9,544 research outputs found
The mass estimate in narrow-line Seyfert 1 galaxies
It is possible that narrow-line Seyfert 1 galaxies (NLS1s) are in the early
stage of active galactic nuclei (AGNs) evolution. It is important to estimate
the mass of supermassive black hole (SBH) in NLS1s. Here we considered the
different kinds of methods to estimate the SBH masses in NLS1s. The virial mass
from the H linewidth assuming random orbits of broad line regions (BLRs)
is consistent with that from the statured soft X-ray luminosity, which showed
that most of NLS1s are in the super-Eddington accretion state. The mass from
the [O III] linewidth is systematically larger than that from above two
methods. It is necessary to measure he bulge stellar dispersion and/or bulge
luminosity in NLS1s.Comment: 2 Pages, 1 figure, in Prof. IAU Symposium No. 222, "The interplay
among Black Holes, Stars and ISM in Galactic Nuclei ", eds. T.
Storchi-Bergmann, Luis Ho and H. R. Schmit
General Fast Sampling Theorems for Nonlinear Systems
This paper is concerned with the gap metric approach to controller discretisation problems for continuous-time nonlinear systems with disturbances in both input and output channels. The principal idea is to construct a discrete controller based on a given stabilizing continuous time controller via a fast sampling and hold procedure and to calculate the gap between the two controllers. It is expected that, under general conditions, the computed gap depends on the discrete sample size and the faster the sample rate, the smaller the gap and, therefore, existing gap metric robust stability theorems can be applied to obtain both stability and performance results for the appropriately discretised controller. This is shown for the case of memoryless controllers and for a more general class of controllers specified by stable, causal operators. In both cases, both regional and global results are obtained under respective local and global incremental stability assumptions on the controllers
Twin roll casting and melt conditioned twin-roll casting of magnesium alloys
Recently, BCAST at Brunel University has developed a MCAST (melt conditioning by advanced shear technology) process for conditioning liquid metal at temperature either above or bellow the alloy liquidus using a high shear twin-screw mechanism. The MCAST process has now been combined with the twin roll casting (TRC) process to form an innovative technology, namely, the melt conditioned twin roll casting (MC-TRC) process for casting Al-alloy and Mg-alloy strips. During the MC-TRC process, liquid alloy with a specified temperature is continuously fed into the MCAST machine. By intensive shearing under the high shear rate and high intensity of turbulence, the liquid is transformed into conditioned melt with uniform temperature and composition throughout the whole volume. The conditioned melt is then fed continuously into the twin-roll caster for strip production. The experimental results show that the AZ91D MC-TRC strips with different thicknesses have fine and uniform microstructure. The strip consists of equiaxed grains with a mean size of 60-70μm. The strip displays extremely uniform grain size and composition throughout the whole cross-section. Investigation also shows that both TRC and MC-TRC processes with reduced deformation are effective to reduce the formation of defects, particularly the formation of the central line segregations
Black hole masses in narrow-line Seyfert 1 galaxies
The masses of central supermassive black holes in a soft X-ray selected
sample of the narrow-line Seyfert 1 galaxies (NLS1s) are estimated by some
different methods to test their theoretical models. Apart from the methods
using the H linewidth and the [O III] linewidth, soft X-ray excess as a
prominent character of NLS1s is used to estimate the black hole masses. The
virial mass derived from the H linewidth assuming random orbits of
broad-line reigns (BLRs) is consistent with that from the soft X-ray bump
luminosity for NLS1s but with a larger scatter. The virial black hole masses
showed that most of NLS1s are in the super-Eddington accretion state while most
of broad-line Seyfert 1 galaxies (BLS1s) are not. We found that the black hole
mass estimated from [O III] linewidth is not in agreement with above two
methods. Using the Eddington limit relation for the super-Eddington accretion
suggested by Wang (2004), we found that there are 16 NLS1s satisfied with this
Eddington limit relation. The masses of these 16 NLS1s derived from X-ray
luminosity are systematically larger than that from H linewidth assuming
random BLRs orbits. If the mass derived from X-ray luminosity is true, the mean
disk inclination to the line of sight in these 16 NLS1s is about ,
which provided new support for the pole-on orientation effect in NLS1s.Comment: 6 pages, 3 figures, accepted by MNRA
On X-ray variability in narrow line and broad line AGNs
We assembled a sample of broad line and narrow lines AGNs observed by ASCA,
whose excess variances have been determined. The central black hole masses in
this sample can be obtained from the reverberation mapping method and the width
of H emission line. Using the black hole masses and the bolometric
luminosity, the Eddington ratio can also be obtained. We confirmed the strong
anti-correlation between X-ray variability and the central black mass founded
by Lu & Yu. We further found that narrow line Seyfert 1 galaxies (NLS1s) also
follow this relation with a larger scatter and there is only a weak correlation
between the X-ray variability and the Eddington ratio, which suggest that rapid
variability and narrow lines in NLS1s are mainly due to small central black
holes in NLS1s, not the difference of circumnuclear gas around NLS1s. A strong
correlation was found between the hard X-ray photon index and the Eddington
ratio. If the suggestion of two distinct accretion classes, namely ADAF and
thin disk accretion, in AGNs (Lu & Yu) is correct, the strong photon
index-Eddington ration correlation showed that there exists a kind of two zone
accretion disk, in which the outer zone is a thin disk, and the inner zone is
an ADAF disk. Otherwise, the accretion process is the thin disk accretion and
the ADAF accretion is not required.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
- …