Differential energy spectrum of VHE photons above 160 GeV for 1ES 1218+304. The markers indicate measured data points and the continuous line is a power-law fit. Downward pointing arrows correspond to upper flux limits (99% probability) for bins with significances below two standard deviations. See Figure 4 (below) for more details.
1ES1218+304 - Discovery of a new VHE Blazar
Reference: V. A. Acciari et al. (The VERITAS Collaboration), The Astrophysical Journal, 695: 1370, 2009
The AGN 1ES 1218+304 is an X-ray-bright high-frequency-peaked (HBL) object located at a redshift z = 0.182. At this distance, significant attenuation from the interaction of high-energy gamma rays with the low-energy photons of the extragalactic background light (EBL) is expected. Gamma rays from extragalactic sources with energies above 100 GeV are expected to interact with optical low-energy photons as they travel through intergalactic space, leading to a cutoff in the measured gamma-ray spectrum. The optical depth of the attenuation is a complicated function of the gamma-ray photon energy, the distance to the source (redshift z), and the cross-section for pair production, and it is related to the density and spectral energy distribution of the cosmic background radiation. The physical importance of the EBL lies in its relation to galaxy formation and evolution and to the star-formation history of the universe.
This AGN was predicted to be a good TeV candidate based on the position of its synchrotron peak at high energy and sufficient radio-to-optical flux. It was first detected at very-high energies by the MAGIC telescope (Albert et al. ApJ 642, L119, 2006). VERITAS observed this source during the first three months of 2007 and detected a gamma-ray signal with a statistical significance of 10.4 standard deviations. The photon spectrum between 160 GeV and 1.8 TeV (see figure) was well described by a power law with an index of Γ = 3.08 ± 0.34 (stat) ± 0.2 (sys). The integral flux was Φ(E > 200 GeV) = (12.2 ± 2.6) ×1012 cm-2 s-1, corresponding to ≈6% of that of the Crab Nebula. No evidence for very-high-energy flux variability was detected during this observation period. Using lower limits on the density of the extragalactic background light in the near to mid-infrared, a limit on the range of intrinsic energy spectra for 1ES 1218+304 was calculated. The intrinsic photon spectrum was shown to have an index harder than Γ = 2.32 ± 0.37 (stat). 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 Γ = 1.86 ± 0.37 (stat).
Figures from paper (click to get full size image):