Magnetars in Binaries as the Engine of Actively Repeating Fast Radio Bursts

The association between FRB 20200428D and the Galactic magnetar SGR J1935+2154 makes magnetars the leading engine of cosmological fast radio bursts (FRBs). However, there is a list of puzzles for this magnetar-for-all-FRBs scenario: known Galactic magnetars are all isolated, and none of them are active repeaters; some cosmological repeaters have extremely high repetition rates but without any measurable spin-related periodicity; some show long-term periodic active windows; and some show diverse rotation measure (RM) evolution patterns, such as quasiperiodic fluctuations, sign reversals, and abrupt RM flares. Here we propose a unified theoretical paradigm for FRBs within the framework of the magnetar engine: most active repeating FRBs originate from magnetars in binary systems with nearly aligned rotation and magnetic axes, some of which have a triple-aligned geometry, i.e., an alignment with the orbital axis as well, whereas apparent nonrepeaters and inactive repeaters originate from magnetars in isolated systems or in binaries with a misaligned geometry. By studying various magnetar formation channels using population synthesis, we show that a few percent of magnetars in the Universe can be in binary systems, most with a massive star companion and some with aligned geometry. We suggest that such binary systems can account for the rich phenomenology of active repeaters. We suggest that the existence of a companion helps to maintain the aligned geometry and that the companion may play an active role in triggering FRBs in an active repeater source.