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A. C.Fabian D. R.Ballantyne A.Merloni S.Vaughan K.Iwasawa Th.Boller 《Monthly notices of the Royal Astronomical Society》2002,331(3):L35-L39
A model for the inner regions of accretion flows is presented where, owing 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 Γ=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 Γ>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 α line. The idea needs to be tested with further broad-band XMM-Newton observations of NLS1s. 相似文献
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K. Iwasawa S. Ettori A. C. Fabian A. C. Edge H. Ebeling 《Monthly notices of the Royal Astronomical Society》2000,313(3):515-523
We report the discovery of highly distorted X-ray emission associated with the nearby cluster Zw 1718.10108, one of the dominant members of which is the powerful radio galaxy 3C353. This cluster has been missed by previous X-ray cluster surveys because of its low Galactic latitude ( b =19.5°), despite its brightness in the hard X-ray band (210 keV flux of 1.21011 erg cm2 s1 ). Our optical charge-coupled device image of the central part of the cluster reveals many member galaxies which are dimmed substantially by heavy Galactic extinction. We have measured redshifts of three bright galaxies near the X-ray emission peak and they are all found to be around z =0.028. The ASCA gas imaging spectrometer and ROSAT high-resolution imager images show three aligned X-ray clumps embedded in low surface-brightness X-ray emission extended by 30 arcmin. The averaged temperature measured with ASCA is kT =4.3±0.2 keV, which appears to be hot for the bolometric luminosity when compared with the temperatureluminosity correlation of galaxy clusters. The irregular X-ray morphology and evidence for a non-uniform temperature distribution suggest that the system is undergoing a merger of substructures. Since the sizes and luminosities of the individual clumps are consistent with those of galaxy groups, Zw 1718.10108 is interpreted as an on-going merger of galaxy groups in a dark matter halo forming a cluster of galaxies and thus is in a transition phase of cluster formation. 相似文献
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S. Vaughan K. Iwasawa A. C. Fabian K. Hayashida 《Monthly notices of the Royal Astronomical Society》2005,356(2):524-530
An analysis of the X-ray variability of the low-luminosity Seyfert nucleus NGC 4395, based on a long XMM–Newton observation, is presented. The power spectrum shows a clear break from a flat spectrum (α≈ 1) to a steeper spectrum (α≈ 2) at a frequency f br = 0.5–3.0 × 10−3 Hz , comparable to the highest characteristic frequency found previously in a Seyfert galaxy. This extends the measured M BH − f br values to lower M BH than previous studies of Seyfert galaxies, and is consistent with an inverse scaling of variability frequency with black hole mass. The variations observed are among the most violent seen in an active galactic nuclei to date, with the fractional rms amplitude ( F var ) exceeding 100 per cent in the softest band. The amplitude of the variations seems intrinsically higher in NGC 4395 than most other Seyfert galaxies, even after accounting for the differences in characteristic frequencies. The origin of this difference is not clear, but it is unlikely to be a high accretion rate ( L / L Edd ≲ 20 per cent for NGC 4395). The variations clearly follow the linear rms–flux relation, further supporting the idea that this is a ubiquitous characteristics of accreting black holes. The variations are highly coherent between different energy bands with any frequency-dependent time delay limited to ≲1 per cent. 相似文献
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We discuss the iron K line complex in NGC 1068. The line complex basically consists of three components, as previously reported.
A new analysis of the ASCA data shows that cold reflection appears to dominate the observed X-ray emission above 4 keV based
on the detection of ‘Compton shoulder’, a weak red wing of the 6.4 keV fluorescence iron K line, and a very flat continuum.
The other two weaker lines at higher energies can be identified with FeXXV and FeXXVI and suggest an highly ionized X-ray
mirror as well, although the line energies are systematically lower than those expected from the resonant lines, consistent
with a redshift by 1.5%.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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M. Guainazzi G. Matt L. A. Antonelli L. Bassani A. C. Fabian R. Maiolino A. Marconi F. Fiore K. Iwasawa L. Piro 《Monthly notices of the Royal Astronomical Society》1999,310(1):10-20
We present an X-ray spectroscopic study of the bright Compton-thick Seyfert 2 galaxies NGC 1068 and the Circinus Galaxy, performed with BeppoSAX . Matt et al. 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 NGC 1068 spectrum is rich in emission lines, mainly owing to K α transitions of He-like elements from oxygen to iron, plus a K α 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, N warm , of the warm scatterer less than a few×1021 cm−2 . In the Circinus Galaxy, the absence of highly ionized iron is consistent with a scattering medium with U X ≲5 and N warm ∼ a few×1022 cm−2 . In both cases the neutral iron line is most naturally explained as fluorescence in the medium responsible for the Compton reflection continuum. In NGC 1068 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. 相似文献
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We present an X-ray spectroscopic study of the prototype far-infrared galaxy NGC 6240 from ASCA . The soft X-ray spectrum (below 2 keV) shows clear signatures of thermal emission well described by a multitemperature optically thin plasma, which probably originates in a powerful starburst. Strong hard X-ray emission is also detected with ASCA and its spectrum above 3 keV is extremely flat with a prominent iron K line complex, very similar to that seen in the Seyfert 2 galaxy NGC 1068 but about an order of magnitude more luminous ( L 3−10keV ≈ 1.4 × 1042 erg s−1 ). The hard X-ray spectrum indicates that only reflected X-rays of an active galactic nucleus (AGN) buried in a heavy obscuration ( N H > 2 × 1024 cm−2 ) are visible. This is evidence for an AGN in NGC 6240, emitting possibly at a quasar luminosity (∼ 1045 erg s−1 ), and suggests its significant contribution to the far-infrared luminosity. 相似文献