Unveiling the nature of the highly obscured active galactic nucleus in NGC 5643 withXMM-Newton

We present results from an XMM–Newton observation of the nearby Seyfert 2 galaxy NGC 5643. The nucleus exhibits a very flat X‐ray continuum above 2 keV, together with a prominent Kα fluorescent iron line. This indicates heavy obscuration. We measure an absorbing column density NH in the range 6–10 × 1023 cm−2, either directly covering the nuclear emission, or covering its Compton reflection. In the latter case, we might be observing a rather unusual geometry for the absorber, whereby reflection from the inner far side of a torus is in turn obscured by its near side outer atmosphere. The nuclear emission might be then either covered by a Compton‐thick absorber, or undergoing a transient state of low activity. A second source (christened ‘X‐1’ in this paper) at the outskirts of the NGC 5643 optical surface outshines the nucleus in X‐rays. If belonging to NGC 5643, it is the third brightest (LX∼ 4 × 1040 erg s−1) known ultraluminous X‐ray source. Comparison with past large aperture spectra of NGC 5643 unveils dramatic X‐ray spectral changes above 1 keV. We interpret these as due to variability of the active nucleus and of source X‐1 intrinsic X‐ray powers by factors of ≥10 and 5, respectively.