EDP Sciences
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Volume 387, Number 3, June I 2002
Page(s) 838 - 849
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20020444

A&A 387, 838-849 (2002)
DOI: 10.1051/0004-6361:20020444

The BeppoSAX broad-band spectrum and variability of the Seyfert 1 NGC 3783

A. De Rosa1, L. Piro1, F. Fiore2, P. Grandi1, L. Maraschi3, G. Matt4, F. Nicastro5 and P. O. Petrucci6

1  Istituto di Astrofisica Spaziale, C.N.R.,Via Fosso del Cavaliere, Roma, Italy
2  Osservatorio Astronomico di Roma, Monteporzio Catone, Italy
3  Osservatorio Astronomico di Brera, Milano, Italy
4  Dipartimento di Fisica, Università degli Studi "Roma Tre", Via della Vasca Navale 84, 00146 Roma, Italy
5  Harvard-Smithsonian Center of Astrophysics, Cambridge, MA 02138 USA
6  Laboratoire d'Astrophysique, Grenoble, France

(Received 16 November 2001 / Accepted 20 March 2002 )

BeppoSAX observed the Seyfert 1 galaxy NGC 3783 for ~5 days from 1998 June 6 to 1998 June 10. The average flux during the observation was $F_{2-10\,\rm keV}\sim 6\times 10^{-11} \;\rm erg\ cm^{-2}\ s^{-1}$. The long exposure provided a very high quality spectrum that exhibits all the typical features of Seyfert 1s. The high energy cut-off of the intrinsic continuum, at $E_{\rm c}=340\pm^{560}_{107}$ keV, was detected for the first time in this object. During the observation the source showed modest flux variations (~20% in 2-10 keV) and spectral variability that can be completely explained by a change of the intrinsic slope $\Delta \Gamma\sim 0.1$. NGC 3783 is one of the few Seyfert 1s that exhibits a soft emission component in the BeppoSAX spectrum. This soft excess is well reproduced by a black body component with temperature $kT\sim0.2$ keV. A warm absorber is also required to fit the soft X-ray spectrum. Both reprocessing features, the Compton reflection hump and the Fe emission line, were detected in our observation. In addition to a narrow iron line (as observed by Chandra) a broad component with $\sigma=0.72\pm^{1.28}_{0.27}$ keV and $EW=115\pm76$ eV, is required to fit the line. The observed Compton hump and the Fe line are well reproduced with the ionized ( $\log\xi=2.73\pm^{0.04}_{0.05}$) disc reflection model of Ross & Fabian. In this case the soft excess observed below 2 keV can be accounted for by the disc emissivity and no additional soft X-rays component is required to fit the spectrum.

Key words: galaxies: individual: NGC 3783 -- galaxies: Seyfert -- X-rays: galaxies

Offprint request: A. De Rosa, derosa@ias.rm.cnr.it

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