A unified picture of Galactic and cosmological fast radio bursts

Abstract

The discovery of a fast radio burst (FRB) in our galaxy associated with a magnetar (neutron star with strong magnetic field) has provided a critical piece of information to help us finally understand these enigmatic transients. We show that the volumetric rate of Galactic-FRB like events is consistent with the faint end of the cosmological FRB rate, and hence they most likely belong to the same class of transients. The Galactic FRB had an accompanying X-ray burst but many X-ray bursts from the same object had no radio counterpart. Their relative rates suggest that for every FRB there are roughly 102–103 X-ray bursts. The radio lightcurve of the Galactic FRB had two spikes separated by 30 ms in the 400-800 MHz frequency band. This is an important clue and highly constraining of the class of models where the radio emission is produced outside the light-cylinder of the magnetar. We suggest that magnetic disturbances close to the magnetar surface propagate to a distance of a few tens of neutron star radii where they damp and produce radio emission. The coincident hard X-ray spikes associated with the two FRB pulses seen in this burst and the flux ratio between the two frequency bands can be understood in this scenario. This model provides a unified picture for faint bursts like the Galactic FRB as well as the bright events seen at cosmological distances.

The physical mechanisms of fast radio bursts

“…The X-rays are about four orders of magnitude more energetic than the radio emission. This radio efficiency can be accommodated within both the pulsar-like 79 and GRB-like 138 models (the latter model predicted this efficiency). There exist two hard peaks in the lightcurve of the X-ray burst that have a separation of about 30 ms (refs.…”

“…Since the observed FRB duration is much longer than the bunch disperse timescale, this model requires a continuous generation of sparks throughout the duration of each FRB. The application of this mechanism to FRBs has been discussed by various authors 79,95,96,111,112,125 . One critical requirement is that there should be a strong electric field parallel to the local magnetic field (E ) in the FRB emission region 95 .…”

“…200428, by Occam's Razor 79,125,139 . A GRB-like model interprets the observed X-rays as the superposition of magnetospheric and shock emissions, with the latter component consistent with the relatively hard spectra during the two lightcurve peaks 138 .…”

“…One issue is that the number of the very active repeating FRB sources in the sky is too small to be consistent with the number of magnetars in the universe. This may require the existence of two populations of magnetars in a unified magnetar model for FRBs 79 : a large population of regular magnetars (similar to Galactic magnetars) that contribute to the bulk of FRBs and a small population of special magnetars (those born from extreme explosions such as GRBs and superluminous supernovae) that power active repeaters. The FRB host galaxy data seem to be consistent with such a picture 82 .…”

“…Cartoon pictures of the two general types of FRB radiation models. a, Pulsarlike models that invoke the magnetosphere of a compact object79 . b, GRB-like models that invoke relativistic shocks launched from a compact object101 .…”