Black Hole Spin-Orbit Misalignment in Galactic X-ray Binaries
  In black hole X-ray binaries, a misalignment between the spin axis of the
black hole and the orbital angular momentum can occur during the supernova
explosion that forms the compact object. In this letter we present population
synthesis models of Galactic black hole X-ray binaries, and study the
probability density function of the misalignment angle, and its dependence on
our model parameters. In our modeling, we also take into account the evolution
of misalignment angle due to accretion of material onto the black hole during
the X-ray binary phase. The major factor that sets the misalignment angle for
X-ray binaries is the natal kick that the black hole may receive at its
formation. However, large kicks tend to disrupt binaries, while small kicks
allow the formation of XRBs and naturally select systems with small
misalignment angles. Our calculations predict that the majority (>67%) of
Galactic field BH XRBs have rather small (>10 degrees) misalignment angles,
while some systems may reach misalignment angles as high as ~90 degrees and
even higher. This results is robust among all population synthesis models. The
assumption of small small misalignment angles is extensively used to
observationally estimate black hole spin magnitudes, and for the first time we
are able to confirm this assumption using detailed population synthesis
calculations.
