11,119 research outputs found
The geometry of sound rays in a wind
We survey the close relationship between sound and light rays and geometry.
In the case where the medium is at rest, the geometry is the classical geometry
of Riemann. In the case where the medium is moving, the more general geometry
known as Finsler geometry is needed. We develop these geometries ab initio,
with examples, and in particular show how sound rays in a stratified atmosphere
with a wind can be mapped to a problem of circles and straight lines.Comment: Popular review article to appear in Contemporary Physic
On variability and spectral distortion of the fluorescent iron lines from black-hole accretion discs
We investigate properties of iron fluorescent line arising as a result of
illumination of a black hole accretion disc by an X-ray source located above
the disc surface. We study in details the light-bending model of variability of
the line, extending previous work on the subject.
We indicate bending of photon trajectories to the equatorial plane, which is
a distinct property of the Kerr metric, as the most feasible effect underlying
reduced variability of the line observed in several objects. A model involving
an X-ray source with a varying radial distance, located within a few central
gravitational radii around a rapidly rotating black hole, close to the disc
surface, may explain both the elongated red wing of the line profile and the
complex variability pattern observed in MCG--6-30-15 by XMM-Newton.
We point out also that illumination by radiation which returns to the disc
(following the previous reflection) contributes significantly to formation of
the line profile in some cases. As a result of this effect, the line profile
always has a pronounced blue peak (which is not observed in the deep minimum
state in MCG--6-30-15), unless the reflecting material is absent within the
innermost 2--3 gravitational radii.Comment: 24 pages, 22 figures. Accepted for publication in MNRA
Kink Arrays and Solitary Structures in Optically Biased Phase Transition
An interphase boundary may be immobilized due to nonlinear diffractional
interactions in a feedback optical device. This effect reminds of the Turing
mechanism, with the optical field playing the role of a diffusive inhibitor.
Two examples of pattern formation are considered in detail: arrays of kinks in
1d, and solitary spots in 2d. In both cases, a large number of equilibrium
solutions is possible due to the oscillatory character of diffractional
interaction.Comment: RevTeX 13 pages, 3 PS-figure
Interestingness of traces in declarative process mining: The janus LTLPf Approach
Declarative process mining is the set of techniques aimed at extracting behavioural constraints from event logs. These constraints are inherently of a reactive nature, in that their activation restricts the occurrence of other activities. In this way, they are prone to the principle of ex falso quod libet: they can be satisfied even when not activated. As a consequence, constraints can be mined that are hardly interesting to users or even potentially misleading. In this paper, we build on the observation that users typically read and write temporal constraints as if-statements with an explicit indication of the activation condition. Our approach is called Janus, because it permits the specification and verification of reactive constraints that, upon activation, look forward into the future and backwards into the past of a trace. Reactive constraints are expressed using Linear-time Temporal Logic with Past on Finite Traces (LTLp f). To mine them out of event logs, we devise a time bi-directional valuation technique based on triplets of automata operating in an on-line fashion. Our solution proves efficient, being at most quadratic w.r.t. trace length, and effective in recognising interestingness of discovered constraints
Energetics of X-ray Cavities and Radio Lobes in Galaxy Clusters
We describe the formation and evolution of X-ray cavities in the hot gas of
galaxy clusters. The cavities are formed only with relativistic cosmic rays
that eventually diffuse into the surrounding gas. We explore the evolution of
cavities formed with a wide range of cosmic ray diffusion rates. In previous
numerical simulations cavities are formed by injecting ultra-hot but
non-relativistic gas which increases the global thermal energy, offsetting
radiative losses in the gas and helping to solve the cooling flow problem.
Contrary to these results, we find that X-ray cavities formed solely by cosmic
rays have a global cooling effect. As the cluster gas is displaced by cosmic
rays, a global expansion of the cluster gas occurs with associated cooling that
exceeds the heating by shock waves as the cavity forms. Most cosmic rays in our
cavity evolutions do not move beyond the cooling radius even after 1 Gyr. The
gas density is depressed by cosmic rays, becomes buoyant, and undergoes a
significant outward mass transfer within the cooling radius, carrying cosmic
rays and relatively low entropy gas to distant regions in the cluster where it
remains for times exceeding the local cooling time in the hot gas. This
post-cavity mass outflow due to cosmic ray buoyancy may contribute toward
solving the cooling flow problem. We describe the energetics, size, stability
and buoyant rise of X-ray cavities in detail, showing how each depends on the
rate of cosmic ray diffusion.Comment: 17 pages, 8 figures, accepted by Ap
Atomic Deuterium Adsorbed on the Surface of Liquid Helium
We investigate deuterium atoms adsorbed on the surface of liquid helium in
equilibrium with a vapor of atoms of the same species. These atoms are studied
by a sensitive optical method based on spectroscopy at a wavelength of 122 nm,
exciting the 1S-2P transition. We present a direct measurement of the
adsorption energy of deuterium atoms on helium and show evidence for the
existence of resonantly enhanced recombination of atoms residing on the surface
to molecules.Comment: 6 pages 4 figure
Control-volume representation of molecular dynamics
A Molecular Dynamics (MD) parallel to the Control Volume (CV) formulation of
fluid mechanics is developed by integrating the formulas of Irving and
Kirkwood, J. Chem. Phys. 18, 817 (1950) over a finite cubic volume of molecular
dimensions. The Lagrangian molecular system is expressed in terms of an
Eulerian CV, which yields an equivalent to Reynolds' Transport Theorem for the
discrete system. This approach casts the dynamics of the molecular system into
a form that can be readily compared to the continuum equations. The MD
equations of motion are reinterpreted in terms of a
Lagrangian-to-Control-Volume (\CV) conversion function , for
each molecule . The \CV function and its spatial derivatives are used to
express fluxes and relevant forces across the control surfaces. The
relationship between the local pressures computed using the Volume Average (VA,
Lutsko, J. Appl. Phys 64, 1152 (1988)) techniques and the Method of Planes
(MOP, Todd et al, Phys. Rev. E 52, 1627 (1995)) emerges naturally from the
treatment. Numerical experiments using the MD CV method are reported for
equilibrium and non-equilibrium (start-up Couette flow) model liquids, which
demonstrate the advantages of the formulation. The CV formulation of the MD is
shown to be exactly conservative, and is therefore ideally suited to obtain
macroscopic properties from a discrete system.Comment: 19 pages, 15 figure
Phytoplankton chlorophyte structure as related to ENSO events in a saline lowland river (Salado River, Buenos Aires, Argentina)
We analyzed the phytoplankton present in the lower sector of the Salado River (Buenos Aires, Argentina) for 10 years (1995â2005) and detected significant changes occurring in chlorophyte abundance and species richness during La Niña event (1998â1999), which period was analyzed throughout the entire basin (main stream and tributaries). We compared the physicochemical and biologic variables between two El NiñoâLa NiñaâSouthern Oscillation (ENSO) periods â El Niño (March 1997âJanuary 1998) and La Niña (May 1998âMay 1999) â to identify possible indicators of a relationship between climatic anomalies and chlorophyte performance. Chlorophyte density increased during the La Niña. Under normal or extreme hydrologic conditions, mobile (Chlamydomonas spp.) and nonmobile (Monoraphidium spp.) chlorophytes codominated. These species belonged to Reynolds's functional groups X1 and X2, those typical of nutrient-enriched environments. Comparative analyses between El Niño and La Niña periods indicated significant differences in physicochemical (K+, dissolved polyphenols, particulate reactive phosphorus, alkalinity, pH) and biologic (species diversity and richness, phytoplankton and chlorophyte total densities) variables between the two periods at all basin sites. During the La Niña condition, species richness was greater owing to interconnected shallow lakes and drainage-channel inputs, while the Shannon diversity index was lower because of the high abundance values of Monoraphidium minutum. A detailed analysis of the chlorophytes in the entire basin, indicated that changes in density and species dominance occurred on a regional scale although diverse chlorophyte assemblages were identified in the different sectors of the Salado River basin. After La Niña event, the entire basin had the potential to revert to the previous density values, showing the resilience to global environmental changes and the ability to reestablish the general conditions of stability.Fil: Solari, LĂa Cristina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de LimnologĂa "Dr. RaĂșl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de LimnologĂa; ArgentinaFil: Gabellone, Nestor Adrian. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de LimnologĂa "Dr. RaĂșl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de LimnologĂa; ArgentinaFil: Claps, Maria Cristina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de LimnologĂa "Dr. RaĂșl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de LimnologĂa; ArgentinaFil: Casco, Maria Adela. Universidad Nacional de la Plata. Facultad de Ciencias Naturales y Museo. Division Ficologia; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Quaini, Karina Paola. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de LimnologĂa "Dr. RaĂșl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de LimnologĂa; ArgentinaFil: Neschuk, Nancy Carolina. DirecciĂłn Provincial de Saneamiento y Obras HidrĂĄulicas del Ministerio de Infraestructura, Vivienda y Servicios PĂșblicos de la Provincia de Buenos Aires; Argentin
Testing Comptonizing coronae on a long BeppoSAX observation of the Seyfert 1 galaxy NGC 5548
We test accurate models of Comptonization spectra over the high quality data
of the BeppoSAX long look at NGC 5548, allowing for different geometries of the
scattering region, different temperatures of the input soft photon field and
different viewing angles. We find that the BeppoSAX data are well represented
by a plane parallel or hemispherical corona viewed at an inclination angle of
30. For both geometries the best fit temperature of the soft photons
is close to 15 eV. The corresponding best fit values of the hot
plasma temperature and optical depth are 250--260 keV and
0.16--0.37 for the slab and hemisphere respectively. These values
are substantially different from those derived fitting the data with a
power-law + cut off approximation to the Comptonization component (kT_{\rm
e}\lta 60 keV, 2.4). This is due to the fact that accurate
Comptonization spectra in anisotropic geometries show "intrinsic" curvature
which reduces the necessity of a high energy cut-off. The Comptonization
parameter derived for the slab model {is} larger than predicted for a two phase
plane parallel corona in energy balance, suggesting that a more
``photon-starved'' geometry is necessary. The spectral softening detected
during a flare which occurred in the central part of the observation
corresponds to a decrease of the Comptonization parameter, probably associated
with an increase of the soft photon luminosity, the {hard} photon luminosity
remaining constant.Comment: 36 pages, 9 figures, accepted by Ap
- âŠ