot*sin(i)=123 +/- 4 km/sec (1 sigma errors). Assuming synchronous rotation,our measured value of the projected rotational velocity implies a mass ratio ofQ=M_1/M_2=1.50 +/- 0.08 (1sigma). The lack of X-ray eclipses implies an upperlimit to the inclination of i<70.7 deg. On the other hand, the large amplitudeof the folded light curve (about 0.5 mag) implies a large inclination (i>60deg). Using the above mass function, mass ratio, and inclination range, themass of the compact object is in the range 8.73 < M_1 < 11.70 solar masses andthe mass of the secondary star is in the range 5.49 < M_2 < 8.14 solar masses(90% confidence). The mass of the compact object is well above the maximum massof a stable neutron star and we conclude that V4641 Sgr contains a black hole.Comment: 17 pages, 12 figures, emulateapj5.sty, submitted to Ap