We present an improved dynamical model of the X-ray binary and microquasar XTE J1550-564 based on new moderate-resolution optical spectroscopy and near-infrared photometry obtained with the 6.5m Magellan Telescopes at Las Campanas Observatory .
Twelve spectra of the source were obtained using the Magellan Echellette Spectrograph between 2008 May 6 and August 4 .
In addition , several hundred images of the field were obtained between 2006 May and 2009 July in the J and K _ { S } filters using the PANIC camera .
The agreement between the 2006/2007 and 2008 J and K _ { S } light curves is not perfect , and the differences can plausibly be attributed to a hot spot on the accretion disk during the 2006/2007 observations .
By combining our new radial velocity measurements with previous measurements obtained 2001 May at the 8.2m VLT and with light curves , we find an orbital period of P = 1.5420333 \pm 0.0000024 days and a radial velocity semiamplitude of K _ { 2 } = 363.14 \pm 5.97 km s ^ { -1 } , which together imply an optical mass function of f ( M ) = 7.65 \pm 0.38 M _ { \odot } .
We find that the projected rotational velocity of the secondary star is 55 \pm 5 km s ^ { -1 } , which implies a very extreme mass ratio of Q \equiv M / M _ { 2 } \approx 30 .
Using a model of a Roche lobe-filling star and an azimuthally symmetric accretion disk , we fit simultaneously optical light curves from 2001 , near-infrared light curves from 2008 and all of the radial velocity measurements to derive system parameters .
We find an inclination of 74.7 \pm 3.8 ^ { \circ } and component masses of M _ { 2 } = 0.30 \pm 0.07 M _ { \odot } and M = 9.10 \pm 0.61 M _ { \odot } for the secondary star and black hole , respectively .
We note that these results depend on the assumption that in 2008 , the disk does not have a hot spot , and that the fraction of light contributed by the accretion disk did not change between the spectroscopic and photometric observations .
By considering two measured values of the disk fraction and by modeling various combinations of NIR and optical light curves , we show that our adopted black hole mass is probably not seriously in error , where the black hole mass ranges between M = 8.91 \pm 1.10 M _ { \odot } and M = 13.94 \pm 1.64 M _ { \odot } .
The radius of the secondary star for the adopted model is 1.75 \pm 0.12 R _ { \odot } .
Using this radius , the average K _ { S } magnitude , and an extinction of A _ { K } = 0.507 \pm 0.050 mag , we find a distance of 4.38 ^ { +0.58 } _ { -0.41 } kpc , which is in good agreement with a recent distance estimate based on HI absorption lines .