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Register a new userThe BeppoSAX X-ray view of reflection-dominated Seyfert Galaxies
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We present new results from BeppoSAX observations of reflection-dominated Seyfert galaxies, and namely: 1) the Compton-thick Seyfert 2s NGC1068 and Circinus Galaxy; 2) the Seyfert 1 NGC4051, whose nucleus was observed on May 1998 to have switched off, leaving only a residual reflection component as an echo of its past activity. Our main focus in this paper is on the soft X-ray continuum properties and on the X-ray line spectroscopy.
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In this paper we present the current status of a XMM-Newton program to observe an optically-defined, complete and unbiased sample of Compton-thick Seyfert 2 galaxies. The main goal of this project is the measurement of the occurrence rate of transition between transmission- (i.e.: Compton-thin), and reflection-dominated spectral states. These transitions potentially provide information on the distribution of the obscuring matter surrounding the nucleus, and on the duty-cycle of the AGN activity. With about 2/3 of the whole sample being observed, we detected 1 further transition out of 8 observed objects, confirming previous suggestions that these transitions occur on time-scales ~50-100 years.
The Seyfert 2 galaxy NGC 7674 has been observed within the BeppoSAX Core Programme with the onboard narrow field instruments between 0.1 and 100 keV. The broad-band spectrum shows four most relevant spectral components: a) a soft excess below ~2 keV; b) a prominent (EW~1 keV) Fe line; c) a flat (Gamma~1.1) 2-10 keV continuum; d) a 4.5sigma detection above 13 keV. The flat power law spectrum can be very well explained within the current AGN unified models assuming a steep (Gamma~2) intrinsic spectrum scattered by warm gas for the low energy band and totally reflected by optically thick cold matter (plausibly a molecular torus) for the high energy band. The case of NGC 7674 adds to the increasing number of so called Compton-thick Seyfert 2 galaxies in which the direct emission is totally absorbed and the X-ray luminosity is thus at least one or two orders of magnitude larger than what inferred from the observed flux.
We present a X-ray spectroscopic study of the bright Compton-thick Seyfert 2s NGC1068 and the Circinus Galaxy, performed with BeppoSAX. Matt et al. (1997 and 1998) interpreted the spectrum above 4 keV as the superposition of Compton reflection and warm plasma scattering of the nuclear radiation. When this continuum is extrapolated downwards to 0.1 keV, further components arise. The NGC1068 spectrum is rich in emission lines, mainly due to K-alpha transitions of He-like elements from oxygen to iron, plus a K-alpha fluorescent line from neutral iron. If the ionized lines originate in the warm scatterer, its thermal and ionization structure must be complex. From the continuum and line properties, we estimate a column density, of the warm scatterer less than a few 10^21 atoms/cm/cm. In the Circinus Galaxy, the absence of highly ionized iron is consistent with a scattering medium with ionization parameter U<5 and density about a few times 10^22 atoms/cm/cm. In both cases the neutral iron line is most naturally explained as fluorescence in the medium responsible for the Compton reflection continuum. In NGC1068 an optically thin plasma emission with kT~500 eV and strongly sub-solar metallicity is required, while such a component is only marginal in the Circinus Galaxy. We tentatively identify this component as emission of diffuse hot gas in the nuclear starbursts. Possible causes for the metal depletion are discussed.
A model for the inner regions of accretion flows is presented where, due to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes occur. We expect this situation to be most relevant when the accretion rate is close to Eddington and the disc is radiation-pressure dominated, and so may apply to Narrow-Line Seyfert~1 (NLS1) galaxies. In this scenario, the hard X-ray source is obscured for most observers, and so the detected X-ray emission would be dominated by reflection off the walls of the sheets. A simple Comptonization calculation shows that the large photon-indices characteristic of NLS1s would be a natural outcome of two reprocessors closely surrounding the hard X-ray source. We test this model by fitting the XMM-Newton spectrum of the NLS1 1H 0707-495 between 0.5 and 11 keV with reflection dominated ionized disc models. A very good fit is found with three different reflectors each subject to the same Gamma=2.35 power-law. An iron overabundance is still required to fit the sharp drop in the spectrum at around 7 keV. We note that even a small corrugation of the accretion disc may result in Gamma > 2 and a strong reflection component in the observed spectrum. Therefore, this model may also explain the strength and the variability characteristics of the MCG-6-30-15 Fe K line. The idea needs to be tested with further broadband XMM-Newton observations of NLS1s.
BeppoSAX observed NGC4258 on December 1998, when its 2-10 keV luminosity was ~1E41 erg/s. 100% variability is observed in the 3-10 keV band on timescales of a half a day while 20% variability is observed on timescales of one hour. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5+/-1.2)E22 cm-2; this component is detected up to 70 keV with S/N>=3 and with the steep power law energy index of 1.11+/-0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in ADAF models with strong winds, is ruled out by the data. The ratio between the nuclear radio and the X-ray luminosities is <=1E-5, similar to that of radio quiet AGN. X-ray variability and spectral shape, radio to X-ray and NIR to X-ray luminosity ratios suggest that the nucleus of NGC4258 could be a scaled-down version of a Seyfert nucleus. The soft (E<=2keV) X-ray emission is complex. There are at least two thermal-like components, with T1=0.6+/-0.1 keV and T2>=1.3 keV. The cooler (L(0.1-2.4keV)=1E40 erg/s) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI. The second component, which can be modeled equally well by an unobscured power law model, has L(0.1-2.4keV)~7E39 erg/s, consistent with that expected from discrete X-ray sources in the host galaxy. NGC4258 and other maser AGNs show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of maser emission from X-ray illumination. So a large column density gas may be a necessary property of masing AGNs.
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