Free-free background radiation from accreting primordial black holes

TL;DR

This paper investigates the free-free background radiation produced by accreting primordial black holes (PBHs), analyzing how PBH mass and abundance influence the radiation, and concludes that current emissions are too weak for observational constraints.

Contribution

It provides a detailed analysis of free-free emission from PBHs, including effects of dark matter halos and PBH abundance, highlighting challenges in constraining PBHs via this radiation.

Findings

Free-free emission depends on PBH mass and abundance.

Growth of dark matter halos around PBHs enhances emission.

Emission is too weak to constrain PBH abundance with current observations.

Abstract

Baryonic gas falling onto a primordial black hole (PBH) emits photons via the free-free process. These photons can contribute the diffuse free-free background radiation in the frequency range of the cosmic microwave background radiation (CMB). We show that the intensity of the free-free background radiation from PBHs depends on the mass and abundance of PBHs. In particular, considering the growth of a dark matter (DM) halo around a PBH by non-PBH DM particles strongly enhances the free-free background radiation. Large PBH fraction increase the signal of the free-free emission. However, large PBH fraction also can heat the IGM gas and, accordingly, suppresses the accretion rate. As a result, the free-free emission decreases when the PBH fraction is larger than 0.1. We find that the free-free emission from PBHs in the CMB and radio frequency is much lower than the CMB blackbody spectrum and the observed free-free emission component in the background radiation. Therefore, it is difficult to obtain the constraint from the free-free emission observation. However further theoretical understanding and observation on the free-free emission from cosmological origin is helpful to study the PBH abundance with the stellar mass.