2,338 research outputs found
The converging inflow spectrum is an intrinsic signature for a black hole: Monte-Carlo simulations of Comptonization on free-falling electrons
An accreting black hole is, by definition, characterized by the drain.
Namely, the matter falls into a black hole much the same way as water
disappears down a drain - matter goes in and nothing comes out. As this can
only happen in a black hole, it provides an unique way to see it. The accretion
proceeds almost in free fall close to the black hole horizon. In this paper we
calculate (by using Monte -Carlo simulations) the specific features of X-ray
spectra formed as a result of upscattering of the soft (disk) photons in the
converging inflow (CI) within about 3 Schwarzschild radii of the black hole.
The full relativistic treatment has been implemented to reproduce these
spectra. We show that spectra in the soft state of black hole systems can be
described as the sum of a thermal (disk) component and the convolution of some
fraction of this component with the CI upscattering spread function. The latter
boosted photon component is seen as an extended power-law at energies much
higher than the characteristic soft photons energy. We demonstrate the
stability of the power spectral index (alpha= 1.8) over a wide range of the
plasma temperature 0-10 keV and mass accretion rates (higher than 2 in
Eddington units). We also demonstrate that the sharp high energy cutoff occurs
at energies of 200-400 keV which are related to the average rest energy of
electrons impinging upon the horizon. The spectrum is practically identical to
the standard thermal Comptonization spectrum when the CI plasma temperature is
getting of order of 50 keV (hard state of BHS). Also, the change of spectral
shapes from the soft to the hard X-ray state is clearly to be related with the
temperature of the bulk flow. These Monte-Carlo simulated CI spectra are then a
inevitable stamp of the BHS.Comment: 30 pages TeX format, 6 PS figures, accepted for ApJ Main Journa
Surface diffusion coefficients by thermodynamic integration: Cu on Cu(100)
The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated
using thermodynamic integration within the transition state theory. The results
are found to be in excellent agreement with the essentially exact values from
molecular-dynamics simulations. The activation energy and related entropy are
shown to be effectively independent of temperature, thus establishing the
validity of the Arrhenius law over a wide range of temperatures. Our study
demonstrates the equivalence of diffusion rates calculated using thermodynamic
integration within the transition state theory and direct molecular-dynamics
simulations.Comment: 4 pages (revtex), two figures (postscript
PAH Emission from Ultraluminous Infrared Galaxies
We explore the relationships between the Polycyclic Aromatic Hydrocarbon
(PAH) feature strengths, mid-infrared continuum luminosities, far-infrared
spectral slopes, optical spectroscopic classifications, and silicate optical
depths within a sample of 107 ULIRGs observed with the Infrared Spectrograph on
the Spitzer Space Telescope. The detected 6.2 micron PAH equivalent widths
(EQWs) in the sample span more than two orders of magnitude (0.006-0.8 micron),
and ULIRGs with HII-like optical spectra or steep far-infrared spectral slopes
(S_{25} / S_{60} < 0.2) typically have 6.2 micron PAH EQWs that are half that
of lower-luminosity starbursts. A significant fraction (~40-60%) of HII-like,
LINER-like, and cold ULIRGs have very weak PAH EQWs. Many of these ULIRGs also
have large (tau_{9.7} > 2.3) silicate optical depths. The far-infrared spectral
slope is strongly correlated with PAH EQW, but not with silicate optical depth.
In addition, the PAH EQW decreases with increasing rest-frame 24 micron
luminosity. We argue that this trend results primarily from dilution of the PAH
EQW by continuum emission from dust heated by a compact central source,
probably an AGN. High luminosity, high-redshift sources studied with Spitzer
appear to have a much larger range in PAH EQW than seen in local ULIRGs, which
is consistent with extremely luminous starburst systems being absent at low
redshift, but present at early epochs.Comment: 15 pages, 9 Figures; Accepted for publication in Ap
Three-body correlations in Borromean halo nuclei
Three-body correlations in the dissociation of two-neutron halo nuclei are
explored using a technique based on intensity interferometry and Dalitz plots.
This provides for the combined treatment of both the n-n and core-n
interactions in the exit channel. As an example, the breakup of 14Be into
12Be+n+n by Pb and C targets has been analysed and the halo n-n separation
extracted. A finite delay between the emission of the neutrons in the reaction
on the C target was observed and is attributed to 13Be resonances populated in
sequential breakup.Comment: 5 pages, 4 figures, submitted to PR
Gaussian multiplicative Chaos for symmetric isotropic matrices
Motivated by isotropic fully developed turbulence, we define a theory of
symmetric matrix valued isotropic Gaussian multiplicative chaos. Our
construction extends the scalar theory developed by J.P. Kahane in 1985
Competition-based model of pheromone component ratio detection in the moth
For some moth species, especially those closely interrelated and sympatric, recognizing a specific pheromone component concentration ratio is essential for males to successfully locate conspecific females. We propose and determine the properties of a minimalist competition-based feed-forward neuronal model capable of detecting a certain ratio of pheromone components independently of overall concentration. This model represents an elementary recognition unit for the ratio of binary mixtures which we propose is entirely contained in the macroglomerular complex (MGC) of the male moth. A set of such units, along with projection neurons (PNs), can provide the input to higher brain centres. We found that (1) accuracy is mainly achieved by maintaining a certain ratio of connection strengths between olfactory receptor neurons (ORN) and local neurons (LN), much less by properties of the interconnections between the competing LNs proper. An exception to this rule is that it is beneficial if connections between generalist LNs (i.e. excited by either pheromone component) and specialist LNs (i.e. excited by one component only) have the same strength as the reciprocal specialist to generalist connections. (2) successful ratio recognition is achieved using latency-to-first-spike in the LN populations which, in contrast to expectations with a population rate code, leads to a broadening of responses for higher overall concentrations consistent with experimental observations. (3) when longer durations of the competition between LNs were observed it did not lead to higher recognition accuracy
Alterations in vascular function in primary aldosteronism - a cardiovascular magnetic resonance imaging study
Introduction: Excess aldosterone is associated with increased cardiovascular risk. Aldosterone has a permissive effect on vascular fibrosis. Cardiovascular magnetic resonance imaging (CMR) allows study of vascular function by measuring aortic distensibility. We compared aortic distensibility in primary aldosteronism (PA), essential hypertension (EH) and normal controls and explored the relationship between aortic distensibility and pulse wave velocity (PWV).<p></p>
Methods: We studied PA (n=14) and EH (n=33) subjects and age-matched healthy controls (n=17) with CMR, including measurement of aortic distensibility, and measured PWV using applanation tonometry. At recruitment, PA and EH patients had similar blood pressure and left ventricular mass.<p></p>
Results: Subjects with PA had significantly lower aortic distensibilty and higher PWV compared to EH and healthy controls. These changes were independent of other factors associated with reduced aortic distensibility, including aging. There was a significant relationship between increasing aortic stiffness and age in keeping with physical and vascular aging. As expected, aortic distensibility and PWV were closely correlated.<p></p>
Conclusion: These results demonstrate that PA patients display increased arterial stiffness compared to EH, independent of vascular aging. The implication is that aldosterone invokes functional impairment of arterial function. The long-term implications of arterial stiffening in aldosterone excess require further study.<p></p>
Jasmonate promotes auxin-induced adventitious rooting in dark-grown Arabidopsis thaliana seedlings and stem thin cell layers by a cross-talk with ethylene signalling and a modulation of xylogenesis
Background: Adventitious roots (ARs) are often necessary for plant survival, and essential for successful micropropagation. In Arabidopsis thaliana dark-grown seedlings AR-formation occurs from the hypocotyl and is enhanced by application of indole-3-butyric acid (IBA) combined with kinetin (Kin). The same IBA + Kin-treatment induces AR-formation in thin cell layers (TCLs). Auxin is the main inducer of AR-formation and xylogenesis in numerous species and experimental systems. Xylogenesis is competitive to AR-formation in Arabidopsis hypocotyls and TCLs. Jasmonates (JAs) negatively affect AR-formation in de-etiolated Arabidopsis seedlings, but positively affect both AR-formation and xylogenesis in tobacco dark-grown IBA + Kin TCLs. In Arabidopsis the interplay between JAs and auxin in AR-formation vs xylogenesis needs investigation. In de-etiolated Arabidopsis seedlings, the Auxin Response Factors ARF6 and ARF8 positively regulate AR-formation and ARF17 negatively affects the process, but their role in xylogenesis is unknown. The cross-talk between auxin and ethylene (ET) is also important for AR-formation and xylogenesis, occurring through EIN3/EIL1 signalling pathway. EIN3/EIL1 is the direct link for JA and ET-signalling. The research investigated JA role on AR-formation and xylogenesis in Arabidopsis dark-grown seedlings and TCLs, and the relationship with ET and auxin. The JA-donor methyl-jasmonate (MeJA), and/or the ET precursor 1-aminocyclopropane-1-carboxylic acid were applied, and the response of mutants in JA-synthesis and -signalling, and ET-signalling investigated. Endogenous levels of auxin, JA and JA-related compounds, and ARF6, ARF8 and ARF17 expression were monitored. Results: MeJA, at 0.01 μM, enhances AR-formation, when combined with IBA + Kin, and the response of the early-JA-biosynthesis mutant dde2–2 and the JA-signalling mutant coi1–16 confirmed this result. JA levels early change during TCL-culture, and JA/JA-Ile is immunolocalized in AR-tips and xylogenic cells. The high AR-response of the late JA-biosynthesis mutant opr3 suggests a positive action also of 12-oxophytodienoic acid on AR-formation. The crosstalk between JA and ET-signalling by EIN3/EIL1 is critical for AR-formation, and involves a competitive modulation of xylogenesis. Xylogenesis is enhanced by a MeJA concentration repressing AR-formation, and is positively related to ARF17 expression. Conclusions: The JA concentration-dependent role on AR-formation and xylogenesis, and the interaction with ET opens the way to applications in the micropropagation of recalcitrant species
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