AN IMPROVED DYNAMICAL MODEL FOR THE MICROQUASAR XTE J1550–564

Abstract

We present an improved dynamical model of the X-ray binary and microquasar\nXTE J1550-564 based on new moderate-resolution optical spectroscopy and\nnear-infrared photometry. By combining our new radial velocity measurements\nwith previous measurements obtained 2001 May at the 8.2m VLT and with light\ncurves, we find an orbital period of P=1.5420333 +/- 0.0000024 days and a\nradial velocity semiamplitude of K_2=363.14 +/- 5.97$ km/sec, which together\nimply an optical mass function of f(M)=7.65 +/- 0.38 solar masses. We find that\nthe projected rotational velocity of the secondary star is 55 +/- 5 km/sec,\nwhich implies a very extreme mass ratio of Q=M/M_2=30. Using a model of a Roche\nlobe-filling star and an azimuthally symmetric accretion disk, we fit\nsimultaneously optical light curves from 2001, near-infrared light curves from\n2008 and all of the radial velocity measurements to derive system parameters.\nWe find an inclination of 74.7 +/- 3.8 deg and component masses of M_2=0.30 +/-\n0.07 solar masses and M=9.10 +/- 0.61 solar masses for the secondary star and\nblack hole, respectively. The radius of the secondary star for the adopted\nmodel is 1.75 +/- 0.12 solar radii. Using this radius, the average K_S\nmagnitude, and an extinction of A_K=0.507 +/- 0.050 mag, we find a distance of\n4.38^{+0.58}_{-0.41} kpc, which is in good agreement with a recent distance\nestimate based on HI absorption lines (abstract shortened).\n