Discovery of Variability in the Very High Energy Gamma-Ray Emission of
  1ES 1218+304 with VERITAS
  We present results from an intensive VERITAS monitoring campaign of the
high-frequency peaked BL Lac object 1ES 1218+304 in 2008/2009. Although 1ES
1218+304 was detected previously by MAGIC and VERITAS at a persistent level of
~6% of the Crab Nebula flux, the new VERITAS data reveal a prominent flare
reaching ~20% of the Crab. While very high energy (VHE) flares are quite common
in many nearby blazars, the case of 1ES 1218+304 (redshift z = 0.182) is
particularly interesting since it belongs to a group of blazars that exhibit
unusually hard VHE spectra considering their redshifts. When correcting the
measured spectra for absorption by the extragalactic background light, 1ES
1218+304 and a number of other blazars are found to have differential photon
indices less than 1.5. The difficulty in modeling these hard spectral energy
distributions in blazar jets has led to a range of theoretical gamma-ray
emission scenarios, one of which is strongly constrained by these new VERITAS
observations. We consider the implications of the observed light curve of 1ES
1218+304, which shows day scale flux variations, for shock acceleration
scenarios in relativistic jets, and in particular for the viability of
kiloparsec-scale jet emission scenarios.
