A broadband X-ray view of NGC4945

TL;DR

This study uses BeppoSAX data to analyze the X-ray emission of NGC4945, revealing a heavily absorbed nucleus with rapid variability and extended emission likely from binaries or hot gas, emphasizing X-ray's role in detecting absorbed AGN.

Contribution

First detailed broadband X-ray analysis of NGC4945, showing nuclear variability without spectral change and clarifying the origin of extended emission.

Findings

Nuclear emission emerges above ~7 keV with high absorption (N_H ~ 10^24/cm^2).

Rapid variability (~10^4 s) without spectral change suggests intrinsic nuclear activity.

Extended emission likely from unresolved binaries or hot gas related to starburst activity.

Abstract

We present the results of a BeppoSAX observation of the nearby spiral galaxy NGC4945 in the 0.1-200 keV energy band. The nuclear X-ray emission emerges above ~7 keV, through an absorber with column density N_H ~ a few 10^24/cm/cm. Its remarkable variability (doubling/halving timescale ~10^4 s) is not associated with any appreciable spectral changes, ruling out variations of the interposed absorber or changes in the primary continuum shape. The intense iron K-ALPHA fluorescent emission line is likely to be produced by the same nuclear absorbing matter. Our estimate of the intrinsic nuclear power suggests that the active nucleus dominates the bolometric energy output of NGC4945, if its nucleus has a typical quasar L_X/L(bol) ratio. This supports the idea that X-rays are the best energy band to search for absorbed AGN. The 0.1-5 keV emission is extended along the plane of the galaxy, and most likely due to a population of unresolved binaries, as believed to happen in several early-type galaxies. Alternatively, hot gas associated with a starburst-driven superwind outflow could substantially contribute to the extended emission above 1 keV.