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 K₂ = 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 ≈ 0.08 mag. We model the light curves as ellipsoidal variations in the secondary star and derive extreme inclination limits of 20° ≤ i ≤ 40°. The formal 3 σ limits on the inclination from a simultaneous fit to the V and I light curves are 24° ≤ i ≤ 36° for a mass ratio Q ≡ M₁/M₂ ≥ 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 ≤ M₂ ≤ 2.6 M_☉), then the best fits for the 3 σ inclination range (24° ≤ i ≤ 36°) and the 3 σ mass function range (0.16 ≤ f(M) ≤ 0.28 M_☉) imply a primary mass in the range 2.7 ≤ M₁ ≤ 7.5 M_☉. The mass of the compact object in 4U 1543-47 is likely to be in excess of ≈ 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.