Suzaku wide-band X-ray Spectroscopy of the Seyfert 2 AGN in NGC 4945

TL;DR

This study used Suzaku's wide-band X-ray spectroscopy to analyze the obscured active nucleus of NGC 4945, revealing detailed properties of the reflection and absorption features, and providing insights into the geometry of the AGN's surrounding material.

Contribution

First detection of neutral nickel Kα line and detailed broadband spectral modeling of the reflection and absorption in NGC 4945's AGN.

Findings

Neutral iron Kβ and nickel Kα lines detected for the first time.

Reflection likely originates from a disk-like structure viewed edge-on.

Intrinsic emission varies by a factor of two within 20 ks.

Abstract

Suzaku observed a nearby Seyfert 2 galaxy NGC4945, which hosts one of the brightest active galactic nuclei above 20 keV. Combining data from the X-ray CCD camera (XIS) and the Hard X-ray Detector (HXD), the AGN intrinsic nuclear emission and its reprocessed signals were observed simultaneously. The intrinsic emission is highly obscured with an absorbing column of $\sim 5 \times 10^{24}$ cm$^{-2}$, and was detectable only above $\sim 10$ keV. The spectrum below 10 keV is dominated by reflection continuum and emission lines from neutral/ionized material. Along with a neutral iron K$\alpha$ line, a neutral iron K$\beta$ and a neutral nickel K$\alpha$ line were detected for the first time from this source. The neutral lines and the cold reflection continuum are consistent with both originating in the same location. The Compton down-scattered shoulder in the neutral Fe-K$\alpha$ line is $\sim 10%$ in flux of the narrow core, which confirms that the line originates from reflection rather than transmission. The weakness of the Compton shoulder also indicates that the reflector is probably seen nearly edge-on. Flux of the intrinsic emission varied by a factor of $\sim 2$ within $\sim 20$ ks, which requires the obscuring material to be geometrically thin. Broadband spectral modeling showed that the solid angle of the neutral reflector is less than a few $\times 10^{-2} \times 2\pi$. All this evidence regarding the reprocessed signals suggests that a disk-like absorber/reflector is viewed from a near edge-on angle.