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

Abstract

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 N_H 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 (L_X~ 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.