14,952 research outputs found
Implications of automatic photon quenching on compact gamma-ray sources
Aims: We investigate photon quenching in compact non-thermal sources. This
involves photon-photon annihilation and lepton synchrotron radiation in a
network that can become non-linear. As a result the gamma-ray luminosity of a
source cannot exceed a critical limit that depends only on the radius of the
source and on the magnetic field. Methods: We perform analytic and numerical
calculations that verify previous results and extend them so that the basic
properties of photon quenching are investigated. Results: We apply the above to
the 2006 TeV observations of quasar 3C279 and obtain the parameter space of
allowed values for the radius of the emitting source, its magnetic field
strength and the Doppler factor of the flow. We argue that the TeV observations
favour either a modest Doppler factor and a low magnetic field or a high
Doppler factor and a high magnetic field.Comment: 10 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Temporal signatures of leptohadronic feedback mechanisms in compact sources
The hadronic model of Active Galactic Nuclei and other compact high energy
astrophysical sources assumes that ultra-relativistic protons,
electron-positron pairs and photons interact via various hadronic and
electromagnetic processes inside a magnetized volume, producing the
multiwavelength spectra observed from these sources. A less studied property of
such systems is that they can exhibit a variety of temporal behaviours due to
the operation of different feedback mechanisms. We investigate the effects of
one possible feedback loop, where \gamma-rays produced by photopion processes
are being quenched whenever their compactness increases above a critical level.
This causes a spontaneous creation of soft photons in the system that result in
further proton cooling and more production of \gamma-rays, thus making the loop
operate. We perform an analytical study of a simplified set of equations
describing the system, in order to investigate the connection of its temporal
behaviour with key physical parameters. We also perform numerical integration
of the full set of kinetic equations verifying not only our analytical results
but also those of previous numerical studies. We find that once the system
becomes `supercritical', it can exhibit either a periodic behaviour or a damped
oscillatory one leading to a steady state. We briefly point out possible
implications of such a supercriticality on the parameter values used in Active
Galactic Nuclei spectral modelling, through an indicative fitting of the VHE
emission of blazar 3C 279.Comment: 19 pages, 20 figures, accepted for publication in MNRA
Viscosity dependence of the rates of diffusional processes
It is shown that the rates of diffusion-controlled processes may have a solvent vicosity independent part as well as a viscosity dependent part. Some relevant experiments involving intramolecular polypeptide movements are discussed, and implications for some experiments on diffusion in membranes are outlined
A self-consistent hybrid Comptonization model for broad-band spectra of accreting supermassive black holes
The nature of the broad-band spectra of supermassive accreting black holes in
active galactic nuclei (AGNs) is still unknown. The hard X-ray spectra of
Seyferts as well as of Galactic stellar-mass black holes (GBHs) are well
represented by thermal Comptonization, but the origin of the seed photons is
less certain. The MeV tails observed in GBHs provide evidence in favour of
non-thermal electron tails and it is possible that such electrons are also
present in the X-ray emitting regions of AGNs. Using simulations with the
kinetic code that self-consistently models electron and photon distributions,
we find that the power-law-like X-ray spectra in AGNs can be explained in terms
of the synchrotron self-Compton radiation of hybrid thermal/non-thermal
electrons, similarly to the hard/low state of GBHs. Under a very broad range of
parameters the model predicts a rather narrow distribution of photon spectral
slopes consistent with that observed from LINERs and Seyferts at luminosities
less than 3 per cent of the Eddington luminosity. The entire infrared to X-ray
spectrum of these objects can be described in terms of our model, suggesting a
tight correlation between the two energy bands. We show that the recently found
correlation between slope and the Eddington ratio at higher luminosities can be
described by the increasing fraction of disc photons in the emitting region,
which may be associated with the decreasing inner radius of the optically thick
accretion disc. The increasing flux of soft photons is also responsible for the
transformation of the electron distribution from nearly thermal to almost
completely non-thermal. The softer X-ray spectra observed in narrow-line
Seyfert galaxies may correspond to non-thermal Comptonization of the disc
photons, predicting that no cutoff should be observed up to MeV energies in
these sources, similarly to the soft-state GBHs.Comment: MNRAS publishe
Heat transfer to a gas containing a cloud of particles
Heat transfer to gas containing particle clou
Comments on theory of volume reflection and radiation in bent crystals
Recent theoretical results on charged particle interaction with planarly
oriented thin bent crystals are reviewed, with the emphasis on dynamics in the
continuous potential. Influence of boundary conditions on the volume-reflected
beam profile is discussed. Basic properties of coherent bremsstrahlung in a
bent crystal are highlighted.Comment: 10 pages, 4 figure
The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Nucleon
The Gerasimov-Drell-Hearn sum rule is one of several dispersive sum rules
that connect the Compton scattering amplitudes to the inclusive photoproduction
cross sections of the target under investigation. Being based on such universal
principles as causality, unitarity, and gauge invariance, these sum rules
provide a unique testing ground to study the internal degrees of freedom that
hold the system together. The present article reviews these sum rules for the
spin-dependent cross sections of the nucleon by presenting an overview of
recent experiments and theoretical approaches. The generalization from real to
virtual photons provides a microscope of variable resolution: At small
virtuality of the photon, the data sample information about the long range
phenomena, which are described by effective degrees of freedom (Goldstone
bosons and collective resonances), whereas the primary degrees of freedom
(quarks and gluons) become visible at the larger virtualities. Through a rich
body of new data and several theoretical developments, a unified picture of
virtual Compton scattering emerges, which ranges from coherent to incoherent
processes, and from the generalized spin polarizabilities on the low-energy
side to higher twist effects in deep inelastic lepton scattering.Comment: 32 pages, 19 figures, review articl
Radio light curves during the passage of cloud G2 near Sgr A*
We calculate radio light curves produced by the bow shock that is likely to
form in front of the G2 cloud when it penetrates the accretion disk of Sgr A*.
The shock acceleration of the radio-emitting electrons is captured
self-consistently by means of first-principles particle-in-cell simulations. We
show that the radio luminosity is expected to reach maximum in early 2013,
roughly a month after the bow shock crosses the orbit pericenter. We estimate
the peak radio flux at 1.4 GHz to be 1.4 - 22 Jy depending on the assumed orbit
orientation and parameters. We show that the most promising frequencies for
radio observations are in the 0.1<nu<1 GHz range, for which the bow shock
emission will be much stronger than the intrinsic radio flux for all the models
considered.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Microparticle manipulation using laser-induced thermophoresis and thermal convection flow
We demonstrate manipulation of microbeads with diameters from 1.5 to 10 ”m and Jurkat cells within a thin fluidic device using the combined effect of thermophoresis and thermal convection. The heat flow is induced by localized absorption of laser light by a cluster of single walled carbon nanotubes, with no requirement for a treated substrate. Characterization of the system shows the speed of particle motion increases with optical power absorption and is also affected by particle size and corresponding particle suspension height within the fluid. Further analysis shows that the thermophoretic mobility (DT) is thermophobic in sign and increases linearly with particle diameter, reaching a value of 8 ”m2 sâ1 Kâ1 for a 10 ”m polystyrene bead
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