Gravitational self-lensing of Fast Radio Bursts in neutron star magnetospheres: II. Applications to strong repeaters and the CHIME population

Abstract

Paper I in this series introduced a model in which seed radio bursts produced by a hotspot anchored in the magnetosphere of a highly-magnetic neutron star (NS) are greatly amplified by strong gravitational self-lensing and thus give rise to Fast Radio Bursts (FRBs). Key features of the FRB population such as the observed dichotomy between repeating and non-repeating sources, their large luminosities and the high-energy power-law distribution of their bursts naturally arise in the model from the amplification dependence on the relative orientation of the rotation axis with respect to the hotspot and the line of sight. Here we compare the model predictions with Five-hundred-meter Aperture Spherical radio Telescope (FAST) data from repeaters and with the general population of FRBs. We find that the burst energy distribution from FRB 20121102A can be explained by assuming two antipodal hotspots in the NS magnetosphere, both producing seed bursts with the same log-normal energy distribution. This scenario implies a well-aligned system geometry, with the rotation axis, line of sight, and hotspot sites separated by $\lesssim 2$°. Similar constraints are found for FRB 20201124A and FRB 20220912A, and weaker ones for FRB 20190520B, owing to its smaller burst sample. We also show that precession of the NS rotation axis can explain the time evolution of the burst energy distribution from FRB 20121102A as well as its temporary disappearance. In application to a cosmological population of randomly-oriented sources the model predicts distance and fluence distributions of FRBs in good agreement with those from a completeness-selected subsample of the first CHIME/FRB catalogue, provided the energy distribution of seed bursts spans a range of ${\sim10^{35}-10^{38}}$ erg.