Magnetic field generation from primordial black hole distributions

TL;DR

This paper develops a statistical framework to estimate magnetic field fluctuations from primordial black hole populations and assesses the viability of two seed field generation mechanisms against observational constraints.

Contribution

It introduces a novel formalism for calculating magnetic seed fields from PBH distributions considering different mass functions and mechanisms.

Findings

Monochromatic PBH distributions cannot produce sufficient seed fields.

Extended PBH distributions also fail to generate adequate seed fields via the Biermann battery.

Magnetic monopole accretion could produce seed fields compatible with current constraints.

Abstract

We introduce a statistical method for estimating magnetic field fluctuations generated from primordial black hole (PBH) populations. To that end, we consider monochromatic and extended Press-Schechter PBH mass functions, such that each constituent is capable of producing its own magnetic field due to some given physical mechanism. Assuming linear correlation between magnetic field fluctuations and matter over-densities, our estimates depend on the mass function, the physical field generation mechanism by each PBH constituent, and the characteristic PBH separation. After computing the power spectrum of magnetic field fluctuations, we apply our formalism to study the plausibility that two particular field generation mechanisms could have given rise to the expected seed fields according to current observational constraints. The first mechanism is the Biermann battery and the second one is due to the accretion of magnetic monopoles at PBH formation, constituting magnetic PBHs. Our results show that, for monochromatic distributions, it does not seem to be possible to generate sufficiently intense seed fields in any of the two field generation mechanisms. For extended distributions, it is also not possible to generate the required seed field by only assuming a Biermann battery mechanism. In fact, we report an average seed field by this mechanism of about 10^{-47} G, at z = 20. For the case of magnetic monopoles we instead assume that the seed values from the literature are achieved and calculate the necessary number density of monopoles. In this case we obtain values that are below the upper limits from current constraints.