Abstract
Aims:To investigate the very high energy (VHE: >100 GeV) γ-ray emission from the high-frequency peaked BL Lac 1ES 0229+200. Methods: Observations of 1ES 0229+200 at energies above 580 GeV were performed with the High Energy Stereoscopic System (HESS) in 2005 and 2006. Results: 1ES 0229+200 is discovered by HESS to be an emitter of VHE photons. A signal is detected at the 6.6σ level in the HESS observations (41.8 h live time). The integral flux above 580 GeV is (9.4±1.5_stat±1.9_syst) × 10-13 cm-2 s-1, corresponding to ~1.8% of the flux observed from the Crab Nebula. The data show no evidence for significant variability on any time scale. The observed spectrum is characterized by a hard power law (Γ = 2.50±0.19_stat±0.10_syst) from 500 GeV to ~15 TeV. Conclusions: The high-energy range and hardness of the observed spectrum, coupled with the object's relatively large redshift (z = 0.1396), enable the strongest constraints so far on the density of the Extragalactic Background Light (EBL) in the mid-infrared band. Assuming that the emitted spectrum is not harder than Γ_int ≈ 1.5, the HESS data support an EBL spectrum ∝λ-1 and density close to the lower limit from source counts measured by Spitzer, confirming the previous indications from the HEGRA data of 1ES 1426+428 (z=0.129). Irrespective of the EBL models used, the intrinsic spectrum of 1ES 0229+200 is hard, thus locating the high-energy peak of its spectral energy distribution above a few TeV.
Highlights
The active galactic nucleus (AGN) 1ES 0229+200 was initially discovered in the Einstein IPC Slew Survey (Elvis et al 1992) and later identified as a BL Lac object (Schachter et al 1993)
Unless the intrinsic BL Lac spectrum is considerably harder4 than 1.5, the Extragalactic Background Light (EBL) density in the MIR range follows a spectrum ∝λ−1 and is very likely close to the lower limits given by galaxy counts as determined by Spitzer and ISO data
This implies that the sources resolved by Spitzer seem to account for the bulk of the diffuse 5−10 μm background
Summary
The active galactic nucleus (AGN) 1ES 0229+200 was initially discovered in the Einstein IPC Slew Survey (Elvis et al 1992) and later identified as a BL Lac object (Schachter et al 1993). It is classified as a high-frequency peaked BL Lac (HBL) due to its X-ray-to-radio flux ratio (Giommi et al 1995). Based on its spectral energy distribution (SED) 1ES 0229+200 is suggested as a potential source of VHE γ-rays (Stecker et al 1996; Costamante & Ghisellini 2002). Despite several attempts, it has not been previously detected in the VHE regime. The Whipple (de la Calle Perez et al 2003; Horan et al.2004), HEGRA (Aharonian et al 2004a), and Milagro
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