3,913 research outputs found
Constraints on a strong X-ray flare in the Seyfert galaxy MCG-6-30-15
We discuss implications of a strong flare event observed in the Seyfert
galaxy MCG-6-30-15 assuming that the emission is due to localized magnetic
reconnection. We conduct detailed radiative transfer modeling of the
reprocessed radiation for a primary source that is elevated above the disk. The
model includes relativistic effects and Keplerian motion around the black hole.
We show that for such a model setup the observed time-modulation must be
intrinsic to the primary source. Using a simple analytical model we then
investigate time delays between hard and soft X-rays during the flare. The
model considers an intrinsic delay between primary and reprocessed radiation,
which measures the geometrical distance of the flare source to the reprocessing
sites. The observed time delays are well reproduced if one assumes that the
reprocessing happens in magnetically confined, cold clouds.Comment: 4 pages, 2 figures, proceedings of a talk given at the symposium 238
at the IAU General Assembly 200
X-ray Variability of AGN and the Flare Model
Short-term variability of X-ray continuum spectra has been reported for
several Active Galactic Nuclei. Significant X-ray flux variations are observed
within time scales down to 10^3-10^5 seconds. We discuss short variability time
scales in the frame of the X-ray flare model, which assumes the release of a
large hard X-ray flux above a small portion of the accretion disk. The
resulting observed X-ray spectrum is composed of the primary radiation and of a
reprocessed Compton reflection component that we model with numerical radiative
transfer simulations. The incident hard X-rays of the flare will heat up the
atmosphere of the accretion disk and hence induce thermal expansion.
Eventually, the flare source will be surrounded by an optically thick medium,
which should modify the observed spectra.Comment: 4 pages, 3 figures, accepted proceedings for a talk at the conference
"AGN variability from the X-rays to the radio", June 2004, Crimean
Observator
Iron lines from transient and persisting hot spots on AGN accretion disks
[abridged] We model the X-ray reprocessing from a strong co-rotating flare
above an accretion disk in active galactic nuclei. We explore the horizontal
structure and evolution of the underlying hot spot. To obtain the spectral
evolution seen by a distant observer, we apply a general relativity ray-tracing
technique. We concentrate on the energy band around the iron K-line, where the
relativistic effects are most pronounced. Persistent flares lasting for a
significant fraction of the orbital time scale and short, transient flares are
considered. In our time-resolved analysis, the spectra recorded by a distant
observer depend on the position of the flare/spot with respect to the central
black hole. If the flare duration significantly exceeds the light travel time
across the spot, then the spot horizontal stratification is unimportant. On the
other hand, if the flare duration is comparable to the light travel time across
the spot radius, the lightcurves exhibit a typical asymmetry in their time
profiles. The sequence of dynamical spectra proceeds from more strongly to less
strongly ionized re-emission. At all locations within the spot the spectral
intensity increases towards edge-on emission angles, revealing the limb
brightening effect. Future X-ray observatories with significantly larger
effective collecting areas will enable to spectroscopically map out the
azimuthal irradiation structure of the accretion disk and to localize
persistent flares. If the hot spot is not located too close to the marginally
stable orbit of the black hole, it will be possible to probe the reflecting
medium via the sub-structure of the iron K-line. Indications for transient
flares will only be obtained from analyzing the observed lightcurves on the
gravitational time scale of the accreting supermassive black hole.Comment: 15 pages, 8 figures, accepted by Astronomy & Astrophysic
Activity and abundance of methane-oxidizing bacteria in secondary forest and manioc plantations of Amazonian Dark Earth and their adjacent soils
The oxidation of atmospheric CH4 in upland soils is mostly mediated by uncultivated groups of microorganisms that have been identified solely by molecular markers, such as the sequence of the pmoA gene encoding the ?-subunit of the particulate methane monooxygenase enzyme. The objective of this work was to compare the activity and diversity of methanotrophs in Amazonian Dark Earth soil (ADE, Hortic Anthrosol) and their adjacent non-anthropic soil. Secondly, the effect of land use in the form of manioc cultivation was examined by comparing secondary forest and plantation soils. CH4 oxidation potentials were measured and the structure of the methanotroph communities assessed by quantitative PCR (qPCR) and amplicon pyrosequencing of pmoA genes. The oxidation potentials at low CH4 concentrations (10 ppm of volume) were relatively high in all the secondary forest sites of both ADE and adjacent soils. CH4 oxidation by the ADE soil only recently converted to a manioc plantation was also relatively high. In contrast, both the adjacent soils used for manioc cultivation and the ADE soil with a long history of agriculture displayed lower CH4 uptake rates. Amplicon pyrosequencing of pmoA genes indicated that USC?, Methylocystis and the tropical upland soil cluster (TUSC) were the dominant groups depending on the site. By qPCR analysis it was found that USC? pmoA genes, which are believed to belong to atmospheric CH4 oxidizers, were more abundant in ADE than adjacent soil. USC? pmoA genes were abundant in both forested and cultivated ADE soil, but were below the qPCR detection limit in manioc plantations of adjacent soil. The results indicate that ADE soils can harbor high abundances of atmospheric CH4 oxidizers and are potential CH4 sinks, but as in other upland soils this activity can be inhibited by the conversion of forest to agricultural plantations
The puzzle of the soft X-ray excess in AGN: absorption or reflection?
The 2-10 keV continuum of AGN is generally well represented by a single power
law. However, at smaller energies the continuum displays an excess with respect
to the extrapolation of this power law, called the ''soft X-ray excess''. Until
now this soft X-ray excess was attributed, either to reflection of the hard
X-ray source by the accretion disk, or to the presence of an additional
comptonizing medium, giving a steep spectrum. An alternative solution proposed
by Gierlinski and Done (2004) is that a single power law well represents both
the soft and the hard X-ray emission and the impression of the soft X-ray
excess is due to absorption of a primary power law by a relativistic wind. We
examine the advantages and drawbacks of reflection versus absorption models,
and we conclude that the observed spectra can be well modeled, either by
absorption (for a strong excess), or by reflection (for a weak excess). However
the physical conditions required by the absorption models do not seem very
realistic: we would prefer an ''hybrid model''.Comment: 4 pages, 3 figures, abstracts SF2A-2005, published by EDP-Sciences
Conference Serie
The analysis of solar models: Neutrinos and oscillations
Tests of solar neutrino flux and solar oscillation frequencies were used to assess standard stellar structure theory. Standard and non-standard solar models are enumerated and discussed. The field of solar seismology, wherein the solar interior is studied from the measurement of solar oscillations, is introduced
Activity and abundance of methane-oxidizing bacteria in secondary forest and manioc plantations of Amazonian Dark Earth and their adjacent soils.
The oxidation of atmospheric CH4 in upland soils is mostly mediated by uncultivated groups of microorganisms that have been identified solely by molecular markers, such as the sequence of the pmoA gene encoding the -subunit of the particulate methane monooxygenase enzyme. The objective of this work was to compare the activity and diversity of methanotrophs in Amazonian Dark Earth soil (ADE, Hortic Anthrosol) and their adjacent non-anthropic soil
Intracule functional models. II. Analytically integrable kernels
We present, within the framework of intracule functional theory (IFT), a class of kernels whose correlation integrals can be found in closed form. This approach affords three major advantages over other kernels that we have considered previously; ease of implementation, computational efficiency, and numerical stability. We show that even the simplest member of the class yields reasonable estimates of the correlation energies of 18 atomic and 56 molecular systems and we conclude that this kernel class will prove useful in the development of future IFT models
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