Implications of a "Fast Radio Burst" from a Galactic Magnetar

Abstract

A luminous radio burst was recently detected in temporal coincidence with a hard X-ray flare from the Galactic magnetar SGR 1935+2154 with a time and frequency structure consistent with cosmological fast radio bursts (FRB) and a fluence within a factor of 10 of the least energetic extragalactic FRB previously detected. Although active magnetars are commonly invoked FRB sources, several distinct mechanisms have been proposed for generating the radio emission which make different predictions for the accompanying higher frequency radiation. We show that the properties of the coincident radio and X-ray flares from SGR 1935+2154, including their approximate simultaneity and relative fluence E radio /E X ∼ 10 −5 , as well as the duration and spectrum of the X-ray emission, are consistent with extant predictions for the synchrotron maser shock model. Rather than arising from the inner magnetosphere, the X-rays are generated by (incoherent) synchrotron radiation from thermal electrons heated at the same internal shocks which produce the coherent maser emission as ultra-relativistic flare ejecta collides with a slower particle outflow (e.g. as generated by earlier flaring activity) on a radial scale ∼ 10 11 cm. Although the rate of SGR 1935+2154-like bursts in the local universe is not sufficient to contribute appreciably to the extragalactic FRB rate, the inclusion of an additional population of more active magnetars with stronger magnetic fields than the Galactic population can explain both the FRB rate as well as the repeating fraction, however only if the population of active magnetars are born at a rate that is at least two-orders of magnitude lower than that of SGR 1935+2154-like magnetars. This may imply that the more active magnetar sources are not younger magnetars formed in a similar way to the Milky Way population (e.g. via ordinary supernovae), but instead through more exotic channels such as superluminous supernovae, accretion-induced collapse or neutron star mergers.

Plasmoid ejection by Alfvén waves and the fast radio bursts from SGR 1935+2154

“…The possibility of blast waves from SGR 1935+2154 is also discussed by Margalit et al (2020a), without specifying the mechanism of the low-energy explosion. They consider a blast wave hitting a slowly expanding baryonic cloud at r ∼ 10 11 cm, and find that it would generate radio and X-ray emission with E radio /E X ∼ 10 −5 , consistent with observations.…”

Pair separation in parallel electric field in magnetar magnetosphere and narrow spectra of fast radio bursts

“…Various FRB models can be divided into "far-way" models and "close-in" models based on the distance of the emission region from the neutron star (Lu et al 2020). The former suggests that the energy is dissipated via an outflow interacting with the ambient medium, and radio emission is produced by certain synchrotron maser mechanisms (Lyubarsky 2014;Waxman 2017;Beloborodov 2017Beloborodov , 2019Metzger et al 2019;Margalit et al 2020;Yu et al 2020). The latter suggests that the radio emission is from the magnetosphere of a neutron star (Pen & Connor 2015;Cordes & Wasserman 2016;Kumar et al 2017;Zhang 2017;Lu & Kumar 2018;Yang & Zhang 2018;Kumar & Bošnjak 2020;Wang et al 2020;Lyutikov & Popov 2020;Lu et al 2020;Dai 2020).…”

The FRB-SGR Connection

“…Any explanation of FRB as products of SGR must be consistent with "cosmological" FRB whose radio emission is several orders of magnitude more energetic than that of FRB 200428 and with the radio-to-gamma ray fluence ratio of FRB 200428 more than five orders of magnitude greater than that of SGR 1806−20. A number of theoretical interpretations have been suggested (Lu, Kumar & Zhang 2020;Lyutikov & Popov 2020;Margalit et al 2020;Wang, Xu & Chen 2020).…”