What does FRB light-curve variability tell us about the emission mechanism?. (arXiv:2007.07265v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Beniamini_P/0/1/0/all/0/1">Paz Beniamini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kumar_P/0/1/0/all/0/1">Pawan Kumar</a>
A few fast radio bursts’ (FRBs) light-curves have exhibited large intrinsic
modulations of their flux on extremely short ($t_{rm r}sim 10mu$s) time
scales, compared to pulse durations ($t_{rm FRB}sim1$ms). 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
($gtrsim 10^{10}$cm) from the compact object. We describe different physical
set-ups that can account for the observed $t_{rm r}/t_{rm FRB}ll 1$ 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 $f_1(nu_1)$ is observed at $t_1$ then
at a lower frequency $nu_2<nu_1$ the flux should be at least
$(nu_2/nu_1)^2f_1$ at a slightly later time ($t_2=t_1nu_1/nu_2$)
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 $sim mu$s timescales,
this will provide strong evidence in favor of magnetospheric models.
A few fast radio bursts’ (FRBs) light-curves have exhibited large intrinsic
modulations of their flux on extremely short ($t_{rm r}sim 10mu$s) time
scales, compared to pulse durations ($t_{rm FRB}sim1$ms). 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
($gtrsim 10^{10}$cm) from the compact object. We describe different physical
set-ups that can account for the observed $t_{rm r}/t_{rm FRB}ll 1$ 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 $f_1(nu_1)$ is observed at $t_1$ then
at a lower frequency $nu_2<nu_1$ the flux should be at least
$(nu_2/nu_1)^2f_1$ at a slightly later time ($t_2=t_1nu_1/nu_2$)
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 $sim mu$s timescales,
this will provide strong evidence in favor of magnetospheric models.
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