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Astron. Astrophys. 356, 463-474 (2000)


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A broad-band X-ray view of NGC 4945

M. Guainazzi 1,2, G. Matt 3, W.N. Brandt 4, L.A. Antonelli 5, P. Barr 1 and L. Bassani 6

1 Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2 XMM SOC, VILSPA, ESA, Apartado 50727, 28080 Madrid, Spain
3 Università degli Studi "Roma Tre", Dipartimento di Fisica, Via della Vasca Navale 84, 00046 Roma, Italy
4 The Pennsylvania State University, Department of Astronomy and Astrophysics, 525 Davey Lab, University Park, PA 16802, USA
5 Osservatorio Astronomico di Roma, Via dell'Osservatorio, 00044 Monteporzio Catone, Italy
6 Istituto Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy

Received 23 September 1999 / Accepted 10 January 2000

Abstract

We present the results of a BeppoSAX observation of the nearby spiral galaxy NGC 4945 in the 0.1-200 keV energy band. The nuclear X-ray emission emerges above [FORMULA]7 keV, through an absorber with column density [FORMULA] a few [FORMULA] cm-2. Its remarkable variability (doubling/halving timescale [FORMULA]104 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 [FORMULA] fluorescent emission line is likely to be produced by the same nuclear absorbing matte. Our estimate of the intrinsic nuclear power suggests that the active nucleus dominates the bolometric energy output ofNGC 4945, if its nucleus has a typical quasar [FORMULA] 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.

Key words: galaxies: active – galaxies: individual: NGC 4945 – galaxies: nuclei – galaxies: spiral – X-rays: galaxies

Send offprint requests to: M. Guainazzi

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000

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