Efficiency of cosmic ray reflections from an ultrarelativistic shock wave

The process of cosmic ray acceleration up to energies in excess of $10^{20}$ eV at relativistic shock waves with large Lorentz factors, $\Gamma \gg 1$ requires $\sim \Gamma^2$ particle energy gains at single reflections from the shock (cf. Gallant & Achterberg 1999). In the present comment, applying numerical simulations we address an efficiency problem arising for such models. The actual efficiency of the acceleration process is expected to be substantially lower than the estimates of previous authors.Comment: 3 pages, 3 figures, accepted for publication in MNRAS (Letters

Prompt high-energy neutrinos from gamma-ray bursts in photospheric and synchrotron self-Compton scenarios

We investigate neutrino emission from gamma-ray bursts (GRBs) under alternative scenarios for prompt emission (the photospheric and synchrotron self-Compton scenarios) rather than the classical optically thin synchrotron scenario. In the former scenario, we find that neutrinos from the pp reaction can be very important at energies around 10-100 TeV. They may be detected by IceCube/KM3Net and useful as a probe of baryon acceleration around/below the photosphere. In the latter scenario, we may expect about EeV pgamma neutrinos produced by soft photons. Predicted spectra are different from that in the classical scenario, and neutrinos would be useful as one of the clues to the nature of GRBs (the jet composition, emission radius, magnetic field and so on).Comment: 5 pages, 3 figures, replaced to match the final version published as PRD Rapid Communication, 78, 101302. Minor typos fixe

Low-temperature dynamics of weakly localized Frenkel excitons in disordered linar chains

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible non-equilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or non resonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive non-monotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intra-band relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes.Comment: 15 pages, 8 figure

Comment on the first-order Fermi acceleration at ultra-relativistic shocks

The first-order Fermi acceleration process at an ultra-relativistic shock wave is expected to create a particle spectrum with the unique asymptotic spectral index sigma_{gamma >> 1} approximately 2.2. Below, we discuss this result and differences in its various derivations, which -- explicitly or implicitly -- always require highly turbulent conditions downstream of the shock. In the presence of medium amplitude turbulence the generated particle spectrum can be much steeper than the above asymptotic one. We also note problems with application of the pitch angle diffusion model for particle transport near the ultra-relativistic shocks.Comment: Substantially modified and shorted version, accepted to A&

Exciton Dephasing and Thermal Line Broadening in Molecular Aggregates

Using a model of Frenkel excitons coupled to a bath of acoustic phonons in the host medium, we study the temperature dependence of the dephasing rates and homogeneous line width in linear molecular aggregates. The model includes localization by disorder and predicts a power-law thermal scaling of the effective homogeneous line width. The theory gives excellent agreement with temperature dependent absorption and hole-burning experiments on aggregates of the dye pseudoisocyanine.Comment: 11 pages, 3 PostScript figure

Thermal effects in exciton harvesting in biased one-dimensional systems

The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by a trap. Previously, we have shown that in the low temperature limit different harvesting scenarios as a function of the applied bias strength (slope of the energy gradient towards the trap) are possible \cite{Vlaming07}. This paper generalizes the results for both homogeneous and disordered chains to nonzero temperatures. We show that thermal effects are appreciable only for low bias strengths, particularly so in disordered systems, and lead to faster harvesting.Comment: 8 pages, 2 fugures, to appear in Journal of Luminescenc