Mergers of Maximally Charged Primordial Black Holes

TL;DR

This paper explores the astrophysical signatures of near-extremal primordial black holes, showing how their mergers and evaporation could produce observable gamma-ray and gravitational wave signals, constraining their abundance.

Contribution

It introduces the potential observability of gamma-ray flux and gravitational wave background from charged primordial black holes, providing new bounds and detection prospects.

Findings

Diffuse photon flux from evaporating black holes can be observed for masses below 10^{12} g.

Upper bounds on primordial black hole abundance are derived from gamma-ray observations.

Gravitational wave background from black hole formation may be detectable by future observatories.

Abstract

Near-extremal primordial black holes stable over cosmological timescales may constitute a significant fraction of the dark matter. Due to their charge the coalescence rate of such black holes is enhanced inside clusters and the non-extremal merger remnants are prone to Hawking evaporation. We demonstrate that if these clusters of near-extremal holes contain a sufficient number of members to survive up to low redshift, the hard photons from continued evaporation begin to dominate the high energy diffuse background. We find that the diffuse photon flux can be observed for a monochromatic mass spectrum of holes lighter than about $10^{12}\rm g$. We place upper bounds on their abundance respecting the current bounds set by gamma ray telescopes. Furthermore, the gravitational wave background induced at the epoch of primordial black hole formation may be detectable by future planned and proposed ground-based and space-borne gravitational wave observatories operating in the mHz to kHz frequency range and can be an important tool for studying light charged primordial black holes over masses in the range $\rm 10^{12}g - 10^{19}g$.