94 research outputs found
Power density spectrum of NGC 5548 and the nature of its variability
We derive power density spectra in the optical and X-ray band in the
timescale range from several years down to a day. We suggest that the
optical power density spectrum consists of two separate components: long
timescale variations and short timescale variations, with the dividing
timescale around 100 days. The shape of the short timescale component is
similar to X-ray power density spectrum which is consistent with the
interpretation of short timescale optical variations being caused by X-ray
reprocessing. We show that the observed optical long timescale variability is
consistent with thermal pulsations of the accretion disc.Comment: Accepted for publication in The Monthly Notices of the Royal
Astronomical Societ
Models of optical/UV continuum in AGN. Constraints from NGC 5548 monitoring campaign
We analyse the data from the optical/IUE observational campaign of the
Seyfert galaxy NGC 5548 in the context of 10 phenomenological models. On the
basis of the optical/UV data as well as constraints from the X-ray observations
we can favour one model of the nucleus: an accretion disc with an inner radius
cutoff surrounded by a hot corona. The second acceptable model for optical/UV
data is a distribution of optically thin clouds. However, X-ray constraints
which were crucial in the analysis of disc type models could not have been
applied; further development of this model is necessary.Comment: TeX file, uses epsf.sty and mn.tex, 12 pages, 6 Postscript figures,
accepted for publication in MNRAS (Sept. 96
Variability of accretion flow in the core of the Seyfert galaxy NGC 4151
We analyze observations of the Seyfert galaxy NGC 4151 covering 90 years in
the optical band and 27 years in the 2-10 keV X-ray band. We compute the
Normalized Power Spectrum Density (NPSD), the Structure Function (SF) and the
Autocorrelation Function (ACF) for these data. The results show that the
optical and X-ray variability properties are significantly different. X-ray
variations are predominantly in the timescale range of 5 - 1000 days. The
optical variations have also a short timescale component which may be related
to X-ray variability but the dominant effect is the long timescale variability,
with timescales longer than 10 years. We compare our results with
observations of NGC 5548 and Cyg X-1. We conclude that the long timescale
variability may be caused by radiation pressure instability in the accretion
disk, although the observed timescale in NGC 4151 is by a factor of few longer
than expected. X-ray variability of this source is very similar to what is
observed in Cyg X-1 but scaled with the mass of the black hole, which suggests
that the radiation pressure instability does not affect considerably the X-ray
production.Comment: 21 pages, 19 figures, 4 tables, accepted for publication in MNRA
Universal spectral shape of high accretion rate AGN
The spectra of quasars and NLS1 galaxies show surprising similarity in their
spectral shape. They seem to scale only with the accretion rate. This is in
contradiction with the simple expectations from the standard disk model which
predicts lower disk temperature for higher black hole mass. Here we consider
two mechanisms modifying the disk spectrum: the irradiation of the outer disk
due to the scattering of the flux by the extended ionized medium (warm absorber
and the development of the warm Comptonizing disk skin under the effect of the
radiation pressure instability. Those two mechanisms seem to lead to a spectrum
which indeed roughly scales, as observed, only with the accretion rate. The
scenario applies only to objects with relatively high luminosity to the
Eddington luminosity ratio for which disk evaporation is inefficient.Comment: 14 pages, 14 figures, 1 table, accepted for publication in A&
Subaru High-Dispersion Spectroscopy of Narrow-Line Region in the Seyfert Galaxy NGC 4151
We report on a study of forbidden emission-line spectrum of nearby Seyfert
1.5 galaxy NGC 4151 based on the high-resolution (R ~ 45,000) optical spectrum
obtained by using the High Dispersion Spectrograph boarded on the Subaru
Telescope. The profile parameters such as the emission-line widths, the
velocity shifts from the recession velocity of the host galaxy, and the
asymmetry indices, for emission lines including very faint ones such as [Ar
IV]4712,4740 and [Fe VI]5631,5677 are investigated. Statistically significant
correlations between the measured profile parameters and the critical densities
of transitions are found while there are no meaningful correlations between the
profile parameters and the ionization potentials of ions. By comparing the
results with photoionization model calculations, we remark that a simple
power-law distribution of the gas density which is independent of the radius
from the nucleus cannot explain the observed correlation between the
emission-line widths and the critical densities of the transitions. Taking the
additional dense gas component expected to exist at the innermost of the
narrow-line region into account, the observed correlations between the
emission-line width and the critical density of the transitions can be
understood since high-critical-density emission lines can arise at such
relatively inner regions even if their ionization potentials are low. The
observed correlation between the blueshift amounts of emission lines and the
critical densities of the ions is also explained if such dense gas clouds
located closer to the nucleus have larger outflowing velocities.Comment: 19 pages and 1 separate jpeg figure. Accepted for publication in A
High Energy Neutrinos from Quasars
We review and clarify the assumptions of our basic model for neutrino
production in the cores of quasars, as well as those modifications to the model
subsequently made by other workers. We also present a revised estimate of the
neutrino background flux and spectrum obtained using more recent empirical
studies of quasars and their evolution. We compare our results with other
thoeretical calculations and experimental upper limits on the AGN neutrino
background flux. We also estimate possible neutrino fluxes from the jets of
blazars detected recently by the EGRET experiment on the Compton Gamma Ray
Observatory. We discuss the theoretical implications of these estimates.Comment: 14 pg., ps file (includes figures), To be published in Space Science
Review
The geoaccumulation index and enrichment factor of mercury in mangrove sediment of Port Klang, Selangor, Malaysia.
Mangrove areas are important to the ecosystem. One of its crucial functions is as a sink of pollutants, especially metal ions. However, the accumulation of metals in mangrove sediment can generate negative impacts on plant growth, microbial activity, and soil fertility. Apart from that, the severity of the impact is highly influenced by the type of metal found in the sediment and the quality of sediment itself. One of the metals that have adverse effects on the environment is mercury. The objectives of this study are to determine the concentration and distribution of mercury and to assess the enrichment of mercury in Port Klang mangrove sediment by using geoaccumulation index and enrichment factor. Sediment samples were collected from 30 sampling points that cover Langat River and Klang River estuaries, Lumut Straits, Pulau Klang, and Pulau Indah. During sampling, water parameters such as pH, salinity, electrical conductivity, and total dissolved solids were measured in situ, whereas the total mercury in sediment samples was determined at the laboratory using inductively coupled plasma mass spectrometry. In this study, mercury was found to be concentrated along Lumut Strait especially in the mixing zone near the confluence of Langat River and at the jetty to Pulau Ketam. The geoaccumulation index and enrichment factor (calculated using logarithmized data of the reference element) found that three stations were enriched with mercury. In addition, geoaccumulation index was also observed to be more objective compared to enrichment factor whose results were influenced by the concentration of reference element used
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