The X-/γ-Ray Correlation in NGC 4945 and the Nature of Its γ-Ray Source

Abstract

Abstract We report hints for the correlation between the X-ray and γ-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. We have divided the Fermi/LAT observations of NGC 4945 into two data sets, comprising events detected during the low (L) and high (H) level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. Both data sets contain an equal amount of 3.8 years of LAT data and NGC 4945 is detected with a similar statistical significance of ∼ 15 σ in L and 14 σ in H. However, the slope of the γ-ray spectrum hardens with the increase of the X-ray flux, with the photon index Γ = 2.47 ± 0.07 in L and 2.11 ± 0.08 in H. The change is confirmed by a systematic variation of the spectral energy distribution as well as a substantial reversal of the γ-ray signal in significance maps for low and high γ-ray energies. The X-/γ-ray correlation indicates that the γ-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. We discuss possible locations of the γ-ray source. We also compare NGC 4945 with other starburst galaxies detected by LAT and we note similarities between those with active nuclei, e.g., unlikely high efficiencies of γ-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the γ-ray emission.