Speaker
Description
We report hints for the correlation between the X-ray and $\gamma$-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. We have divided Fermi/LAT observations of NGC 4945 into two datasets, 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 datasets contain an equal amount of 3.8 years of LAT data and NGC 4945 is detected with a similar statistical significance of ∼ 15$\sigma$ in L and 14$\sigma$ in H. However, the slope of the $\gamma$-ray spectrum hardens with increase of the X-ray flux, with the photon index $\Gamma$ = 2.47 ± 0.07 in L and 2.11 ± 0.08 in H. The change is confirmed by systematic variation of the spectral energy distribution as well as a substantial reversal of the $\gamma$-ray signal in significance maps for low and high $\gamma$-ray energies.
The X/$\gamma$-ray correlation indicates that the $\gamma$-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium.
