864 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
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
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 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
Angiotensin converting enzyme gene polymorphism is associated with severity of coronary artery disease in men with high total cholesterol levels
This study examines whether renin-angiotensin-aldosterone system gene polymorphisms: ACE (encoding for angiotensin converting enzyme) c.2306-117_404 I/D, AGTR1 (encoding for angiotensin II type-1 receptor) c.1080*86A>C and CYP11B2 (encoding for aldosterone synthase) c.-344C>T are associated with the extension of coronary atherosclerosis in a group of 647 patients who underwent elective coronary angiography. The extension of CAD was evaluated using the Gensini score. The polymorphisms were determined by PCR and RFLP assays. The associations between genotypes and the extent of coronary atherosclerosis were tested by the Kruskal-Wallis test, followed by pairwise comparisons using Wilcoxon test. The population has been divided into groups defined by: sex, smoking habit, past myocardial infarction, BMI (>, †25), age (>, †55), diabetes mellitus, level of total cholesterol (>, †200 mg/dl), LDL cholesterol (>, †130 mg/dl), HDL cholesterol (>, †40 mg/dl), triglycerides (>, †150 mg/dl). Significant associations between the ACE c.2306-117_404 I/D polymorphism and the Gensini score in men with high total cholesterol levels (PKruskal-Wallisâ=â0.008; Padjustedâ=â0.009), high level of LDL cholesterol (PKruskal-Wallisâ=â0.016; Padjustedâ=â0.028) and low level of HDL cholesterol (PKruskal-Wallisâ=â0.04; Padjustedâ=â0.055) have been found. No association between the AGTR1 c.1080*86A>C and CYP11B2 c.-344C>T and the Gensini score has been found. These results suggest that men who carry ACE c.2306-117_404 DD genotype and have high total cholesterol, high LDL cholesterol and low HDL cholesterol levels may be predisposed to the development of more severe CAD
Performance of a small size telescope (SST-1M) camera for gamma-ray astronomy with the Cherenkov Telescope Array
The foreseen implementations of the Small Size Telescopes (SST) in CTA will
provide unique insights into the highest energy gamma rays offering fundamental
means to discover and under- stand the sources populating the Galaxy and our
local neighborhood. Aiming at such a goal, the SST-1M is one of the three
different implementations that are being prototyped and tested for CTA. SST-1M
is a Davies-Cotton single mirror telescope equipped with a unique camera
technology based on SiPMs with demonstrated advantages over classical
photomultipliers in terms of duty-cycle. In this contribution, we describe the
telescope components, the camera, and the trigger and readout system. The
results of the commissioning of the camera using a dedicated test setup are
then presented. The performances of the camera first prototype in terms of
expected trigger rates and trigger efficiencies for different night-sky
background conditions are presented, and the camera response is compared to
end-to-end simulations.Comment: All CTA contributions at arXiv:1709.0348
Using muon rings for the optical throughput calibration of the SST-1M prototype for the Cherenkov Telescope Array
Imaging Atmospheric Cherenkov Telescopes (IACTs) are ground-based instruments
devoted to the study of very high energy gamma-rays coming from space. The
detection technique consists of observing images created by the Cherenkov light
emitted when gamma rays, or more generally cosmic rays, propagate through the
atmosphere. While in the case of protons or gamma-rays the images present a
filled and more or less elongated shape, energetic muons penetrating the
atmosphere are visualised as characteristic circular rings or arcs. A
relatively simple analysis of the ring images allows the reconstruction of all
the relevant parameters of the detected muons, such as the energy, the impact
parameter, and the incoming direction, with the final aim to use them to
calibrate the total optical throughput of the given IACT telescope. We present
the results of preliminary studies on the use of images created by muons as
optical throughput calibrators of the single mirror small size telescope
prototype SST-1M proposed for the Cherenkov Telescope Array.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
DigiCam - Fully Digital Compact Read-out and Trigger Electronics for the SST-1M Telescope proposed for the Cherenkov Telescope Array
The SST-1M is one of three prototype small-sized telescope designs proposed
for the Cherenkov Telescope Array, and is built by a consortium of Polish and
Swiss institutions. The SST-1M will operate with DigiCam - an innovative,
compact camera with fully digital read-out and trigger electronics. A high
level of integration will be achieved by massively deploying state-of-the-art
multi-gigabit transmission channels, beginning from the ADC flash converters,
through the internal data and trigger signals transmission over backplanes and
cables, to the camera's server link. Such an approach makes it possible to
design the camera to fit the size and weight requirements of the SST-1M
exactly, and provide low power consumption, high reliability and long lifetime.
The structure of the digital electronics will be presented, along with main
physical building blocks and the internal architecture of FPGA functional
subsystems.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
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