Greybody factors of charged black holes with axion hair

TL;DR

This paper investigates how axion hair on charged black holes affects greybody factors, revealing potential observational signatures for axions and magnetic monopoles through deviations from classical black hole models.

Contribution

It introduces the impact of axion hair on greybody factors of charged black holes, highlighting the dependence on electric-magnetic charge ratios and breaking of duality.

Findings

Deviations from Reissner-Nordstrom black holes are significant at higher multipole moments.

Greybody factors can serve as probes for axion presence and magnetic monopoles.

The effective potential depends on the electric to magnetic charge ratio.

Abstract

We study the greybody factor of charged hairy black holes (BHs) that arise due to the presence of an axion coupled to the electromagnetic field. Specifically, we consider spin-0 and spin-1 test particles propagating in the background of BHs with axion hair, where the spacetime geometry is modified compared to that of the Reissner-Nordstrom (RN) BH. In contrast to the RN solution, with a given total BH charge, the effective potential for test particles depends on the ratio of electric to magnetic charges. In other words, charged BHs with axion hair breaks the electric-magnetic duality present in the RN solution. We compute the transmission coefficient of test particles plunging into the charged hairy BH and find that the deviation from the RN solution is particularly evident for higher multipole moments. Precise measurements of greybody factors can thus serve as probes for the possible existence of axions coupled to the electromagnetic field, as well as potential signatures of magnetic monopoles.