  We derive stringent constraints on the persistent source associated with FRB
121102: Size $10^{17}$ cm $<R<10^{18}$ cm, age $<300$ yr, characteristic
electron energy $\varepsilon_e\sim0.3$ GeV, total energy $\sim10^{49}$ erg. The
hot radiating plasma is confined by a cold plasma of mass $M_c<0.03
(R/10^{17.5}{\rm cm})^4 M_\odot$. The source is nearly resolved, and may be
resolved by 10 GHz observations. The fact that $\varepsilon_e\sim m_p c^2$
suggests that the hot plasma was created by the ejection of a mildly
relativistic, $M\sim10^{-5} M_\odot$ shell, which propagated into an extended
ambient medium or collided with a pre-ejected shell of mass $M_c$. The inferred
plasma properties are inconsistent with typical "magnetar wind nebulae" model
predictions.
  We suggest a physical mechanism for the generation of FRBs (independent of
the persistent source model): Ejection from an underlying compact object,
$R_s\sim10^{6}$ cm, of highly relativistic shells, with energy $E_s=10^{41}$
erg and Lorentz factor $\gamma_s$~$10^3$, into a surrounding e-p plasma with
density $n\sim0.1/cm^3$ (consistent with that inferred for the plasma producing
the persistent emission associated with FRB 121102). Such shell ejections with
energy typical for FRB events lead to plasma conditions appropriate for strong
synchrotron maser emission at the GHz range, $\nu_{
coh.}\sim0.5(E/10^{41}erg)^{1/4}$ GHz. In this model, a significant fraction of
the deposited energy is converted to an FRB with duration $R_s/c$, accompanied
by ~10 MeV photons carrying less energy than the FRB.
  The inferred energy and mass associated with the source are low compared to
those of typical supernova ejecta. This may suggest some type of a "weak
stellar explosion", where a neutron star is formed with relatively low mass and
energy ejection. However, the current upper limit on R does not allow one to
rule out $M_c\sim1M_\odot$.
