Jet Formation in General-Relativistic, Magnetized Bondi-Hoyle-Lyttleton Accretion

A black hole (BH) travelling through a uniform, gaseous medium is described by Bondi-Hoyle-Lyttleton (BHL) accretion. If the wind is magnetized, then the black hole can produce relativistic outflows. We performed seven three-dimensional simulations of BHL accretion onto rotating black holes using the general-relativistic magnetohydrodynamics code H-AMR, where we mainly varied the strength of a background magnetic field that threads the gaseous wind. We found that the wind continuously drags magnetic flux onto the BH, which accumulates near the event horizon and causes the flow to become magnetically arrested. Depending on the strength of the background magnetic field, the BHs can sometimes launch relativistic jets with high enough power to escape the inner accretion flow, become bent by the wind, 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. Whenever a jet is active, the jet power saturates to values at the Blandford-Znajek jet power. 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 more realistic descriptions of BHs propagating through magnetized media, which can approximately describe a wide variety of astrophysical environments.