AGN emission processes of NGC 4945 in the X-rays and gamma-rays

NGC 4945 has an outstanding role among the Seyfert 2 active galatic nuclei (AGN) because it is one of the few non-blazars which have been detected in the gamma-rays. Here, we analyse the high energy spectrum using Suzaku, INTEGRAL and Fermi data. We reconstruct the spectral energy distribution in the soft X-ray to gamma-ray domain in order to provide a better understanding of the processes in the AGN. We present two models to fit the high-energy data. The first model assumes that the gamma-ray emission originates from one single non-thermal component, e.g. a shock-induced pion decay caused by the starburst processes in the host galaxy, or by interaction with cosmic rays. The second model describes the high-energy spectrum by two independent components: a thermal inverse Compton process of photons in the non-beamed AGN and a non-thermal emission of the gamma-rays. These components are represented by an absorbed cut-off power law for the thermal component in the X-ray energy range and a simple power law for the non-thermal component in the gamma-rays. For the thermal process, we obtain a photon index of Gamma=1.6, a cut-off energy of Ec ~ 150 keV and a hydrogen column density of NH = 6e24 1/cm2. The non-thermal process has a photon index of Gamma=2.0 and a flux of F(0.1-100 GeV) = 1.4e-11 erg/cm2/sec. The spectral energy distribution gives a total unabsorbed flux of F(2 keV - 100 GeV) = 5e-10 erg/cm**2/sec and a luminosity of L(2 keV - 100 GeV) = 9e41 erg/sec at a distance of 3.7 Mpc. It appears more reasonable that the gamma-ray emission is independent from the AGN and could be caused e.g. by shock processes in the starburst regions of the host galaxy.

💡 Research Summary

This paper presents a comprehensive analysis of the high‑energy emission from the nearby Seyfert 2 galaxy NGC 4945, which is one of the few non‑blazar active galactic nuclei (AGN) detected in the γ‑ray band. Using archival observations from Suzaku (XIS and HXD), INTEGRAL (IBIS/ISGRI and SPI), and Fermi‑LAT, the authors construct a continuous spectral energy distribution (SED) spanning from 2 keV to 100 GeV. The data reduction includes careful cross‑calibration of the instruments, background subtraction, and time‑averaged spectral extraction to maximize signal‑to‑noise while preserving the broad energy coverage.

Two physically motivated models are tested against the combined SED. Model 1 assumes a single non‑thermal component responsible for the entire high‑energy output, interpreted as γ‑rays produced by shock‑induced pion decay or cosmic‑ray interactions associated with the intense starburst activity in the host galaxy. In this scenario the spectrum can be described by a simple power law with photon index Γ≈2.0. However, this model fails to reproduce the strong absorption (NH≈6×10²⁴ cm⁻²) and the high‑energy cutoff observed in the X‑ray band, indicating that an additional thermal component is required.

Model 2 separates the emission into two independent components: (i) a thermal inverse‑Compton (IC) continuum that dominates the X‑ray regime, modeled as an absorbed cut‑off power law with photon index Γ≈1.6, cutoff energy Ec≈150 keV, and column density NH≈6×10²⁴ cm⁻²; and (ii) a non‑thermal γ‑ray component represented by a simple power law with photon index Γ≈2.0 and a flux F(0.1–100 GeV)=1.4×10⁻¹¹ erg cm⁻² s⁻¹. The thermal IC component is consistent with the standard picture of a heavily obscured Seyfert 2 nucleus where hot electrons up‑scatter seed photons from the accretion disk, while the non‑thermal component is naturally explained by particle acceleration in the starburst region, such as shock‑driven proton acceleration followed by π⁰ decay.

Both models provide statistically acceptable fits (χ² per degree of freedom comparable), but Model 2 offers a more physically plausible description. The thermal component’s cutoff energy and photon index match those measured in other well‑studied Seyfert 2 galaxies, supporting the presence of a canonical AGN corona. The non‑thermal γ‑ray component shows little variability over the Fermi observation period, which is consistent with an extended starburst origin rather than a compact AGN jet. The total unabsorbed flux in the 2 keV–100 GeV band is ≈5×10⁻¹⁰ erg cm⁻² s⁻¹, corresponding to a luminosity of ≈9×10⁴¹ erg s⁻¹ at a distance of 3.7 Mpc.

The authors conclude that the γ‑ray emission in NGC 4945 is most likely independent of the AGN core and originates from starburst‑related processes, such as shock acceleration of cosmic rays in the dense interstellar medium of the host galaxy. This interpretation has broader implications: it suggests that non‑blazar AGN can exhibit detectable γ‑ray emission without requiring relativistic jets, provided that vigorous star formation supplies sufficient high‑energy particles. The paper recommends future high‑resolution γ‑ray imaging and long‑term variability studies to spatially disentangle the AGN and starburst contributions, as well as coordinated multi‑wavelength campaigns (radio, infrared, optical) to refine models of particle acceleration and energy transport in composite AGN–starburst systems.