The Astrophysical Journal Letters, Volume 709, pp. L163-L167 (2010)
Abstract
We present results from an intensive VERITAS monitoring campaign of the high-frequency peaked BL Lac (HBL) 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 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 very high energy (VHE) spectra considering their redshifts. When correcting the measured spectra for absorption by the extragalactic background light (EBL), 1ES 1218+304 and a number of other blazars are found to have differential photon indices Gamma 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.
