Constraints on Pulsed Emission Model for Repeating FRB 121102

Abstract

Recent localization of the repeating Fast Radio Burst (FRB) 121102 revealed the distance of its host galaxy and luminosities of bursts. Only from the luminosities and durations of 30 recursive bursts observed in 2012-2016, we investigated constraints on the young neutron star (NS) model of giant radio pulse (GRP) like emission; i.e., (a) the FRB intrinsic luminosity is supported by the spin-down energy, and (b) the FRB duration is shorter than the NS rotation period. We found that the allowed region of NS parameters is very limited, the dipole magnetic field \(\sim10^{13}\) G and the initial rotational period \(\lesssim3\) ms. These constraints do not depend on the properties of the possible persistent radio counterpart and the circumburst environments such as surrounding materials. The NS age in this model is \(\lesssim3.3\) yr, not inconsistent with the observed time span of the repeating burst activities (3-4 yr). Anisotropy of the pulsed emission and the braking index of the NS do not affect this age constraint. Since the NS is older than the spin-down timescale, this hypothetical GRP-like model expects a rapid radio flux decay (\(\lesssim1\) Jy within a few years) as the spin-down luminosity decreases. The continuous monitoring will give a hint of discrimination of this model. If no flux evolution will be seen, we need to consider an alternative model, e.g., the magnetically powered flare.