The Mass of the Black Hole in XTE J1118+480
  We present contemporaneous, broadband, near-infrared spectroscopy (0.9-2.45
micron) and H-band photometry of the black hole X-ray binary, XTE J1118+480. We
determined the fractional dilution of the NIR ellipsoidal light curves of the
donor star from other emission sources in the system by comparing the
absorption features in the spectrum with field stars of known spectral type. We
constrained the donor star spectral type to K7 V - M1 V and determined that the
donor star contributed 54+/-27% of the H-band flux at the epoch of our
observations. This result underscores the conclusion that the donor star cannot
be assumed to be the only NIR emission source in quiescent X-ray binaries. The
H-band light curve shows a double-humped asymmetric modulation with extra flux
at orbital phase 0.75. The light curve was fit with a donor star model light
curve, taking into account a constant second flux component based on the
dilution analysis. We also fit models that included emission from the donor
star, a constant component from the accretion disk, and a phase-variable
component from the bright spot where the mass accretion stream impacts the
disk. These simple models with reasonable estimates for the component physical
parameters can fully account for the observed light curve, including the extra
emission at phase 0.75. From our fits, we constrained the binary inclination to
68 <= i <= 79 deg. This leads to a black hole mass of 6.9 <= M_BH <= 8.2 solar
masses. Long-term variations in the NIR light curve shape in XTE J1118+480 are
similar to those seen in other X-ray binaries and demonstrate the presence of
continued activity and variability in these systems even when in full
quiescence.
