Constraining Electromagnetic Signals from Black Holes with Hair

TL;DR

This paper investigates electromagnetic signals from 'hairy' black holes during mergers, constraining their properties through gamma-ray observations and analyzing potential signals like GW150914, to understand black hole electromagnetic emissions.

Contribution

It introduces a model-independent framework to constrain electromagnetic emissions from hairy black holes during mergers using gravitational wave and gamma-ray data.

Findings

Upper bounds on electromagnetic energy fraction: $oldsymbol{ ext{epsilon}<10^{-5}-10^{-4}}$ for 10-50 solar mass black holes.

Detection of a gamma-ray burst following GW150914 is consistent with electromagnetic emission from a hairy black hole with $oldsymbol{ ext{epsilon} ext{~}10^{-7}-10^{-6}}$.

Abstract

We constrain a broad class of "hairy" black hole models capable of directly sourcing electromagnetic radiation during a binary black hole merger. This signal is generic and model-independent since it is characterized by the black hole mass ($M$) and the fraction of that mass released as radiation ($\epsilon$). For field energy densities surpassing the Schwinger limit, this mechanism triggers pair-production to produce a gamma-ray burst. By cross-referencing gravitational wave events with gamma-ray observations, we place upper bounds of $\epsilon<10^{-5}-10^{-4}$ for $10-50$ $M_\odot$ black holes depending on the black hole mass. We discuss the weak detection of a gamma-ray burst following GW150914 and show that this event is consistent with rapid electromagnetic emission directly from a "hairy" black hole with $\epsilon\sim10^{-7}-10^{-6}$. Below the Schwinger limit, ambient charged particles are rapidly accelerated to nearly the speed of light by the strong electromagnetic field. For 1-50 $M_\odot$ black holes and $\epsilon$ ranging from $10^{-20}$ to $10^{-7}$, the typical proton energies are $\sim20$ GeV-20 TeV and electron energies are $\sim0.01-10$ GeV. At these energies, cosmic ray protons and electrons quickly diffuse into the Milky Way's background magnetic field, making it difficult to identify a point source producing them. Overall, constraining $\epsilon$ in this less energetic regime becomes difficult and future constraints may need to consider specific models of "hairy" black holes.