INTEGRAL observations of six AGN in the Galactic Plane

Free access article

Issue		A&A Volume 444, Number 2, December III 2005


Page(s)		431 - 441
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20053875

A&A 444, 431-441 (2005)
DOI: 10.1051/0004-6361:20053875

INTEGRAL observations of six AGN in the Galactic Plane

S. Soldi^{1, 2}, V. Beckmann^{3, 4}, L. Bassani⁵, T. J.-L. Courvoisier^{1, 2}, R. Landi^{5, 6}, A. Malizia⁵, A. J. Dean⁷, A. De Rosa⁸, A. C. Fabian⁹ and R. Walter^{1, 2}

¹ INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland
e-mail: Simona.Soldi@obs.unige.ch
² Observatoire de Genève, 51 chemin des Mailletes, 1290 Sauverny, Switzerland
³ NASA Goddard Space Flight Center, Exploration of the Universe Division, Greenbelt, MD 20771, USA
⁴ Joint Center for Astrophysics, Department of Physics, University of Maryland, Baltimore County, MD 21250, USA
⁵ IASF, CNR/INAF, via Gobetti 101, 40129 Bologna, Italy
⁶ Dipartimento di Fisica, Università di Bologna, Viale C. Berti Pichat 6/2, 40127 Bologna, Italy
⁷ School of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
⁸ IASF, CNR/INAF, Via Fosso del Cavaliere 100, 00133 Rome, Italy
⁹ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

(Received 21 July 2005 / Accepted 22 August 2005)

Abstract
We present results on approximately one year of INTEGRAL observations of six AGN detected during the regular scans of the Galactic Plane. The sample is composed by five Seyfert 2 objects (MCG -05-23-16, NGC 4945, the Circinus galaxy, NGC 6300, ESO 103-G35) and the radio galaxy Centaurus A. The continuum emission of each of these sources is well represented by a highly absorbed ( $N_{\rm H}>10^{22} \, \rm cm^{-2}$ ) power law, with average spectral index $\Gamma = 1.9 \pm 0.3$ . A high energy exponential cut-off at $E_{\rm c} \sim 50 \rm\,~keV$ is required to fit the spectrum of the Circinus galaxy, whereas a lower limit of 130 keV has been found for NGC 4945 and no cut-off has been detected for NGC 6300 in the energy range covered by these INTEGRAL data. The flux of Centaurus A was found to vary by a factor of ~2 in 10 months, showing a spectral change between the high and low state, which can be modelled equally well by a change in the absorption ( $N_{\rm H}$ from 17 to $33 \times 10^{22} \rm\, cm^{-2}$ ) or by the presence of a cut-off at $\ga$ 120 keV in the low state spectrum. A comparison with recently reprocessed BeppoSAX/PDS data shows a general agreement with INTEGRAL results. The high energy cut-off in the hard X-ray spectra appears to be a common but not universal characteristic of Seyfert 2 and to span a wide range of energies.

Key words: galaxies: active -- galaxies: Seyfert -- gamma rays: observations -- X-rays: galaxies

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