| 1. |
- Perry, J. J., et al.
(författare)
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Hysteresis in broad-line regions of active galactic nuclei
- 1994
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Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 271:3, s. 561-572572
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Tidskriftsartikel (refereegranskat)abstract
- The response of emission-line luminosities to changes in the observed continuum in active galaxies is commonly used for reverberation mapping (RM) of the emission-line region. The method assumes regularity in the continuum source and linearity in the line response to the continuum, and that the phase-space distribution of the line-emitting gas is constant in time. The wealth of information contained in the detailed line profiles is under-utilized in current methods of RM. The authors propose a new method to improve this situation. The authors define normalized profiles and show how their use enables one to explore details of the emission-line region that have hitherto eluded detection. Furthermore, the authors investigate the possibility of testing the validity of the assumptions as used in the standard application of RM. The authors present new methods, the hysteresis test and the Q concept - also based on the line profiles - to test the basic assumptions of RM. The test fails when applied to NGC 4151. The authors show that there is good evidence either for significantly extended and complex emission-line regions, or for changes in the mass distribution over time-scales of months or more - i.e. on time-scales comparable to the length of typical observing campaigns
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| 2. |
- Romeo, Alessandro, 1961
(författare)
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Stability of thick two-component galactic discs
- 1992
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Ingår i: Monthly Notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 256:2, s. 307-320
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Tidskriftsartikel (refereegranskat)abstract
- The thickness scales and the local parameters relevant to both the equilibrium and stability of two-component galactic disks are studied. The results obtained, as regards the vertical structure at equilibrium of two-component galactic disks, are used to investigate their local linear stability properties. Under reasonable assumptions, finite-thickness corrections to the local dispersion relation can be expressed in terms of two reduction factors weakening the response of the two components or, equivalently, lowering their equilibrium surface densities. An ansatz for such reduction factors, justified by extending the analysis performed for one-component purely stellar disks, is made, and the corresponding two-fluid marginal stability curve is studied in standard star-dominated and peculiar gas-dominated regimes. It is found that the stabilizing role of finite-thickness effects can partially counterbalance the destabilizing role of the cold interstellar gas in linear regimes.
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| 3. |
- van Groningen, Ernst
(författare)
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Further evidence for Raman scattering in RR Tel
- 1993
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Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 264:4, s. 975-979979
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Tidskriftsartikel (refereegranskat)abstract
- Raman scattering of O VIlambdalambda13O2, 1308 photons on the ground level of neutral hydrogen has been suggested as the cause of the broad emission features at 6825 and 7088 Aring in the spectra of symbiotic stars. The authors report on the detection of other Raman lines in the optical spectrum of the symbiotic star RR Tel. The lines detected are two He II Balmer lines that are Raman-scattered to wavelengths close to hydrogen Hbeta and Hgamma. Another possible detection is scattered C III lambda977, which appears at about 4977 Aring in the optical spectrum
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| 4. |
- Zhang, F. J., et al.
(författare)
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THE SUBMILLIARCSECOND RADIO STRUCTURES OF THE 2 BL LAC OBJECTS MKN421 AND 0735 + 178
- 1991
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Ingår i: Monthly notices of the Royal Astronomical Society. - Oxford : Oxford University Press. - 0035-8711 .- 1365-2966. ; 248:3, s. 566-571
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Tidskriftsartikel (refereegranskat)abstract
- Very high-resolution (approximately 0.15 mas) VLBI hybrid maps of the two BL Lac objects Mkn421 and 0735 + 178 have been produced at lambda-1.35 cm. The structures of both sources are simple. Only an unresolved core is found in Mkn421. It is magnetic field dominated, and the motion of relativistic electrons is strongly restrained by the magnetic field. Relativistic electrons with relatively low energy are responsible for the synchrotron radiation. The maps of 0735 + 178 show that the radio structure at lambda-1.35 cm is dominated by a strong core, and there is a weak component outside. We can put a lower limit to the angular velocity of the outer component (0.32 +/- 0.03) mas yr-1, corresponding to an apparent transverse speed of (5.4 +/- 0.6)c which tallies with previous results. The outer component has an extension towards the south-east, a tendency also seen in the 6-cm VLBI maps. With our 1.35-cm VLBI observations, we demonstrate that it is possible to study a new component emerging from its core during an outburst and to study its behaviour on its way out.
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