Constraining the Charge of a Black Hole with Electromagnetic Radiation from a Black Hole-Neutron Star System

TL;DR

This paper proposes a method to constrain the electric charge of black holes in black hole-neutron star systems by analyzing electromagnetic emissions, providing upper limits on the charge-to-mass ratio based on observed data.

Contribution

It introduces a novel approach to estimate black hole charge using electromagnetic signals from charged BH-NS mergers, assuming dipole radiation and synchrotron emission mechanisms.

Findings

Maximum charge-to-mass ratio of BH is about 1.12×10⁻⁶ for GW200105

Maximum charge-to-mass ratio of BH is about 1.53×10⁻⁶ for GW200115

Electromagnetic emission efficiency varies across different bands, highest in X-ray

Abstract

Black hole-neutron star (BH-NS) mergers are expected to emit gravitational-wave (GW) and electromagnetic (EM) counterparts when the NS is tidally disrupted or plunges into the BH. Recently, GW 200105 and GW200115 were claimed as originating in BH-NS mergers, even GW 200105 remains in debate. Several optical source candidates are reported to possible associate with the two GW events, but not confirmed yet. In this work, we assume that the BH is charged (the NS is naturally charged) and try to constrain the charge of the BH by using the possible associated EM emission from the charged BH and NS system working in the inspiral regime. We adopt electric and magnetic dipole radiations for the binaries which power a Poynting-flux-dominated outflow to accelerate electrons. Then, it produces the observed EM radiation via synchrotron radiation. We find that the conversion efficiency in the X-ray band is much higher than that of the ultraviolet (UV), near-infrared, and radio bands. The estimated maximum charge-to-mass ratio (the charge for unit mass) of the BH is $1.12\times 10^{-6}$ and $1.53\times 10^{-6}$ esu for the binary systems of GW200105 and GW200115, respectively, if magnetic field strength $B_{p}\lesssim ~10^{16}$ G and period $P>~1$ ms for the NS spin.