2017ApJ...849...97W -
Astrophys. J., 849, 97-97 (2017/November-2)
The X-/γ-ray correlation in NGC 4945 and the nature of its γ-ray source.
WOJACZYNSKI R. and NIEDZWIECKI A.
Abstract (from CDS):
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 ∼15σ in L and 14σ 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.
Abstract Copyright:
© 2017. The American Astronomical Society. All rights reserved.
Journal keyword(s):
galaxies: individual: NGC 4945 - galaxies: Seyfert - gamma rays: galaxies - radiation mechanisms: nonthermal - radiation mechanisms: nonthermal
Simbad objects:
12
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