Magnetospheres of black hole-neutron star binaries

TL;DR

This study uses force-free simulations to explore the magnetosphere properties of black hole-neutron star binaries, revealing significant electromagnetic emissions, magnetic reconnections, and potential radio signals detectable within 200 Mpc.

Contribution

It provides the first detailed force-free simulation analysis of black hole-neutron star binary magnetospheres across various configurations, highlighting electromagnetic luminosities and reconnection phenomena.

Findings

Electromagnetic luminosities up to 10^{46} erg/s depending on magnetic field strength.

Development of long-distance spiral current sheets and magnetic reconnections near the black hole horizon.

Potential detectable radio signals within 200 Mpc for upcoming observatories.

Abstract

We perform force-free simulations for a neutron star orbiting a black hole, aiming at clarifying the main magnetosphere properties of such binaries towards their innermost stable circular orbits. Several configurations are explored, varying the orbital separation, the individual spins and misalignment angle among the magnetic and orbital axes. We find significant electromagnetic luminosities, $L\sim 10^{42-46} \, [B_{\rm pole}/ 10^{12}{\rm G}]^2 \, {\rm erg/s}$ (depending on the specific setting), primarily powered by the orbital kinetic energy, being about one order of magnitude higher than those expected from unipolar induction. The systems typically develop current sheets that extend to long distances following a spiral arm structure. The intense curvature of the black hole produces extreme bending on a particular set of magnetic field lines as it moves along the orbit, leading to magnetic reconnections in the vicinity of the horizon. For the most symmetric scenario (aligned cases), these reconnection events can release large-scale plasmoids that carry the majority of the Poynting fluxes. On the other hand, for misaligned cases, a larger fraction of the luminosity is instead carried outwards by large-amplitude Alfv{\'e}n waves disturbances. We estimate possible precursor electromagnetic emissions based on our numerical solutions, finding radio signals as the most promising candidates to be detectable within distances of $\lesssim 200$\,Mpc by forthcoming facilities like the Square Kilometer Array.