Precessing flaring magnetar as a source of repeating FRB 180916.J0158+65

Abstract: Recently, CHIME detected periodicity in the bursting rate of the repeating FRB 180916.J0158+65. In a popular class of models, the fast radio bursts (FRBs) are created by giant magnetic flares of a hyper-active magnetar driven by fast ambipolar diffusion in the core. We point out that in this scenario the magnetar is expected to precess freely with a period of hours to weeks. The internal magnetic field $B\sim 10^{16}$G deforms the star, and magnetic flares induce sudden changes in magnetic stresses. The resulting torques and displacements of the principal axes of inertia are capable of pumping a significant amplitude of precession. The anisotropy of the flaring FRB activity, combined with precession, implies a strong periodic modulation of the visible bursting rate. The ultra-strong field invoked in the magnetar model provides: (1) energy for the frequent giant flares, (2) the high rate of ambipolar diffusion, releasing the magnetic energy on the timescale $\sim 10^9$s, (3) the core temperature $T\approx 10^9$K, likely above the critical temperature for neutron superfluidity, (4) strong magnetospheric torques, which efficiently spin down the star, and (5) deformation with ellipticity $\epsilon> 10^{-6}$, much greater than the rotational deformation. These conditions result in a precession with negligible viscous damping, and can explain the observed 16 day period in FRB 180916.J0158+65.

Precessing Flaring Magnetar as a Source of Repeating FRB~180916.J0158+65

The paper explores the potential role of precessing flaring magnetars in the generation of repeating fast radio bursts (FRB), with a specific focus on FRB~180916.J0158+65. The authors propose that the periodicity observed in FRB activity is due to the free precession of a magnetar driven by hyper-active giant magnetic flares. They provide a comprehensive analysis of the physical conditions and mechanisms that could facilitate such precession, suggesting it as a plausible explanation for the 16-day cycle detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME).

Core Model

The pivotal concept in this model centers around the magnetar's intense internal magnetic fields, estimated at $B_{\rm int} \sim 10^{16}$ G, which are said to crucially deform the star and induce fluctuations in magnetic stresses. These internal changes lead to precession stimulating periodic modulations of visible burst rates, as hypothesized for FRB~180916.J0158+65.

Highlights of the Model's Features:

1. Energy Mechanisms: Ultra-strong magnetic fields provide sufficient energy for frequent giant flares that contribute to FRB activity.
2. Ambipolar Diffusion: The fast ambipolar diffusion in the magnetar's core occurs over a timescale of approximately $10^9$ seconds. This process releases magnetic energy, raises the core temperature, and influences precession dynamics.
3. Heat and Temperature Dynamics: The internal temperature of the magnetar is postulated to surpass the critical temperature for neutron superfluidity, influencing its precessional behavior without significant damping from viscosity.
4. Ellipticity: Magnetically induced deformation results in notable ellipticity ($\epsilon \sim 10^{-6}$), impacting the precession mechanics.
5. Rotation and Spindown: The interaction of magnetospheric torques and gravitational forces contributes to the noticeable spindown and precession of the star.

Implications and Speculations

The research suggests that the precession model could provide insights into the periodic modulation of FRB rates observed by CHIME, indicating potential for predicting future variations. If validated, this model might also facilitate understanding of similar periodicities found in other repeating FRBs (e.g., FRB 121102) suggesting variable magnetar activity. Furthermore, observing and measuring changes in the precession period can serve as an empirical test of the model as the star continues to spin down.

Future Investigations in Magnetar Dynamics and FRBs

Making reference to future implications, the study opens paths for comprehensive observational and theoretical work to refine the understanding of FRB production mechanisms. It emphasizes the need for advanced simulations of magnetar dynamics, especially targeting aspects like magnetic relaxation, torque interactions, and realistic 3D interpretations of magnetospheric flare behaviors. Furthermore, monitoring FRBs over extended periods may provide empirical data that either supports or challenges the precession hypothesis.

To conclude, this paper demonstrates a significant stride towards elucidating the enigmatic behaviors of repeating FRBs through the lens of magnetar physics, offering a technically detailed, albeit speculative, framework that encourages further exploration and validation.