Discovery of Variability in the Very High Energy Gamma-Ray Emission of 1ES 1218+304 with VERITAS

TL;DR

This paper reports on VERITAS observations of blazar 1ES 1218+304, revealing a significant gamma-ray flare and providing insights into its spectral properties and jet emission mechanisms.

Contribution

The study presents the first detailed variability analysis of 1ES 1218+304 at VHE, including a prominent flare and implications for gamma-ray emission models.

Findings

Detected a gamma-ray flare reaching ~20% of Crab flux.

Observed very hard VHE spectra with photon indices less than 1.5.

Provided constraints on jet emission scenarios based on variability data.

Abstract

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.