SIMBAD references

Observations of the Orion region with COMPTEL have revealed a source which shows up only in the 3-7MeV range. A detailed counts spectrum was checked for signatures of nuclear excitation lines and indeed, corresponding bumps near 4.5MeV and 6MeV were found. Later OSSE observations agree with the COMPTEL results either if the excitation line emission originates in an extended region of a few degrees size or if the emission is time variable. In this paper we discuss an alternative identification for the observed emission in the 3-7MeV band, namely the flat-spectrum radio quasar (FSRQ) PKS 0539-057 at z=0.84 located 2 degrees to the east of Orion A. Annihilation and e⁺/e^– bremsstrahlung of pair plasma in a jet pointed towards us could well account for the observed gamma-ray spectrum in the COMPTEL energy range. For the case that pair plasma may be repeatedly injected in the jet of this source, some predictions for the lightcurves in the EGRET, COMPTEL and radio range are given. It is proposed that time variability in the 3-7MeV flux could be used to discriminate between the alternative models.