LAUSR

A few fast radio bursts' (FRBs) light-curves have exhibited large intrinsic modulations of their flux on extremely short (t_r ∼ 10μs) time scales, compared to pulse durations (t_(FRB) ∼ 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 t_r/t_(FRB) ≪ 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.