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761.
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763.
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
764.
R.J. Beswick A. Pedlar† A.J. Holloway 《Monthly notices of the Royal Astronomical Society》2002,329(3):620-628
We have observed broad H i absorption in the radio galaxy 3C 293 using Multi-Element Radio Linked Interferometric Network (MERLIN) at 0.2-arcsec angular resolution and the Giant Meterwavelength Radio Telescope (GMRT) at arcsec resolution. Extensive H i is found in absorption across the centre of this peculiar radio galaxy, allowing a detailed study of the dynamics of the neutral gas on linear scales down to ∼160 pc. In optical depth position–velocity diagrams across the central few kpc we detect a distinct velocity gradient of 179 km s−1 arcsec−1 associated with the broad absorption. This is interpreted as a ring of neutral gas rotating around the suspected position of the active galactic nucleus (AGN) . The radius of this high velocity gradient ring is found to be >0.74 arcsec (600 pc), implying an upper limit upon the enclosed mass of , assuming a near edge-on disc with an inclination of i . The optical depth of H i is mapped across the entire central region of 3C 293 showing enhancements of a factor of 4 in the areas that are co-spatial with dust lanes seen in Hubble Space Telescope ( HST ) imaging of this galaxy. 相似文献
765.
766.
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768.
Guang-Zhong Xie Luo-En Chen Huai-Zhen Li Li-Sheng Mao Hong Dai Zhao-Hua Xie Li Ma Shu-Bai Zhou National Astronomical Observatories Yunnan Observatory Chinese Academy of Science Kunming Yunnan Astrophysics Center Yunnan University Kunming Physics Department Yunnan Normal University Kunming Physics Department Yuxi Normal College Yuxi Graduate University of Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(5):463-470
Black hole mass is one of the fundamental physical parameters of active galactic nuclei (AGNs), for which many methods of estimation have been proposed. One set of methods assumes that the broad-line region (BLR) is gravitationally bound by the central black hole potential, so the black hole mass can be estimated from the orbital radius and the Doppler velocity. Another set of methods assumes the observed variability timescale is determined by the orbital timescale near the innermost stable orbit around the Schwarzschild black hole or the Kerr black hole, or by the characteristic timescale of the accretion disk. We collect a sample of 21 AGNs, for which the minimum variability timescales have been obtained and their black hole masses (Mσ) have been well estimated from the stellar velocity dispersion or the BLR size-luminosity relation. Using the minimum variability timescales we estimated the black hole masses for 21 objects by the three different methods, the results are denoted by Ms, Mk and Md, respectively. We compared each of them with Mσindividually and found that: (1) using the minimum variability timescale with the Kerr black hole theory leads to small differences between Mσand Mk, none exceeding one order of magnitude, and the mean difference between them is about 0.53 dex; (2) using the minimum variability timescale with the Schwarzschild black hole theory leads to somewhat larger difference between Mσand Ms: larger than one order of magnitude for 6 of the 21 sources, and the mean difference is 0.74 dex; (3) using the minimum variability timescale with the accretion disk theory leads to much larger differences between Mσand Md, for 13 of the 21 sources the differences are larger than two orders of magnitude; and the mean difference is as high as about 2.01 dex. 相似文献
769.
V. K. Kapahi 《Journal of Astrophysics and Astronomy》1995,16(1):1-36
Radio maps at 5 GHz with an angular resolution of 1 to 2 arcsec and a dynamic range ≳ 200:1 are presented for a sample of
45 radio quasars at redshifts between 0.2 and 1.5. The sources were imaged from observations made with the Very Large Array
with the aim of investigating the epoch dependence of misalignments and asymmetries in their extended radio structure. Maps
of some of the larger radio sources are presented also at a frequency of 1.5 GHz with a typical angular resolution of ≈ 4
arcsec. The radio structure of most of the quasars reported here has been delineated in considerably greater detail than available
in the literature. 相似文献
770.
Jennifer A. Grimes Steve Rawlings Chris J. Willott 《Monthly notices of the Royal Astronomical Society》2005,359(4):1345-1355
We extend our previous analysis which used generalized luminosity functions (GLFs) to predict the number of quasars and galaxies in low-radio-frequency-selected samples as a function of redshift, radio luminosity, narrow-emission-line luminosity and type of unified scheme. Our extended analysis incorporates the observed submillimetre (850-μm) flux densities of radio sources, employs a new method which allows us to deal with non-detections, and focuses on the high-luminosity population. First, we conclude that the submillimetre luminosity L 850 of low-frequency-selected radio sources is correlated with the bolometric luminosity L bol of their quasar nuclei via an approximate scaling relation L 850 ∝ L 0.7±0.2 bol . Secondly, we conclude that there is quantitative evidence for a receding-torus-like physical process for the high-luminosity population within a two-population unified scheme for radio sources; this evidence comes from the fact that radio quasars are brighter in both narrow emission lines and submillimetre luminosity than radio galaxies matched in radio luminosity and redshift. Thirdly, we note that the combination of a receding-torus-like scheme and the assumption that the observed submillimetre emission is dominated by quasar-heated dust yields a scaling relation L 850 ∝ L 1/2 bol which is within the errors of that determined here for radio-selected quasars, and consistent with that inferred for radio-quiet quasars. 相似文献