Distinctive electromagnetic signals caused by gravitational waves (of sub-solar mass primordial black hole binary mergers) interacting with galactic magnetic fields

TL;DR

This paper proposes that gravitational waves from sub-solar mass primordial black hole mergers interact with galactic magnetic fields to produce detectable electromagnetic signals, offering a new method to identify primordial black holes.

Contribution

It introduces a novel mechanism for electromagnetic signals generated by PBH mergers interacting with galactic magnetic fields, which could help distinguish PBHs from astrophysical black holes.

Findings

Perturbed EMWs have constant strength around 10^{-12} Tesla at Earth for within-galaxy sources.

EMW strength depends on source distance for extragalactic PBH mergers, potentially detectable.

Same mass ratio yields identical EMW strength regardless of PBH mass or binary distance.

Abstract

As a candidate of dark matter, and related to many fundamental physics issues, the primordial black hole (PBH) is a crucial topic. However, so far the existence of PBHs is still not confirmed, and currently running GW detectors are still not able to distinguish them from the normal astrophysical BHs. In this article, we propose that the GWs (of PBH binary mergers) could interact with the very widespread background galactic magnetic fields in the Milky way, to produce the perturbed electromagnetic waves (EMWs) with unique characteristics of frequencies, waveforms, spectra and polarizations. In order to be distinguished from astrophysical black holes, only the PBHs with masses less than the solarmass are considered here, and their binary mergers will radiate GWs in frequencies much higher above the plasma frequency of interstellar medium (ISM), so corresponding perturbed EMWs (in the same frequencies to such GWs) can propagate through the ISM until the Earth. Our estimations show that, for the sub-solar mass PBH binary mergers within the Milky way (disk or halo), the strengths of the perturbed EMWs turn into constant levels around 10^{-12} Tesla (for magnetic components) and around 10^{-10}Watt m^{-2} (for energy flux densities) at the Earth, generally for all cases of different PBH masses (and not dependent on the distance of sources), and the same mass ratio of the PBH binary gives the same strength (at the Earth) of perturbed EMWs despite different PBH masses (GW frequencies) or binary distances. Differently, for the PBH binary mergers outside the Milky way, the perturbed EMWs at Earth have lower strengths (and depend on the distance of sources), but for some part of distance range, they would also be detectable. If such EM signals and special EM counterpart of GWs from PBHs could be detected by space- or land-based EMWs detectors, it may provide direct evidence of the PBHs.