The Structure of the Strongly Lensed Gamma-ray Source B2 0218+35
  Strong gravitational lensing is a powerful tool for resolving the high energy
universe. We combine the temporal resolution of Fermi-LAT, the angular
resolution of radio telescopes, and the independently and precisely known
Hubble constant from Planck, to resolve the spatial origin of gamma-ray flares
in the strongly lensed source B2 0218+35. The lensing model achieves 1
milliarcsecond spatial resolution of the source at gamma-ray energies. The data
imply that the gamma-ray flaring sites are separate from the radio core: the
bright gamma-ray flare (MJD: 56160 - 56280) occurred $51\pm8$ pc from the 15
GHz radio core, toward the central engine. This displacement is significant at
the $\sim3\sigma$ level, and is limited primarily by the precision of the
Hubble constant. B2 0218+35 is the first source where the position of the
gamma-ray emitting region relative to the radio core can be resolved. We
discuss the potential of an ensemble of strongly lensed high energy sources for
elucidating the physics of distant variable sources based on data from Chandra
and SKA.
