A comprehensive analysis toward the Fermi-LAT source 4FGL J1846.9-0227: Jets of a proto-planetary nebula producing gamma-rays?

Abstract: Most of the $\gamma$-ray sources in the Fermi-LAT 14-year Source Catalogue are associated with pulsars and blazars. However, unveiling the nature of the still unassociated $\gamma$-ray sources is important for the understanding of high energy emission mechanisms in astrophysical objects. This work presents a comprehensive study towards the region covered by the Fermi source 4FGL J1846.9$-$0227, previously suggested to be a blazar and a massive protostar. Using multiwavelength observations, we analysed several astrophysical objects in the region as possible counterparts of the Fermi-LAT source. Having discarded most of them after a detailed and comprehensive analysis, we suggest that the most likely candidate to be such a counterpart is IRAS 18443$-$0231, a likely proto-planetary nebula. We found that the radio continuum emission at 3 GHz of IRAS 18443$-$0231 shows a compact source related to faint emission with jet-like morphology. Additionally, we identified an associated red-shifted CO molecular outflow. Using data from several catalogues, we obtained radio spectral index values ranging from $-$0.57 to $-$0.39 for IRAS 18443$-$0231, indicating syncrothron emission due to particles accelerated by the jets. We point out that these jets could explain the $\gamma$-ray emission through mechanisms such as proton-proton collisions and relativistic Bremsstrahlung. IRAS 18443$-$0231, lying almost at the centre of the Fermi confidence ellipse and related to the hard X-ray source 4XMM J184700.4$-$022752, would be the first association between a proto-planetary nebula and $\gamma$-ray emission.