Herschel PACS and SPIRE observations of blazar PKS 1510-089: a case for
  two blazar zones
  We present the results of observations of blazar PKS 1510-089 with the
Herschel Space Observatory PACS and SPIRE instruments, together with
multiwavelength data from Fermi/LAT, Swift, SMARTS and SMA. The source was
found in a quiet state, and its far-infrared spectrum is consistent with a
power-law with a spectral index of alpha ~ 0.7. Our Herschel observations were
preceded by two 'orphan' gamma-ray flares. The near-infrared data reveal the
high-energy cut-off in the main synchrotron component, which cannot be
associated with the main gamma-ray component in a one-zone leptonic model. This
is because in such a model the luminosity ratio of the External-Compton and
synchrotron components is tightly related to the frequency ratio of these
components, and in this particular case an unrealistically high energy density
of the external radiation would be implied. Therefore, we consider a
well-constrained two-zone blazar model to interpret the entire dataset. In this
framework, the observed infrared emission is associated with the synchrotron
component produced in the hot-dust region at the supra-pc scale, while the
gamma-ray emission is associated with the External-Compton component produced
in the broad-line region at the sub-pc scale. In addition, the optical/UV
emission is associated with the accretion disk thermal emission, with the
accretion disk corona likely contributing to the X-ray emission.
