Ringdown signatures in the Ernst-Wild geometry: modelling Kerr black holes immersed in a magnetic field

Abstract

We analyse the quasinormal mode (QNM) spectrum for Kerr black holes surrounded by an asymptotically uniform magnetic field, modelled with the Ernst-Wild geometry. A perturbative expansion in both the rotation parameter a and the magnetic field B allows the analysis of perturbations with Kerr-like asymptotics well inside the Melvin radius, and we obtain the spectrum for a variety of scalar QNMs over a range of parameters using the continued fraction method. We then interpolate the low-lying mode spectrum to construct an Ernst-Wild template for the ringdown, and use the LIGO-Virgo-KAGRA analysis tool pyRing to assess the impact of the magnetosphere on the extraction of ringdown signatures from several observed binary black hole mergers.