Very high energy γ-ray emission from RBS 0679

In this paper, we report the Fermi Large Area Telescope (LAT) detection of Very High Energy (VHE; Eγ > 100 GeV) γ-ray emission from the BL Lac object RBS 0679. 5.3 years of LAT observations revealed the presence of three VHE photon events within 0 ∘.1 of RBS 0679, with a subsequent unbinned likelihood analysis finding RBS 0679 to be a source of VHE photons at 6.9 standard deviations (σ). An unbinned likelihood analysis of the 0.1–100 GeV data, binned in 28-d periods, finds both the flux and spectral index to be variable, with a ‘softer-when-brighter’ trend in the global γ-ray characteristics. On the other hand, the 28-d periods in which the VHE photons were detected have spectral indices that are consistent with the 5.3 year average suggesting that the observed VHE emission is not associated with a spectral hardening event. The discovery of RBS 0679 in the 100–300 GeV energy range, combined with the non-detection above 390 GeV with the H.E.S.S. telescope array, suggest RBS 0679 to be an intriguing source that requires further follow-up observations with ground-based γ-ray observatories