DISCOVERY OF VERY HIGH ENERGY γ-RAY EMISSION FROM CENTAURUS A WITH H.E.S.S.

Abstract

We report the discovery of faint very high energy (VHE; E > 100 GeV) γ-ray emission from the radio galaxy Centaurus A in observations performed with the High Energy Stereoscopic System (H.E.S.S.) experiment, an imaging atmospheric Cherenkov telescope array consisting of four telescopes located in Namibia. Centaurus A has been observed for more than 120 hr. A signal with a statistical significance of 5.0σ is detected from the region including the radio core and the inner kpc jets. The integral flux above an energy threshold of ∼250 GeV is measured to be 0.8% of the flux of the Crab Nebula (apparent luminosity: L(>250 GeV) ≈2.6 × 1039 erg s−1, adopting a distance of 3.8 Mpc). The spectrum can be described by a power law with a photon index of 2.7 ± 0.5stat ± 0.2sys. No significant flux variability is detected in the data set. However, the low flux only allows detection of variability on the timescale of days to flux increments above a factor of ∼15–20 (3σ and 4σ, respectively). The discovery of VHE γ-ray emission from Centaurus A reveals particle acceleration in the source to >TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.