956 research outputs found
The electrostatic instability for realistic pair distributions in blazar/EBL cascades
This work revisits the electrostatic instability for blazar-induced pair
beams propagating through IGM with the methods of linear analysis and PIC
simulations. We study the impact of the realistic distribution function of
pairs resulting from interaction of high-energy gamma-rays with the
extragalactic background light. We present analytical and numerical
calculations of the linear growth rate of the instability for arbitrary
orientation of wave vectors. Our results explicitly demonstrate that the finite
angular spread of the beam dramatically affects the growth rate of the waves,
leading to fastest growth for wave vectors quasi-parallel to the beam direction
and a growth rate at oblique directions that is only by a factor of 2-4 smaller
compared to the maximum. To study the non-linear beam relaxation, we performed
PIC simulations that take into account a realistic wide-energy distribution of
beam particles. The parameters of the simulated beam-plasma system provide an
adequate physical picture that can be extrapolated to realistic blazar-induced
pairs. In our simulations the beam looses only 1\% percent of its energy, and
we analytically estimate that the beam would lose its total energy over about
simulation times. Analytical scaling is then used to extrapolate to the
parameters of realistic blazar-induced pair beams. We find that they can
dissipate their energy slightly faster by the electrostatic instability than
through inverse-Compton scattering. The uncertainties arising from, e.g.,
details of the primary gamma-ray spectrum are too large to make firm statements
for individual blazars, and an analysis based on their specific properties is
required.Comment: Accepted for publication in ApJ (2018), in prin
Stellar-to-halo mass relation of cluster galaxies
In the hierarchical formation model, galaxy clusters grow by accretion of
smaller groups or isolated galaxies. During the infall into the centre of a
cluster, the properties of accreted galaxies change. In particular, both
observations and numerical simulations suggest that its dark matter halo is
stripped by the tidal forces of the host.
We use galaxy-galaxy weak lensing to measure the average mass of dark matter
haloes of satellite galaxies as a function of projected distance to the centre
of the host, for different stellar mass bins. Assuming that the stellar
component of the galaxy is less disrupted by tidal stripping, stellar mass can
be used as a proxy of the infall mass. We study the stellar to halo mass
relation of satellites as a function of the cluster-centric distance to measure
tidal stripping.
We use the shear catalogues of the DES science verification archive, the
CFHTLenS and the CFHT Stripe 82 (CS82) surveys, and we select satellites from
the redMaPPer catalogue of clusters. For galaxies located in the outskirts of
clusters, we find a stellar to halo mass relation in good agreement with the
theoretical expectations from \citet{moster2013} for central galaxies. In the
centre of the cluster, we find that this relation is shifted to smaller halo
mass for a given stellar mass. We interpret this finding as further evidence
for tidal stripping of dark matter haloes in high density environments.Comment: 15 pages, 14 figure
The transport of cosmic rays in self-excited magnetic turbulence
The process of diffusive shock acceleration relies on the efficacy with which
hydromagnetic waves can scatter charged particles in the precursor of a shock.
The growth of self-generated waves is driven by both resonant and non-resonant
processes. We perform high-resolution magnetohydrodynamic simulations of the
non-resonant cosmic-ray driven instability, in which the unstable waves are
excited beyond the linear regime. In a snapshot of the resultant field,
particle transport simulations are carried out. The use of a static snapshot of
the field is reasonable given that the Larmor period for particles is typically
very short relative to the instability growth time. The diffusion rate is found
to be close to, or below, the Bohm limit for a range of energies. This provides
the first explicit demonstration that self-excited turbulence reduces the
diffusion coefficient and has important implications for cosmic ray transport
and acceleration in supernova remnants.Comment: 8 pages, 8 figures, accepted for publication in MNRA
Nonthermal Electron Acceleration at Collisionless Quasi-perpendicular Shocks
Shock waves propagating in collisionless heliospheric and astrophysical
plasmas have been studied extensively over the decades. One prime motivation is
to understand the nonthermal particle acceleration at shocks. Although the
theory of diffusive shock acceleration (DSA) has long been the standard for
cosmic-ray acceleration at shocks, plasma physical understanding of particle
acceleration remains elusive. In this review, we discuss nonthermal electron
acceleration mechanisms at quasi-perpendicular shocks, for which substantial
progress has been made in recent years. The discussion presented in this review
is restricted to the following three specific topics. The first is stochastic
shock drift acceleration (SSDA), which is a relatively new mechanism for
electron injection into DSA. The basic mechanism, related in-situ observations
and kinetic simulations results, and how it is connected with DSA will be
discussed. Second, we discuss shock surfing acceleration (SSA) at very high
Mach number shocks relevant to young supernova remnants (SNRs). While the
original proposal under the one-dimensional assumption is unrealistic, SSA has
now been proven efficient by a fully three-dimensional kinetic simulation.
Finally, we discuss the current understanding of the magnetized
Weibel-dominated shock. Spontaneous magnetic reconnection of self-generated
current sheets within the shock structure is an interesting consequence of
Weibel-generated strong magnetic turbulence. We argue that high Mach number
shocks with both Alfven and sound Mach numbers exceeding 20-40 will likely
behave as a Weibel-dominated shock. Despite a number of interesting recent
findings, the relative roles of SSDA, SSA, and magnetic reconnection for
electron acceleration at collisionless shocks and how the dominant particle
acceleration mechanisms change depending on shock parameters remain to be
answered.Comment: To appear in Reviews of Modern Plasma Physics as an invited revie
A Novel approach to quality-of-service provisioning in trusted relay quantum key distribution networks
In recent years, noticeable progress has been made in the development of quantum equipment, reflected through the number of successful demonstrations of Quantum Key Distribution (QKD) technology. Although they showcase the great achievements of QKD, many practical difficulties still need to be resolved. Inspired by the significant similarity between mobile ad-hoc networks and QKD technology, we propose a novel quality of service (QoS) model including new metrics for determining the states of public and quantum channels as well as a comprehensive metric of the QKD link. We also propose a novel routing protocol to achieve high-level scalability and minimize consumption of cryptographic keys. Given the limited mobility of nodes in QKD networks, our routing protocol uses the geographical distance and calculated link states to determine the optimal route. It also benefits from a caching mechanism and detection of returning loops to provide effective forwarding while minimizing key consumption and achieving the desired utilization of network links. Simulation results are presented to demonstrate the validity and accuracy of the proposed solutions
A Process for Co-Designing Educational Technology Systems for Refugee Children
There is a growing interest in the potential for technology to facilitate emergency education of refugee children. However, designing in this space requires knowledge of the displaced population and the contextual dynamics surrounding it. Design should therefore be informed by both existing research across relevant disciplines, and from the practical experience of those who are on the ground facing the problem in real life. This paper describes a process for designing appropriate technology for these settings. The process draws on literature from emergency education, student engagement and motivation, educational technology, and participatory design. We emphasise a thorough understanding of the problem definition, the nature of the emergency, and of socio-cultural aspects that can inform the design process. We describe how this process was implemented leading to the design of a digital learning space for children living in a refugee camp in Greece. This drew on involving different groups of participants such as social-workers, parents, and children
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