X-ray/γ-ray Correlation in Seyfert 2 Galaxy NGC 4945

Abstract

We report the correlation between the X-ray and γ-ray emission found in our analysis of the Fermi/LAT and Swift/BAT data from 8 years of observations of the nearby Seyfert 2 galaxy NGC 4945. Using the BAT light-curves we determined summed periods of low and high X-ray flux and we found that the average γ-ray spectrum is harder in the latter (higher X-ray flux level), with the difference of the γ-ray spectral index of ~0.4. The correlation indicates that the γ-rays are produced in the active nucleus of this Seyfert galaxy rather than by cosmic rays interacting with the interstellar medium (as assumed in the alternative starburst model). We compare NGC 4945 with two other γ-ray loud galaxies showing both starburst and AGN activities, NGC 1068 and Circinus.