New constraints on the mid-IR EBL from the HESS discovery of VHE γ-rays from 1ES 0229+200

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 6.6 σ 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 similar 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 similar 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) ∼ to 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. (authors)