228 research outputs found
New results on source and diffusion spectral features of Galactic cosmic rays: I- B/C ratio
In a previous study (Maurin et al., 2001), we explored the set of parameters
describing diffusive propagation of cosmic rays (galactic convection,
reacceleration, halo thickness, spectral index and normalization of the
diffusion coefficient), and we identified those giving a good fit to the
measured B/C ratio. This study is now extended to take into account a sixth
free parameter, namely the spectral index of sources. We use an updated version
of our code where the reacceleration term comes from standard minimal
reacceleration models. The goal of this paper is to present a general view of
the evolution of the goodness of fit to B/C data with the propagation
parameters. In particular, we find that, unlike the well accepted picture, and
in accordance with our previous study, a Kolmogorov-like power spectrum for
diffusion is strongly disfavored. Rather, the analysis points towards
along with source spectra index . Two
distinct energy dependences are used for the source spectra: the usual
power-law in rigidity and a law modified at low energy, the second choice being
only slightly preferred. We also show that the results are not much affected by
a different choice for the diffusion scheme. Finally, we compare our findings
to recent works, using other propagation models. This study will be further
refined in a companion paper, focusing on the fluxes of cosmic ray nuclei.Comment: 32 pages, 13 figures, accepted in A&
Uncoupling of EGFR–RAS signaling and nuclear localization of YBX1 in colorectal cancer
The transcription factor YBX1 can act as a mediator of signals transmitted via
the EGFR–RAS–MAPK axis. YBX1 expression has been associated with tumor
progression and prognosis in multiple types of cancer. Immunohistochemical
studies have revealed dependency between YBX1 expression and individual EGFR
family members. We analyzed YBX1 and EGFR family proteins in a colorectal
cancer (CRC) cohort and provide functional analyses of YBX1 in the context of
EGFR–RAS–MAPK signaling. Immunohistochemistry for YBX1 and EGFR family
receptors with two antibodies for YBX1 and EGFR were performed and related to
clinicopathological data. We employed Caco2 cells expressing an inducible
KRASV12 gene to determine effects on localization and levels of YBX1. Mouse
xenografts of Caco2-KRASV12 cells were used to determine YBX1 dynamics in a
tissue context. The two different antibodies against YBX1 showed discordant
immunohistochemical stainings in cell culture and clinical specimens.
Expression of YBX1 and EGFR family members were not correlated in CRC.
Analysis of Caco2 xenografts displayed again heterogeneity of YBX1 staining
with both antibodies. Our results suggest that YBX1 is controlled via complex
regulatory mechanisms involving tumor stroma interaction and signal
transduction processes. Our study highlights that YBX1 antibodies have
different specificities, advocating their use in a combined manner
Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
We have calculated the evolution of cosmic ray (CR) modified astrophysical
shocks for a wide range of shock Mach numbers and shock speeds through
numerical simulations of diffusive shock acceleration (DSA) in 1D quasi-
parallel plane shocks. The simulations include thermal leakage injection of
seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion
is assumed. We model shocks similar to those expected around cosmic structure
pancakes as well as other accretion shocks driven by flows with upstream gas
temperatures in the range K and shock Mach numbers spanning
. We show that CR modified shocks evolve to time-asymptotic states
by the time injected particles are accelerated to moderately relativistic
energies (p/mc \gsim 1), and that two shocks with the same Mach number, but
with different shock speeds, evolve qualitatively similarly when the results
are presented in terms of a characteristic diffusion length and diffusion time.
For these models the time asymptotic value for the CR acceleration efficiency
is controlled mainly by shock Mach number. The modeled high Mach number shocks
all evolve towards efficiencies %, regardless of the upstream CR
pressure. On the other hand, the upstream CR pressure increases the overall CR
energy in moderate strength shocks (). (abridged)Comment: 23 pages, 12 ps figures, accepted for Astrophysical Journal (Feb. 10,
2005
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
Kinetic approaches to particle acceleration at cosmic ray modified shocks
Kinetic approaches provide an effective description of the process of
particle acceleration at shock fronts and allow to take into account the
dynamical reaction of the accelerated particles as well as the amplification of
the turbulent magnetic field as due to streaming instability. The latter does
in turn affect the maximum achievable momentum and thereby the acceleration
process itself, in a chain of causality which is typical of non-linear systems.
Here we provide a technical description of two of these kinetic approaches and
show that they basically lead to the same conclusions. In particular we discuss
the effects of shock modification on the spectral shape of the accelerated
particles, on the maximum momentum, on the thermodynamic properties of the
background fluid and on the escaping and advected fluxes of accelerated
particles.Comment: 22 pages, 7 figures, accepted for publication in MNRA
Shock Acceleration of Cosmic Rays - a critical review
Motivated by recent unsuccessful efforts to detect the predicted flux of TeV
gamma-rays from supernova remnants, we present a critical examination of the
theory on which these predictions are based. Three crucial problems are
identified: injection, maximum achievable particle energy and spectral index.
In each case significant new advances in understanding have been achieved,
which cast doubt on prevailing paradigms such as Bohm diffusion and
single-fluid MHD. This indicates that more realistic analytical models, backed
by more sophisticated numerical techniques should be employed to obtain
reliable predictions. Preliminary work on incorporating the effects of
anomalous transport suggest that the resulting spectrum should be significantly
softer than that predicted by conventional theory.Comment: 8 pages, invited review presented at the 17th ECRS, Lodz, July 2000;
to appear in Journal of Physics G: Nuclear and Particle Physic
Single-spacecraft techniques for shock parameters estimation : A systematic approach
Spacecraft missions provide the unique opportunity to study the properties of collisionless shocks utilising in situ measurements. In the past years, several diagnostics have been developed to address key shock parameters using time series of magnetic field (and plasma) data collected by a single spacecraft crossing a shock front. A critical aspect of such diagnostics is the averaging process involved in the evaluation of upstream/downstream quantities. In this work, we discuss several of these techniques, with a particular focus on the shock obliquity (defined as the angle between the upstream magnetic field and the shock normal vector) estimation. We introduce a systematic variation of the upstream/downstream averaging windows, yielding to an ensemble of shock parameters, which is a useful tool to address the robustness of their estimation. This approach is first tested with a synthetic shock dataset compliant with the Rankine-Hugoniot jump conditions for a shock, including the presence of noise and disturbances. We then employ self-consistent, hybrid kinetic shock simulations to apply the diagnostics to virtual spacecraft crossing the shock front at various stages of its evolution, highlighting the role of shock-induced fluctuations in the parameters' estimation. This approach has the strong advantage of retaining some important properties of collisionless shock (such as, for example, the shock front microstructure) while being able to set a known, nominal set of shock parameters. Finally, two recent observations of interplanetary shocks from the Solar Orbiter spacecraft are presented, to demonstrate the use of this systematic approach to real events of shock crossings. The approach is also tested on an interplanetary shock measured by the four spacecraft of the Magnetospheric Multiscale (MMS) mission. All the Python software developed and used for the diagnostics (SerPyShock) is made available for the public, including an example of parameter estimation for a shock wave recently observed in-situ by the Solar Orbiter spacecraft.Peer reviewe
Programmable Quantum Processors based on Spin Qubits with Mechanically-Mediated Interactions and Transport
Solid state spin qubits are promising candidates for quantum information
processing, but controlled interactions and entanglement in large, multi-qubit
systems are currently difficult to achieve. We describe a method for
programmable control of multi-qubit spin systems, in which individual
nitrogen-vacancy (NV) centers in diamond nanopillars are coupled to
magnetically functionalized silicon nitride mechanical resonators in a scanning
probe configuration. Qubits can be entangled via interactions with
nanomechanical resonators while programmable connectivity is realized via
mechanical transport of qubits in nanopillars. To demonstrate the feasibility
of this approach, we characterize both the mechanical properties and the
magnetic field gradients around the micromagnet placed on the nanobeam
resonator. Furthermore, we show coherent manipulation and mechanical transport
of a proximal spin qubit by utilizing nuclear spin memory, and use the NV
center to detect the time-varying magnetic field from the oscillating
micromagnet, extracting a spin-mechanical coupling of 7.7(9) Hz. With realistic
improvements the high-cooperativity regime can be reached, offering a new
avenue towards scalable quantum information processing with spin qubits.Comment: 7 pages, 4 figure
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