Jet Formation in 3D GRMHD Simulations of Bondi-Hoyle-Lyttleton Accretion
  A black hole (BH) travelling through a uniform, gaseous medium is described
by Bondi-Hoyle-Lyttleton (BHL) accretion. If the medium is magnetized, then the
black hole can produce relativistic outflows. We performed the first 3D,
general-relativistic magnetohydrodynamics simulations of BHL accretion onto
rapidly rotating black holes using the code H-AMR, where we mainly varied the
strength of a background magnetic field that threads the medium. We found that
the ensuing accretion continuously drags to the BH the magnetic flux, which
accumulates near the event horizon until it becomes dynamically important.
Depending on the strength of the background magnetic field, the BHs can
sometimes launch relativistic jets with high enough power to drill out of the
inner accretion flow, become bent by the headwind, and escape to large
distances. While for stronger background magnetic fields the jets are
continuously powered, at weaker field strengths they are intermittent, turning
on and off depending on the fluctuating gas and magnetic flux distributions
near the event horizon. We find that our jets reach extremely high efficiencies
of $\sim100-300\%$, even in the absence of an accretion disk. We also
calculated the drag forces exerted by the gas onto to the BH, finding that the
presence of magnetic fields causes drag forces to be much less efficient than
in unmagnetized BHL accretion, and sometimes become negative, accelerating the
BH rather than slowing it down. Our results extend classical BHL accretion to
rotating BHs moving through magnetized media and demonstrate that accretion and
drag are significantly altered in this environment.
