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.
Highlights
The spectacular combined detection of gravitational waves (GW) and electromagnetic (EM) radiation from the binary neutron star (BNS) merger GW170817 [1,2,3] has initiated the new era of multimessenger astronomy
We extend our previous studies [28], paying a particular attention to the effect that the intense curvature in the vicinity of the black hole (BH) has on the magnetosphere: e.g., in the magnetic field topology, the induced electric currents and charge distributions, and in the obtained Poynting luminosity
We find that the extra gravitational framedragging effect contributes to further twisting of the magnetic field lines near the BH with the prograde spin, leading to stronger toroidal discontinuities that reconnect in a region closer to the BH horizon
Summary
The spectacular combined detection of gravitational waves (GW) and electromagnetic (EM) radiation from the binary neutron star (BNS) merger GW170817 [1,2,3] has initiated the new era of multimessenger astronomy. This single multimessenger observation led to important breakthroughs in our understanding of astrophysics and even fundamental physics. It confirmed that BNS mergers give rise to short gamma-ray bursts and the production of heavy elements via r-process; as well as it has provided constraints on proprieties of matter at nuclear densities, an estimation of the Hubble constant, and stringent tests of general relativity, among others (e.g., [4]).
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