VERITAS Observations of the BL Lac Object 1ES 1218+304

TL;DR

This paper reports the detection of very-high-energy gamma-ray emission from the BL Lac object 1ES 1218+304, providing spectral measurements and constraints on its intrinsic energy spectrum and the extragalactic background light.

Contribution

First detection of VHE gamma-ray emission from 1ES 1218+304 with spectral analysis and EBL constraints.

Findings

Gamma-ray spectrum well described by a power law with index 3.08.

No evidence for VHE flux variability during observations.

Intrinsic spectrum likely harder than Gamma = 1.86.

Abstract

The VERITAS collaboration reports the detection of very-high-energy (VHE) gamma-ray emission from the high-frequency-peaked BL Lac object 1ES 1218+304 located at a redshift of z=0.182. A gamma-ray signal was detected with high statistical significance for the observations taken during several months in the 2006-2007 observing season. The photon spectrum between ~160 GeV and ~1.8 TeV is well described by a power law with an index of Gamma = 3.08 +/- 0.34(stat) +/- 0.2(sys). The integral flux above 200 GeV corresponds to ~6% of that of the Crab Nebula. The light curve does not show any evidence for VHE flux variability. Using lower limits on the density of the extragalactic background light (EBL) in the near-IR to mid-IR we are able to limit the range of intrinsic energy spectra for 1ES 1218+304. We show that the intrinsic photon spectrum is harder than a power law with an index of Gamma = 2.32 +/- 0.37. When including constraints from the spectra of 1ES 1101-232 and 1ES 0229+200, the spectrum of 1ES 1218+304 is likely to be harder than Gamma = 1.86 +/- 0.37.