1995A&A...293L...1B

We report evidence for a blazar-type AGN that is exceptionally bright at MeV energies. It is firmly detected by COMPTEL in the 1-10MeV range, but we find only a weak ∼3σ source in the EGRET archival data (>100MeV), which is confirmed by the EGRET team. The source is now listed as one of the marginal detections in the latest EGRET catalogue, being the weakest source in the whole list. It is located about 10deg from the LMC at l~=270deg, b~=-35deg. Within the 99% error region of this new COMPTEL source (GRO J0516-609), two potential AGN counterparts can be identified. These are PKS0506-612 and PKS0522-611, separated by ∼2deg, which are both compact, radio loud, flat-spectrum radio sources, like all AGNs detected at high-energy γ-rays. The former is the preferred counterpart by the weak EGRET detection, which has a smaller error region. The source was in the COMPTEL field-of-view on three occasions during its Phase I sky survey, but it was detected only in the second and third observation. This suggests a flux increase by a factor of about two in the two-month period between the first and second viewing. Evidence for the weak EGRET source is also only found in the second and third observation. The radiated power per decade of energy (νF_ν) strongly peaks at ∼3MeV. If GRO J0516-609 is indeed a blazar, it can probably be ruled out that the MeV γ-rays are mainly due to Comptonization of an external radiation field. "MeV blazars" like GRO J0516-609 may be responsible for the observed MeV bump in the cosmic background spectrum.