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The COMPTEL experiment on the Compton Gamma-Ray Observatory (CGRO), which operates in the 0.75-30 MeV energy range, has surveyed the entire sky since its launch in April, 1991. In addition to the radio galaxy Cen A, MeV emission has been detected from several blazar-type AGN (3C273, 3C279, PKS0528+134, and CTA102/3C454.3). All these blazars have been detected at higher energies by the EGRET experiment onboard GRO and they generally demand a break in their spectrum below the lower bound of EGRET measurements.
We now have evidence for a MeV blazar not previously reported by EGRET. For this new COMPTEL detection, the (two) candidate sources are, as for the high-energy list, compact, flat-spectrum radio-loud sources. Having analysed the public EGRET data to complement that of COMPTEL, we see that the spectrum breaks around a few MeV, where the radiated power strongly peaks.
The existence of such sources could explain the MeV bump in the diffuse $\gamma$-ray background spectrum.
