88,618 research outputs found
Deformation-induced accelerated dynamics in polymer glasses
Molecular dynamics simulations are used to investigate the effects of
deformation on the segmental dynamics in an aging polymer glass. Individual
particle trajectories are decomposed into a series of discontinuous hops, from
which we obtain the full distribution of relaxation times and displacements
under three deformation protocols: step stress (creep), step strain, and
constant strain rate deformation. As in experiments, the dynamics can be
accelerated by several orders of magnitude during deformation, and the history
dependence is entirely erased during yield (mechanical rejuvenation). Aging can
be explained as a result of the long tails in the relaxation time distribution
of the glass, and similarly, mechanical rejuvenation is understood through the
observed narrowing of this distribution during yield. Although the relaxation
time distributions under deformation are highly protocol specific, in each case
they may be described by a universal acceleration factor that depends only on
the strain.Comment: 15 pages, 15 figure
Using infrared/X-ray flare statistics to probe the emission regions near the event horizon of Sgr A*
The supermassive black hole at the centre of the Galaxy flares at least daily
in the infrared (IR) and X-ray bands, yet the process driving these flares is
still unknown. So far detailed analysis has only been performed on a few bright
flares. In particular, the broadband spectral modelling suffers from a strong
lack of simultaneous data. However, new monitoring campaigns now provide data
on thousands of flaring events, allowing a statistical analysis of the flare
properties. In this paper, we investigate the X-ray and IR flux distributions
of the flare events. Using a self-consistent calculation of the particle
distribution, we model the statistical properties of the flares. Based on a
previous work on single flares, we consider two families of models: pure
synchrotron models and synchrotron self-Compton (SSC) models. We investigate
the effect of fluctuations in some relevant parameters (e.g. acceleration
properties, density, magnetic field) on the flux distributions. The
distribution of these parameters is readily derived from the flux distributions
observed at different wavelengths. In both scenarios, we find that fluctuations
of the power injected in accelerated particles plays a major role. This must be
distributed as a power-law (with different indices in each model). In the
synchrotron dominated scenario, we derive the most extreme values of the
acceleration power required to reproduce the brightest flares. In that model,
the distribution of the acceleration slope fluctuations is constrained and in
the SSC scenario we constrain the distributions of the correlated magnetic
field and flow density variations.Comment: 9 pages, 3 tables, 6 figures, MNRAS, June 201
Active Galactic Nuclei Jets and Multiple Oblique Shock Acceleration: Starved Spectra
Shocks in jets and hot spots of Active Galactic Nuclei (AGN) are one
prominent class of possible sources of very high energy cosmic ray particles
(above 10^18eV). Extrapolating their spectrum to their plausible injection
energy from some shock, implies an enormous hidden energy for a spectrum of
index ~-2. Some analyzes suggest the particles' injection spectrum at source to
be as steep as -2.4 to -2.7, making the problem much worse, by a factor of
order 10^6. Nevertheless, it seems implausible that more than at the very best
1/3 of the jet energy, goes into the required flux of energetic particles thus,
one would need to allow for the possibility that there is an energy problem,
which we would like to address in this work. Sequences of consecutive oblique
shock features, or conical shocks, have been theorized and eventually observed
in many AGN jets. Based on that, we use by analogy the 'Comptonisation' effect
and we propose a scenario of a single injection of particles which are
accelerated consecutively by several oblique shocks along the axis of an AGN
jet. We use detailed test-particle approximation Monte Carlo simulations in
order to calculate particle spectra by acceleration at such a shock pattern
while monitoring the efficiency of acceleration, calculating differential
spectra. We find that the first shock of a sequence of oblique shocks,
establishes a low energy power-law spectrum with ~E^-2.7. The consecutive
shocks push the spectrum up in energy, rendering flatter distributions with
steep cut-offs and characteristic depletion at low energies, an effect which
could explain the puzzling apparent extra source power as well as the flat or
inverted spectra from distant flaring sources.Comment: 14 pages, submitted to A&
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