FRB Periodicity: Mild Pulsars in Tight O/B-star Binaries

Periodicities observed in two fast radio burst (FRB) sources (16 days in FRB 180916.J0158+65 and 160 days in FRB 121102) are consistent with that of tight, stellar-mass binary systems. In the case of FRB 180916.J0158+65 the primary is an early OB-type star with the mass-loss rate $\dot{M}\sim {10}^{-8}\mbox{--}{10}^{-7}{M}_{\odot }$ yr−1, and the secondary is a neutron star. The observed periodicity is not intrinsic to the FRB's source, but is due to the orbital phase-dependent modulation of the absorption conditions in the massive star's wind. The observed relatively narrow FRB activity window implies that the primary's wind dynamically dominates that of the pulsar, $\eta ={L}_{\mathrm{sd}}/(\dot{M}{v}_{w}c)\leqslant 1$, where Lsd is the pulsar spin-down, $\dot{M}$ is the primary's wind mass-loss rate, and vw is its velocity. The condition η ≤ 1 requires a mildly powerful pulsar with Lsd ≲ 1037 erg s−1. The observations are consistent with magnetically powered radio emission originating in the magnetospheres of young, strongly magnetized neutron stars, the classical magnetars.