Possible Electromagnetic Phenomena during the Coalescence of Neutron Star–Black Hole Binary Systems

Abstract

Possible models for the generation of electromagnetic (EM) radiation during the coalescence of neutron star-black hole binaries are considered. The mass of the remnant disk around the black hole during the coalescence of neutron stars and black holes is calculated by taking into account the equation of state for neutron stars and the rotation of the binary components before the coalescence. The parameters of binary systems before the coalescence (the mass ratio, the component rotation, the neutron star magnetic field) are calculated by the population synthesis method. The derived mass of the remnant disk around the black hole after the coalescence is used to estimate the kinetic energy of the relativistic jet launched by the Blandford-Znajek mechanism. A disk mass of more than $\sim 0.05 M_\odot$ required for the formation of short gamma-ray bursts is shown to be obtained in no more than 1-10\% of the coalescences (depending on the equation of state). Less efficient common envelopes (a large parameter $\alpha_{CE}$) lead to a noticeably larger percentage of events with astrophysically interesting EM energy release. For binaries with a large mass ratio, in which a magnetized neutron star is not subjected to tidal disruption before the coalescence, the possibility of the formation of an electrically charged rotating black hole (Wald charge) is considered and estimates of the maximum EM power released by such a black hole after the coalescence are made. The conversion of the emitted gravitational waves into electromagnetic ones in the relativistic lepton plasma generated in coalescing pulsar-black hole binaries at the pre-coalescence stage is discussed.