Fast Radio Bursts from Magnetars Born in Binary Neutron Star Mergers and Accretion Induced Collapse

Recently born magnetars are promising candidates for the engines powering fast radio bursts (FRBs). The focus thus far has been placed on millisecond magnetars born in rare core-collapse explosions, motivated by the star-forming dwarf host galaxy of the repeating FRB 121102, which is remarkably similar to the hosts of superluminous supernovae and long gamma-ray bursts. However, long-lived magnetars may also be created in binary neutron star (BNS) mergers, in the small subset of cases with a sufficiently low total mass for the remnant to avoid collapse to a black hole, or in the accretion-induced collapse (AIC) of a white dwarf. A BNS or AIC FRB channel will be characterized by distinct host galaxy and spatial offset distributions which we show are consistent with the recently reported FRB 180924, localized by the Australian Square Kilometre Array Pathfinder to a massive quiescent host galaxy with an offset of about 1.4 effective radii. Using models calibrated to FRB 121102, we make predictions for the dispersion measure, rotation measure, and persistent radio emission from magnetar FRB sources born in BNS mergers or AIC, and show these are consistent with upper limits from FRB 180924. Depending on the rate of AIC, and the fraction of BNS mergers leaving long-lived stable magnetars, the birth rate of repeating FRB sources associated with older stellar populations could be comparable to that of the core-collapse channel. We also discuss potential differences in the repetition properties of these channels, as a result of differences in the characteristic masses and magnetic fields of the magnetars.