Blazar Sheath Illumination of the Outer Molecular Torus: A Resolution of
  the Seed Photon Problem for the far-GeV Blazar flares
  Recent multi-wavelength work led by the Boston University blazar group (e.g.,
Marscher et al. 2010) strongly suggests that a fraction of the blazar flares
seen by the Fermi Large Area Telescope (LAT) take place a few to several pc
away from the central engine. However, at such distances from the central
engine, there is no adequate external photon field to provide the seed photons
required for producing the observed GeV emission under leptonic inverse Compton
(IC) models. A possible solution is a spine-sheath geometry for the emitting
region (MacDonald et al. 2015, but see Nalewajko et al. 2014). Here we use the
current view of the molecular torus (e.g., Elitzur, 2012; Netzer 2015) in which
the torus extends a few pc beyond the dust sublimation radius with dust clouds
distributed with a declining density for decreasing polar angle. We show that
for a spine-sheath blazar jet embedded in the torus, the wide beaming pattern
of the synchrotron radiation of the relatively slow sheath will heat molecular
clouds whose subsequent IR radiation will be seen highly boosted in the spine
comoving frame, and that under reasonable conditions this photon field can
dominate over the sheath photons directly entering the spine. If the sheath is
sufficiently luminous it will sublimate the dust, and if the sheath synchrotron
radiation extends to optical-UV energies (as may happen during flares), this
will illuminate the sublimated dust clouds to produce emission lines that will
vary in unison with the optical-UV continuum, as has been very recently
reported for blazar CTA 102 (Jorstad et al. 2017).
