A simple relationship for the spectro-temporal structure of bursts from FRB 121102

Abstract

We consider a simple dynamical and relativistic model to explain the spectro-temporal structure often displayed by repeating fast radio bursts (FRBs). We show how this model can account for the downward frequency drift in a sequence of sub-bursts of increasing arrival time (the “sad trombone” effect) and their tendency for exhibiting a reduced pulse width with increasing frequency of observation. Most importantly, this model also predicts a systematic inverse relationship between the (steeper) slope of the frequency drift observed within a single sub-burst and its temporal duration. Using already published data for FRB 121102 we find and verify the relationship predicted by this model. We therefore argue that the overall behaviour observed for this object as a function of frequency is consistent with an underlying narrow-band emission process, where the wide-band nature of the measured FRB spectrum is due to relativistic motions. Although this scenario and the simple dynamics we consider could be applied to other theories, they are well-suited for a model based upon Dicke’s superradiance as the physical process responsible for FRB radiation in this and similar sources.

Evidence of a shared spectro-temporal law between sources of repeating fast radio bursts

“…In (16) we introduced a simple dynamical model where a triggering source (e.g., a pulsar or magnetar; see (19)) is located directly behind an FRB source, as seen by an observer. The regions from which FRB signals are emanating are assumed to be moving towards (or away from) the observer, potentially at relativistic speeds.…”

“…For example, one sequence of subbursts detected toward this source reveals an upward central frequency drift with increasing arrival time (a "happy trombone" effect). A few models have anticipated such a possibility for the spectra of FRBs (14,16,17). In particular, in (16) we proposed a simple dynamical relativistic model where a descending or an ascending central frequency drift for a sequence of sub-bursts can be explained based on the intrinsic properties of the corresponding FRB source.…”

“…A few models have anticipated such a possibility for the spectra of FRBs (14,16,17). In particular, in (16) we proposed a simple dynamical relativistic model where a descending or an ascending central frequency drift for a sequence of sub-bursts can be explained based on the intrinsic properties of the corresponding FRB source. But more importantly, our model also predicts that a steeper frequency drift should be present within individual sub-bursts (henceforth the sub-burst drift) where the slope of the FRB signal as displayed in a dynamical spectrum (i.e., frequency vs. time) obeys a simple law scaling inversely with the temporal duration of the sub-burst.…”

Chromatic periodic activity down to 120 megahertz in a fast radio burst

“…As these two FRBs reside in significantly different environments, the behavior may be common across FRBs. The frequencydependence and consistent sign of the drifting phenomenon will likely offer clues to the FRB emission mechanism 29,55,56 .…”

An analysis of the time-frequency structure of several bursts from FRB 121102 detected with MeerKAT

“…Hessels et al 2019). Intrinsic mechanisms include pulsar-like sparking and cosmic-comb models (Wang et al 2019(Wang et al , 2020, radius-to-frequency mapping in pulsars (Lyutikov 2019), the decreasing Lorentz factor of electrons near the surface of a neutron star (Gu et al 2020), decelerating blastwaves from the flare ejecta of young magnetars (Metzger et al 2019), or the (potentially relativistic) motion of highly collimated FRB emission with respect to an observer (Rajabi et al 2020). Propagation effects include scintillation (Simard & Pen 2018;Simard & Ravi 2020) and plasma lensing (Cordes et al 2017).…”