A search for Centaurus A-like features in the spectra of Fermi-LAT detected radio galaxies

ABSTRACT Motivated by the detection of a hardening in the γ-ray spectrum of the radio galaxy Centaurus A, we have analysed $\mathord {\sim }10$ yr of Fermi-Large Area Telescope (LAT) observations of 26 radio galaxies to search for similar spectral features. We find that the majority of the radio galaxies’ γ-ray spectral energy distributions are best fitted with a simple power-law model, and no spectral hardening similar to that found in Centaurus A was detected. We show that, had there been any such spectral features present in our sample of radio galaxies, they would have been seen, but note that seven of the radio galaxies (3C 111, 3C 120, 3C 264, IC 4516, NGC 1218, NGC 2892, and PKS 0625−35) show evidence for flux variability on 6-month time-scales, which makes the detection of any steady spectral features difficult. We find a strong positive correlation (r = 0.9) between the core radio power at 5 GHz and the γ-ray luminosity and, using a simple extrapolation to TeV energies, we expect around half of the radio galaxies studied will be detectable with the forthcoming Cherenkov Telescope Array.