345 research outputs found
On the cosmic ray diffusion in a violent interstellar medium
A variety of the available observational data on the cosmic ray (CR) spectrum, anisotropy and composition are in good agreement with a suggestion on the diffusion propagation of CR with energy below 10(15) eV in the interstellar medium. The magnitude of the CR diffusion coefficient and its energy dependence are determined by interstellar medium (ISM) magnetic field spectra. Direct observational data on magnetic field spectra are still absent. A theoretical model to the turbulence generation in the multiphase ISM is resented. The model is based on the multiple generation of secondary shocks and concomitant large-scale rarefactions due to supernova shock interactions with interstellar clouds. The distribution function for ISM shocks are derived to include supernova statistics, diffuse cloud distribution, and various shock wave propagation regimes. This permits calculation of the ISM magnetic field fluctuation spectrum and CR diffusion coefficient for the hot phase of ISM
Diffusive radiation in Langmuir turbulence produced by jet shocks
Anisotropic distributions of charged particles including two-stream
distributions give rise to generation of either stochastic electric fields (in
the form of Langmuir waves, Buneman instability) or random quasi-static
magnetic fields (Weibel and filamentation instabilities) or both. These
two-stream instabilities are known to play a key role in collisionless shock
formation, shock-shock interactions, and shock-induced electromagnetic
emission. This paper applies the general non-perturbative stochastic theory of
radiation to study electromagnetic emission produced by relativistic particles,
which random walk in the stochastic electric fields of the Langmuir waves. This
analysis takes into account the cumulative effect of uncorrelated Langmuir
waves on the radiating particle trajectory giving rise to angular diffusion of
the particle, which eventually modifies the corresponding radiation spectra. We
demonstrate that the radiative process considered is probably relevant for
emission produced in various kinds of astrophysical jets, in particular, prompt
gamma-ray burst spectra, including X-ray excesses and prompt optical flashes.Comment: 9 pages, 5 figures, MNRAS, accepte
Mechanism of the Efficient Tryptophan Fluorescence Quenching in Human γD-Crystallin Studied by Time-Resolved Fluorescence†
Human γD-crystallin (HγD-Crys) is a two-domain, β-sheet eye lens protein found in the lens nucleus. Its long-term solubility and stability are important to maintain lens transparency throughout life. HγD-Crys has four highly conserved buried tryptophans (Trps), with two in each of the homologous β-sheet domains. In situ, these Trps will be absorbing ambient UV radiation that reaches the lens. The dispersal of the excited-state energy to avoid covalent damage is likely to be physiologically relevant for the lens crystallins. Trp fluorescence is efficiently quenched in native HγD-Crys. Previous steady-state fluorescence measurements provide strong evidence for energy transfer from Trp42 to Trp68 in the N-terminal domain and from Trp130 to Trp156 in the C-terminal domain [Chen, J., et al. (2006) Biochemistry 45, 11552−11563]. Hybrid quantum mechanical−molecular mechanical (QM-MM) simulations indicated that the fluorescence of Trp68 and Trp156 is quenched by fast electron transfer to the amide backbone. Here we report additional information obtained using time-resolved fluorescence spectroscopy. In the single-Trp-containing proteins (Trp42-only, Trp68-only, Trp130-only, and Trp156-only), the highly quenched Trp68 and Trp156 have very short lifetimes, τ ~0.1 ns, whereas the moderately fluorescent Trp42 and Trp130 have longer lifetimes, τ ~3 ns. In the presence of the energy acceptor (Trp68 or Trp156), the lifetime of the energy donor (Trp42 or Trp130) decreased from ~3 to ~1 ns. The intradomain energy transfer efficiency is 56% in the N-terminal domain and is 71% in the C-terminal domain. The experimental values of energy transfer efficiency are in good agreement with those calculated theoretically. The absence of a time-dependent red shift in the time-resolved emission spectra of Trp130 proves that its local environment is very rigid. Time-resolved fluorescence anisotropy measurements with the single-Trp-containing proteins, Trp42-only and Trp130-only, indicate that the protein rotates as a rigid body and no segmental motion is detected. A combination of energy transfer with electron transfer results in short excited-state lifetimes of all Trps, which, together with the high rigidity of the protein matrix around Trps, could protect HγD-Crys from excited-state reactions causing permanent covalent damage
Radio emission from acceleration sites of solar flares
The Letter takes up a question of what radio emission is produced by
electrons at the very acceleration site of a solar flare. Specifically, we
calculate incoherent radio emission produced within two competing acceleration
models--stochastic acceleration by cascading MHD turbulence and regular
acceleration in collapsing magnetic traps. Our analysis clearly demonstrates
that the radio emission from the acceleration sites: (i) has sufficiently
strong intensity to be observed by currently available radio instruments and
(ii) has spectra and light curves which are distinctly different in these two
competing models, which makes them observationally distinguishable. In
particular, we suggest that some of the narrowband microwave and decimeter
continuum bursts may be a signature of the stochastic acceleration in solar
flares.Comment: ApJL, in pres
Local Field effects on the radiative lifetime of emitters in surrounding media: virtual- or real-cavity model?
For emitters embedded in media of various refractive indices, different
macroscopic or microscopic theoretical models predict different dependencies of
the spontaneous emission lifetime on refractive index. Among those models are
the two most promising models: the virtual-cavity model and the real-cavity
model. It is a priori not clear which model is more relevant for a given
situation. By close analysis of the available experimental results and
examining the assumptions underlying the two models, we reach a consistent
interpretation of the experimental results and give the criteria which model
should apply for a given situation.Comment: 12 pages with 4 figure
Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries
In the present paper we discuss the modifications introduced into the
first-order Fermi shock acceleration process due to a finite extent of
diffusive regions near the shock or due to boundary conditions leading to an
increased particle escape upstream and/or downstream the shock. In the
considered simple example of the planar shock wave we idealize the escape
phenomenon by imposing a particle escape boundary at some distance from the
shock. Presence of such a boundary (or boundaries) leads to coupled steepening
of the accelerated particle spectrum and decreasing of the acceleration time
scale. It allows for a semi-quantitative evaluation and, in some specific
cases, also for modelling of the observed steep particle spectra as a result of
the first-order Fermi shock acceleration. We also note that the particles close
to the upper energy cut-off are younger than the estimate based on the
respective acceleration time scale. In Appendix A we present a new
time-dependent solution for infinite diffusive regions near the shock allowing
for different constant diffusion coefficients upstream and downstream the
shock.Comment: LaTeX, 14 pages, 4 postscript figures; Solar Physics (accepted
Spectra of magnetic fluctuations and relativistic particles produced by a nonresonant wave instability in supernova remnant shocks
We model strong forward shocks in young supernova remnants with efficient
particle acceleration where a nonresonant instability driven by the cosmic ray
current amplifies magnetic turbulence in the shock precursor. Particle
injection, magnetic field amplification (MFA) and the nonlinear feedback of
particles and fields on the bulk flow are derived consistently. The shock
structure depends critically on the efficiency of turbulence cascading. If
cascading is suppressed, MFA is strong, the shock precursor is stratified, and
the turbulence spectrum contains several discrete peaks. These peaks, as well
as the amount of MFA, should influence synchrotron X-rays, allowing
observational tests of cascading and other assumptions intrinsic to the
nonlinear model of nonresonant wave growth.Comment: 3 figures, 5 pages. Accepted for publication in ApJ
Diffusive Radiation in One-dimensional Langmuir Turbulence
We calculate spectra of radiation produced by a relativistic particle in the
presence of one-dimensional Langmuir turbulence which might be generated by a
streaming instability in the plasma, in particular, in the shock front or at
the shock-shock interactions. The shape of the radiation spectra is shown to
depend sensitively on the angle between the particle velocity and electric
field direction. The radiation spectrum in the case of exactly transverse
particle motion is degenerate and similar to that of spatially uniform Langmuir
oscillations. In case of oblique propagation, the spectrum is more complex, it
consists of a number of power-law regions and may contain a distinct
high-frequency spectral peak. %at \omega=2\omega\pe \gamma^2. The emission
process considered is relevant to various laboratory plasma settings and for
astrophysical objects as gamma-ray bursts and collimated jets.Comment: 4 pages, 1 figure, accepted for Phys. Rev.
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