Monster shocks and transient black hole pulsars from black hole–neutron star mergers

A majority of black hole-neutron star mergers are expected to leave little or no tidally disrupted remnant material. Magnetic field of the neutron star will transition onto the black hole while leaving a clean magnetosphere during and after the merger. I will present our latest simulations of a black hole-neutron star merger with consistently tracking the common magnetosphere of the binary in a nearly force-free limit, uncovering two types of transients with magnetospheric origins. Fast magnetosonic waves, excited during the late inspiral and merger phases, nonlinearly steepen into radiative monster shocks. The post-merger magnetosphere rearranges into a split monopole geometry shortly after the merger, and the spin of the remnant black hole powers a transient `black hole pulsar' state. For inclined magnetic field configurations, a rotating split monopole field launches a pulsar-like striped wind. Our analysis suggests this black hole pulsar to emit a blueing gamma-ray transient. The exponential decay of the magnetic field at the horizon happens on resistive timescales, where we find that the ringdown of the remnant black hole drives a more rapid shedding of magnetic fields in certain cases.