High energy gamma-ray sources in the VVV survey - II. The AGN counterparts

We identified Active Galactic Nuclei (AGN) candidates as counterparts to unidentified gamma-ray sources (UGS) from the Fermi-LAT Fourth Source Catalogue at lower Galactic latitudes. Our methodology is based on the use of near- and mid-infrared photometric data from the VISTA Variables in the Vía Láctea (VVV) and Wide-field Infrared Survey Explorer (WISE) surveys. The AGN candidates associated with the UGS occupy very different regions from the stars and extragalactic sources in the colour space defined by the VVV and WISE infrared colours. We found 27 near-infrared AGN candidates possibly associated with 14 Fermi-LAT sources using the VVV survey. We also found 2 blazar candidates in the regions of 2 Fermi-LAT sources using WISE data. There is no match between VVV and WISE candidates. We have also examined the K_s light curves of the VVV candidates and applied the fractional variability amplitude (σ_rms) and the slope of variation in the K_s passband to characterise the near-infrared variability. This analysis shows that more than 85 per cent of the candidates have slopes in the K_s passband >10⁻⁴ mag/day and present σ_rms values consistent with a moderate variability. This is in good agreement with typical results seen from type-1 AGN. The combination of YJHK_s colours and K_s variability criteria was useful for AGN selection, including its use in identifying counterparts to Fermi γ-ray sources.