Binary coalescences as sources of Ultra-High Energy Cosmic Rays

TL;DR

This paper proposes that binary black hole and neutron star mergers can produce ultra-high energy cosmic rays through charged particle collisions near black holes embedded in strong magnetic fields, potentially explaining UHECR origins.

Contribution

It introduces a mechanism for UHECR production in binary coalescences involving magnetic fields and charged particles, a novel link between gravitational wave sources and cosmic rays.

Findings

High-energy particle collisions occur in BH-NS and BH-BH binaries with magnetic fields >10^{10} G.

Millions of ultra-high energy collisions may happen before binary mergers.

Binary coalescences could be significant sources of UHECRs, linking GW and cosmic ray observations.

Abstract

Binary coalescences are known sources of gravitational waves (GWs) and they encompass combinations of black holes (BHs) and neutron stars (NSs). Here we show that when BHs are embedded in magnetic fields ($B$s) larger than approximately $10^{10}$ G, charged particles colliding around their event horizons can easily have center-of-mass energies in the range of ultra-high energies ($\gtrsim 10^{18}$ eV) and escape. Such B-embedding and high-energy particles can take place in BH-NS binaries, or even in BH-BH binaries with one of the BHs being charged (with charge-to-mass ratios as small as $10^{-5}$, which do not change GW waveforms) and having a residual accretion disk. Ultra-high center-of-mass energies for particle collisions arise for basically any rotation parameter of the BH when $B \gtrsim 10^{10}$ G, meaning that it should be a common aspect in binaries, especially in BH-NS ones given the natural presence of a $B$ onto the BH and charged particles due to the NS's magnetosphere. We estimate that up to millions of ultra-high center-of-mass collisions may happen before the merger of BH-BH and BH-NS binaries. Thus, binary coalescences can also be efficient sources of ultra-high energy cosmic rays (UHECRs) and constraints to NS/BH parameters would be possible if UHECRs are detected along with GWs.