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

Full text version

ArXiv version: ArXiV:0901.4561

Contact person: Pascal Fortin

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): 

Figure 1: Distribution of excess events as a function of the squared angular distance (theta^2) between the reconstructed shower directions and the nominal position of 1ES 1218+304. The dashed line indicates the boundary of the signal region at 0.158 deg (0.025 deg^2) as determined from optimization on the Crab Nebula. The solid curve shows the expectation for a point source as measured from data on the Crab Nebula.

Figure 2: Significance map of the region around 1ES 1218+304. The white cross indicates the position of the corresponding radio source. The white circle in the upper left corner shows the angular resolution of VERITAS (PSF). The ring background model was used to estimate the background and the significances were calculated using equation 17 in Li and Ma (1983).

Figure 3: Light curve of the integral photon flux above 200 GeV for the source 1ES 1218+304. Each point corresponds to the average daily flux and assumes a spectral shape of dN/dE ~ E^(-Γ) with Γ = 3.08. The error bars represent the statistical uncertainty.

Figure 4: 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, Helene (1983)) for bins with significances below two standard deviations. The dot-dash line shows the differential energy spectrum of 1ES 1218+304 measured by MAGIC (Albert et al. 2006) and the dash line shows the differential energy spectrum of the Crab Nebula measured by VERITAS (September to November 2006) for comparison.

Figure 5: The range of EBL densities that are considered for reconstructing the source spectrum of 1ES 1218+304 is shown. The filled triangles represent lower limits from galaxy counts by (Madau & Pozetti 2000) whereas the open rectangles are lower limits from the Spitzer observatory (Fazio et al. 2004).

Figure 6: Measured spectrum of 1ES1218+304 (circles) and de-absorbed spectrum for the AHA0.45 EBL scenario that produces the softest possible spectrum among all EBL scenarios considered (filled quadrangles). Furthermore we show the corrected spectrum for the SMS baseline model by Malkan & Stecker (1998); Stecker et al. (2006) (hollow quadrangles).