Improved Parameters for the Black Hole Binary System X-Ray Nova MUSCAE 1991

We have continued to observe the black hole binary X-ray Nova Muscae 1991 (GS 1124 - 683, GU Mus, hereafter XN Mus 91) in quiescence over the last 3 years. These observations have yielded refined values of the orbital period (P = 0d.4326058 ±0d.0000031) and the semiamplitude of the absorption line radial velocity curve (K2 = 406±7 km s⁻¹) The implied mass function is f(M) = 3.01±0.15 Msun, which represents the minimum mass of the compact primary star. A reasonable lower limit on the mass of the primary, based on the lack of observed X-ray and optical eclipses and a previous measurement of the binary mass ratio, is M₁ > 4.4 Msun; this mass exceeds the maximum mass of a stable neutron star, reinforcing the argument that XN Mus 91 contains a black hole. The phase-averaged spectrum (Doppler corrected to account for the orbital motion of the secondary) shows strong absorption lines consistent with a dwarf main-sequence star, with an MK type of K3 V-KS V. The disk spectrum, obtained by subtracting the spectrum of a KS V template star from the averaged spectrum, shows strong Balmer emission lines, as well as emission lines of He I and Fe II, which are superimposed on a relatively flat continuum. Ellipsoidal modeling of the light curves indicates that the inclination is in the range 54° ≤ i ≤ 65°. Using the mass function and our previous estimate of the mass ratio, the primary mass is then in the range 5.0 ≤M₁ ≤ 7.5 Msun.