Physical Constraints On Fast Radio Burst
  Fast Radio Bursts (FRBs) are isolated, $\ms$ radio pulses with dispersion
measure (DM) of order $10^3\DMunit$. Galactic candidates for the DM of high
latitude bursts detected at $\GHz$ frequencies are easily dismissed. DM from
bursts emitted in stellar coronas are limited by free-free absorption and those
from HII regions are bounded by the nondetection of associated free-free
emission at radio wavelengths. Thus, if astronomical, FRBs are probably
extra-galactic. FRB 110220 has a scattering tail of $\sim 5.6\pm 0.1 \ms$. If
the electron density fluctuations arise from a turbulent cascade, the
scattering is unlikely to be due to propagation through the diffuse
intergalactic plasma. A more plausible explanation is that this burst sits in
the central region of its host galaxy. Pulse durations of order $\ms$ constrain
the sizes of FRB sources implying high brightness temperatures that indicates
coherent emission. Electric fields near FRBs at cosmological distances would be
so strong that they could accelerate free electrons from rest to relativistic
energies in a single wave period.
