Periodicity in recurrent fast radio bursts and the origin of ultralong period magnetars

The recurrent fast radio burst FRB 180916 was recently shown to exhibit a 16-d period (with possible aliasing) in its bursting activity. Given magnetars as widely considered FRB sources, this period has been attributed to precession of the magnetar spin axis or the orbit of a binary companion. Here, we make the simpler connection to a rotational period, an idea observationally motivated by the 6.7-h period of the Galactic magnetar candidate, 1E 161348–5055. We explore three physical mechanisms that could lead to the creation of ultralong period magnetars: (i) enhanced spin-down due to episodic mass-loaded charged particle winds (e.g. as may accompany giant flares), (ii) angular momentum kicks from giant flares, and (iii) fallback leading to long-lasting accretion discs. We show that particle winds and fallback accretion can potentially lead to a sub-set of the magnetar population with ultralong periods, sufficiently long to accommodate FRB 180916 or 1E 161348–5055. If confirmed, such periods implicate magnetars in relatively mature states (ages 1−10 kyr) and which possessed large internal magnetic fields at birth B_int ≿ 10¹⁶ G. In the low-twist magnetar model for FRBs, such long period magnetars may dominate FRB production for repeaters at lower isotropic-equivalent energies and broaden the energy distribution beyond that expected for a canonical population of magnetars, which terminate their magnetic activity at shorter periods P ≾ 10 s.