2019ApJ...885..117G

The Circinus galaxy is a nearby composite starburst/active galactic nucleus (AGN) system. In this work we re-analyze the GeV emission from Circinus with 10 yr of Fermi-LAT Pass 8 data. In the energy range of 1-500 GeV, the spectrum can be well fitted by a power-law model with a photon index of Γ = 2.20 ± 0.14, and its photon flux is (5.90 ± 1.04) x 10^–10 photons cm^–2 s^–1. Our 0.1-500 GeV flux is several times lower than that reported in previous literature, which is roughly in compliance with the empirical relation for star-forming and Local Group galaxies and might be reproduced by the interaction between cosmic rays and the interstellar medium. The ratio between the γ-ray luminosity and the total infrared luminosity is near the proton calorimetric limit, indicating that Circinus may be a proton calorimeter. However, marginal evidence for variability of the γ-ray emission is found in the timing analysis, which may indicate the activity of an AGN jet. More Fermi-LAT data and future observation of CTA are required to fully reveal the origin of its γ-ray emission.