Very High Energy gamma-ray emission from RBS 0970

Abstract

In this letter I report the Fermi Large Area Telescope (LAT) detection of Very High Energy (VHE; E>100 GeV) gamma-ray emission from the BL Lac object RBS 0970. 5.3 years of LAT observations revealed the presence of 3 VHE photon events within 0.1 degrees of RBS 0970, with a subsequent unbinned likelihood analysis finding RBS 0970 to be a source of VHE photons at the 6.5 sigma level of confidence. The E>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 day periods centered on detection of the VHE photons, revealed that the emission of the lowest energy VHE photons coincided with a hardening of the gamma-ray spectrum. Interestingly, the same analysis did not find any significant gamma-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 gamma-ray observatories.