Very high energy γ -ray emission from RBS 0970.

In this Letter I report the Fermi Large Area Telescope (LAT) detection of very high energy (VHE; E > 100 GeV) γ -ray emission from the BL Lac object RBS 0970. 5.3 years of LAT observations revealed the presence of three VHE photon events within 0. ◦1 of RBS 0970, with a subsequent unbinned likelihood analysis finding RBS 0970 to be a source of VHE photons at the 6.5σ level of confidence. The ≥1 GeV flux, binned in monthly periods, did not indicate any flux brightening of RBS 0970 accompanying the emission of the VHE photons. However, a likelihood analysis of the 0.1−100 GeV flux, binned in 28 d periods centred on detection of the VHE photons, revealed that the emission of the lowest energy VHE photons coincided with a hardening of the γ -ray spectrum. Interestingly, the same analysis did not find any significant γ -ray emission from RBS 0970 during the emission of the highest energy VHE event. The discovery of RBS 0970 as a VHE emitter, combined with the spectral variability, suggest RBS 0970 to be a good candidate for follow-up observations with ground-based γ -ray observatories