Fast Radio Burst Trains from Magnetar Oscillations
  Quasi-periodic oscillations inferred during rare magnetar giant flare tails
were initially interpreted as torsional oscillations of the neutron star (NS)
crust, and have been more recently described as global core+crust
perturbations. Similar frequencies are also present in high signal-to-noise
magnetar short bursts. In magnetars, disturbances of the field are strongly
coupled to the NS crust regardless of the triggering mechanism of short bursts.
For low-altitude magnetospheric magnetar models of fast radio bursts (FRBs)
associated with magnetar short bursts, such as the low-twist model, crustal
oscillations may be associated with additional radio bursts in the encompassing
short burst event (as recently suggested for SGR 1935+2154). Given the large
extragalactic volume probed by wide-field radio transient facilities, this
offers the prospect of studying NS crusts leveraging samples far more numerous
than galactic high-energy magnetar bursts by studying statistics of sub-burst
structure or clustered trains of FRBs. We explore the prospects for
distinguishing NS equation of state models with increasingly larger future sets
of FRB observations. Lower $l$-number eigenmodes (corresponding to FRB time
intervals of $\sim5-50$ ms) are likely less susceptible than high-$l$ modes to
confusion by systematic effects associated with the NS crust physics, magnetic
field, and damping. They may be more promising in their utility, and also may
corroborate models where FRBs arise from mature magnetars. Future observational
characterization of such signals can also determine whether they can be
employed as cosmological "standard oscillators" to constrain redshift, or can
be used to constrain the mass of FRB-producing magnetars when reliable
redshifts are available.
