Orbital parameters for the soft X-ray transient 4U 1543-47: Evidence for a black hole

Spectroscopic observations of the soft X-ray transient 4U 1543-47 reveal a radial velocity curve with a period of P = 1.123 +/- 0.008 days and a semiamplitude of K-2 = 124 +/- 4 km s(-1). The resulting mass function is f(M) = 0.22 +/- 0.02 M.. We classify the secondary star as A2 V, in agreement with previous work, and measure T-eff = 9000 +/- 500 K and E(B-V) = 0.50 +/- 0.05 from fits to synthetic spectra. We derive a distance of d = 9.1 +/- 1.1 kpc if the secondary is on the main sequence. We see no emission lines from the accretion disk, and the measured fractions of disk light in the B- and V-bands are 10% and 21%, respectively. The V and I light curves exhibit two Waves per orbital cycle with amplitudes approximate to 0.08 mag. We model the light curves as ellipsoidal variations in the secondary star and derive extreme inclination limits of 20 degrees less than or equal to i less than or equal to 40 degrees. The formal 3 sigma limits on the inclination from a simultaneous fit to the V and I light curves are 24 degrees less than or equal to i less than or equal to 36 degrees for a mass ratio Q = M-1/M-2 greater than or equal to 1. However, there are systematic effects in the data that the model does not account for, so the above constraints should be treated with caution. We argue that the secondary star is still on the main sequence with the mass transfer being driven by expansion due to normal main sequence evolution. If the secondary star has a mass near the main sequence values for early A stars (2.3 less than or equal to M-2 less than or equal to 2.6 M.), then the best fits for the 3 sigma inclination range (24 degrees less than or equal to i less than or equal to 36 degrees) and the 3 sigma mass function range (0.16 less than or equal to f(M) less than or equal to 0.28 M.) imply a primary mass in the range 2.7 less than or equal to M-1 less than or equal to 7.5 M.. The mass of the compact object in 4U 1543-47 is likely to be in excess of approximate to 3 M., which is widely regarded as the maximum mass of a stable neutron star. Thus we conclude that 4U 1543-47 most likely contains a black hole.