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

TL;DR

This work argues that all FRBs may be powered by magnetars, and that the subset of actively repeating FRBs predominantly originate from magnetars in binaries with nearly aligned spin and magnetic axes ($χ \sim 0$). A population-synthesis framework shows that a few percent of magnetars are expected in binaries, with companion types largely being massive MS stars, enabling a triple-aligned geometry that naturally explains diverse RM evolution (irregular fluctuations, reversals, and jets of RM flares) and the very high burst rates observed. The authors discuss how propagation of FRB waves is facilitated in aligned binaries, and propose companion-induced triggers via wind-perturbed Alfvén dynamics as a mechanism for high repetition rates. The framework provides a unified explanation for several FRB phenomenologies and highlights observational tests that could distinguish binary-aligned magnetars from isolated or misaligned systems.

Abstract

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 quasi-periodic fluctuations, sign reversals, and abrupt RM flares. Here we propose a unified theoretical framework for FRBs within the framework of magnetar engine: Most active repeating FRBs originate from magnetars in binary systems with nearly aligned rotation and magnetic axes, some of which with a triple-aligned geometry, i.e. with an alignment with the orbital axis as well; whereas apparent non-repeaters and inactive repeaters originate from magnetars in isolated systems or in binaries with a misaligned geometry. By studying various magnetar formation channels using population syntheses, 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.

Figures (5)

• Figure 1: Various magnetar formation channels through single and binary evolution. Magnetars with a black-colored magnetic field background are isolated, while those with a light-colored background are in binary systems. The figure also shows the magnetar's spin axis (white) and magnetic axis (orange, when mis-aligned from the spin axis), with the curved orange arrow indicating the flip evolution of the magnetic axis. The dashed lines indicate that, in some cases, the spin and magnetic axes of magnetars differ from what the flowchart shows.
• Figure 2: Fractions of magnetars from different formation channels based on the fiducial model. Different bars represent different magnetar formation fractions among all NSs.
• Figure 3: Fractions of different magnetar companion types, including MS star subtypes.
• Figure 4: Illustration of three possible scenarios explaining the observed RM evolution in different FRBs. In each case, the left panel presents a cartoon showing different phases of a binary system and the surrounding environment of magnetars, with blue solid lines representing the orbits, white solid lines representing magnetic field lines, and white dashed lines representing the line of sight (LOS). The right panel shows the corresponding RM evolution over time (Case I: Irregular RM evolution of FRB 20201124A xuh22; Case II: RM reversals of FRB 20190520B Anna2023; Case III: RM flare and RM reversals in FRB 20220529 Li2025).
• Figure A1: A schematic illustration of the physical configuration surrounding an accreting magnetar. The central sphere represents the magnetar, surrounded by an orange accretion disk. Three unit vectors are shown to indicate key orientations: the $\bm{m}$ vector (light blue) represents the magnetic axis (z-axis), the $\bm{s}$ vector (dark blue) denotes the magnetar's spin axis, and the $\bm{d}$ vector (orange) is normal to the accretion disk plane (accretion disc axis).