# Inspiral of a Spinning Black Hole--Magnetized Neutron Star Binary: Increasing Charge and Electromagnetic Emission

## TL;DR

This paper studies electromagnetic emissions from a spinning black hole and magnetized neutron star binary during inspiral, highlighting how increasing charge and magnetic interactions could produce detectable short X-ray signals.

## Contribution

It introduces a model for EM emission from a charged, spinning black hole--neutron star binary, emphasizing the role of magnetic dipole radiation and reconnection during inspiral.

## Key findings

- Magnetic dipole radiation and reconnection are significant in late inspiral stages.
- Rapid black hole spin and strong neutron star magnetization enhance EM signals.
- Potential for detectable short X-ray transients from such mergers.

## Abstract

The mergers of black hole (BH)--neutron star (NS) binaries have been one of the most interesting topics in astrophysics, because such events have been thought to possibly produce multimessenger signals including gravitational waves and broadband electromagnetic (EM) waves. In this paper, we investigate EM emission from the inspiral of a binary composed of a spinning BH and a magnetized NS. Observationally, the BH is usually more massive than $\sim7M_\odot$ and the NS has a mass $\simeq 1.4M_\odot$. During the inspiral of such a binary, the BH will accumulate more and more charges based on the charging scenario of Wald, even though the BH will eventually swallow the NS whole inevitably. We calculate the emission luminosities and energies through three energy dissipation mechanisms: magnetic dipole radiation, electric dipole radiation, and magnetic reconnection. We show that magnetic dipole radiation due to the spin of the increasingly charged BH and magnetic reconnection in between the BH and the NS could be most significant at the final inspiral stage. We find that if the BH is rapidly rotating and the NS is strongly magnetized, these mechanisms would lead to a detectable EM signal (e.g., a short-duration X-ray transient).