Bounds on PBH fraction in a stimulated axion/ALP decay scenario

TL;DR

This paper explores how stimulated decay of axions or ALPs around black holes can be used to set stringent limits on the abundance of primordial black holes, based on observational flux constraints.

Contribution

It introduces a novel method to constrain PBH fractions using stimulated axion/ALP decay effects near black holes and derives new upper bounds on PBH abundance.

Findings

PBH fraction must be less than 10^{-17} for certain mass ranges

Stimulated decay produces observable radiation exceeding current limits

Constraints are derived for PBHs with masses between 10^{-19} and 10^{-7} solar masses

Abstract

In this work, we show that stimulated decay of axions or axion-like particles (ALP) in black hole superradiance is an efficient way to find and hunt primordial black holes (PBH). When de Broglie's wavelength of the axion/ ALP is comparable or larger than the black hole horizon radius, a large population of them accumulates in the surroundings of the black hole. When these axions or ALPs couple to photons, the bosonic cloud decays into radiation that contributes to the X-ray, visible light, and radio wave background flux that can exceed current observational limits measured at Earth. If the masses are in the interval of $10^{-3}\mathrm{eV}<\mu <1\mathrm{eV}$, to be consistent with current observations of microwave background light, we found that the fraction of primordial black holes should be smaller than $f_{PBH} < 10^{-17}$ for primordial black holes with masses within $10^{-19} M_{\odot}<M_{BH}<10^{-7} M_\odot$.