What does FRB light-curve variability tell us about the emission mechanism?

Abstract

A few fast radio bursts’ (FRBs) light-curves have exhibited large intrinsic modulations of their flux on extremely short (tr ∼ 10μs) time scales, compared to pulse durations (tFRB ∼ 1ms). Light-curve variability timescales, the small ratio of rise time of the flux to pulse duration, and the spectro-temporal correlations in the data constrain the compactness of the source and the mechanism responsible for the powerful radio emission. The constraints are strongest when radiation is produced far (≳ 1010cm) from the compact object. We describe different physical set-ups that can account for the observed tr/tFRB ≪ 1 despite having large emission radii. The result is either a significant reduction in the radio production efficiency or distinct light-curves features that could be searched for in observed data. For the same class of models, we also show that due to high-latitude emission, if a flux f1(ν1) is observed at t1 then at a lower frequency ν2 < ν1 the flux should be at least (ν2/ν1)2f1 at a slightly later time (t2 = t1ν1/ν2) independent of the duration and spectrum of the emission in the comoving frame. These features can be tested, once light-curve modulations due to scintillation are accounted for. We provide the timescales and coherence bandwidths of the latter for a range of possibilities regarding the physical screens and the scintillation regime. Finally, if future highly resolved FRB light-curves are shown to have intrinsic variability extending down to ∼μs timescales, this will provide strong evidence in favor of magnetospheric models.

The physics of fast radio bursts

“…Scintillation phenomena that have been observed in a few cases like FRB 150807 and FRB 121102 match the predictions well [46,47]. Note that the spectral structure of the recent Galactic FRB 200428 could be potentially explained by scintillation [48], however, scintillation is unlikely to explain all the variability features seen in this burst [49]. Besides, the refractive interstellar scintillation (RISS) may occur on a larger scale r ref = r 2 F /r diff , of which the scintillation timescale t scint ≃ r ref /V could be much longer than the FRB burst duration [6].…”

The 60 pc Environment of FRB 20180916B

“…Observations of ∼3-4 µs burst structure place tight constraints on the size of the emitting region (Nimmo et al 2020); in the absence of special relativistic effects, this corresponds to a ∼1 km region, given light-crossingtime arguments. In the context of magnetar models, this short timescale, along with the range of observed temporal timescales spanning 3-4 orders of magnitude from ∼µs−ms (Nimmo et al 2020), is more naturally explained in terms of emission generated relatively close to the neutron star (Beniamini & Kumar 2020;Lyutikov et al 2020;Lu et al 2020) -as opposed to much further out in a relativistic shock (Margalit & Metzger 2018;Beloborodov 2017).…”

Periodic Fast Radio Bursts from Luminous X-ray Binaries

“…Any process that taps into the energy of the ejecta shell by crossing it at close to the speed of light in the co-moving frame (e.g., a reverse shock or magnetic reconnection) will complete once the shell reaches a radius r FRB ∼ Γ 2 Δ. An FRB emitted coincident with this energy release will arrive at an external observer over a timescale  r FRB /2Γ 2 ∼ t f , i.e., roughly matching the original activity time of the central engine 11 (see Beniamini & Kumar 2020 for more details). This follows a similar faithful mapping between engine and prompt emission activity in gamma-ray bursts (e.g., Kumar & Zhang 2015).…”