Radio precursors of monster shocks: a mechanism for fast radio bursts from SGR 1935+2154

Abstract

Kilohertz perturbations in active magnetars evolve into monster radiative shocks at radii $r\sim 10^8$ cm. The shock generates X-rays and a semi-coherent radio precursor, which strongly interacts with the magnetospheric plasma ahead of the shock. We show that this interaction self-regulates the precursor emission and find its self-consistent frequency and luminosity. The precursor frequency falls in the GHz band and its production peaks when the shock expands to $r\approx 10^9$ cm. The resulting GHz burst has a sub-millisecond duration and energy ${\mathcal{E}}_{\mathrm{FRB}}\approx 10^{34}{\mathcal{E}}_{38}^{0.2}$ erg where $\mathcal{E}$ is the energy of the primary magnetosonic disturbance that launched the shock. As the GHz burst propagates to the light cylinder $R_{\mathrm{LC}}\sim 10^{10}$ cm, it faces a threat of being absorbed by the magnetosphere. The burst escapes if the local plasma density at $R_{\mathrm{LC}}$ is $\sim 30$ times lower than typically expected for active magnetars, so distant observers need some luck to see the radio burst. The shock X-rays follow the radio waves with a millisecond delay. Shocks from kilohertz disturbances with energies $\mathcal{E}\sim 10^{38}$ erg generate X-ray and radio bursts similar to the activity detected in SGR 1935+2154.

21 pages, 6 figures, submitted to ApJ