496 research outputs found
Search for ultra high energy cosmic rays from radiogalaxy Virgo A
Active galactic nuclei (AGNs) are considered to be one of the most
appropriate sources of ultra high energy cosmic rays (UHECRs, ). Radiogalaxy Virgo A (M87) in the centre of a cluster of
galaxies Virgo Cluster (VC) can be a prominent source of UHECRs. We investigate
the possible contribution of Virgo A and the VC to the flux of events with
trans-GZK energies - extremely high energy cosmic rays (EHECRs) - from the
recent Auger and Telescope Array (TA) data sets ( and , respectively). We simulate EHECR propagation from Virgo A
and the VC taking into account their deflections in galactic (GMF) and
extragalactic (EGMF) magnetic fields and show that there is no excess of EHECR
arrival directions from images of Virgo A/VC at different EHECR rigidities. By
means of event-by-event analysis we recover the extragalactic arrival
directions of EHECR events detected by Auger and TA for representative set of
nuclei H(p), He, N, Si, Fe, and find evidences of enhanced fluxes of N-Si-Fe
EHECRs from the Local Filament and Hot/Cold Spot regions. The Local Filament
with its enhanced magnetic field is an expected contributor to the UHECR flux
as the closest to the Earth last scattering centre, whereas Hot/Cold Spot
region is a part of a larger arc-like spot, possibly created by diffusively
spreading jet of UHECRs, accelerated in the relativistic jet of Virgo A during
a prominent nuclear outburst about 10 - 12 Myr ago.Comment: This article has been accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
Mind the Gap: interpreting the Minsk II Agreement. IES Policy Brief Issue 2016/3 • March 2016
This Policy Brief is the first of a two-part feature that examines Russia’s ability to influence French, German and EU narratives on the Minsk II agreement and Ukraine’s evolving position in the international system. This first Policy Brief will focus on the competing Russian and Ukrainian narratives surrounding Minsk II. Thus, it will examine Russia’s efforts to portray Ukraine as a ‘failed state’ and a ‘semi-sovereign’ subject, and to blame the West for provoking a ‘civil war’ in the country. It also outlines Ukraine’s counter-narratives that emphasises its sovereignty and the inter-state nature of its war with Russia
The Fantastic Four: A plug 'n' play set of optimal control pulses for enhancing nmr spectroscopy
We present highly robust, optimal control-based shaped pulses designed to
replace all 90{\deg} and 180{\deg} hard pulses in a given pulse sequence for
improved performance. Special attention was devoted to ensuring that the pulses
can be simply substituted in a one-to-one fashion for the original hard pulses
without any additional modification of the existing sequence. The set of four
pulses for each nucleus therefore consists of 90{\deg} and 180{\deg}
point-to-point (PP) and universal rotation (UR) pulses of identical duration.
These 1 ms pulses provide uniform performance over resonance offsets of 20 kHz
(1H) and 35 kHz (13C) and tolerate reasonably large radio frequency (RF)
inhomogeneity/miscalibration of (+/-)15% (1H) and (+/-)10% (13C), making them
especially suitable for NMR of small-to-medium-sized molecules (for which
relaxation effects during the pulse are negligible) at an accessible and widely
utilized spectrometer field strength of 600 MHz. The experimental performance
of conventional hard-pulse sequences is shown to be greatly improved by
incorporating the new pulses, each set referred to as the Fantastic Four
(Fanta4).Comment: 28 pages, 19 figure
Spatio-temporal evolution of the nonresonant instability in shock precursors of young supernova remnants
A nonresonant cosmic-ray-current-driven instability may operate in the shock
precursors of young supernova remnants and be responsible for magnetic-field
amplification, plasma heating and turbulence. Earlier simulations demonstrated
magnetic-field amplification, and in kinetic studies a reduction of the
relative drift between cosmic rays and thermal plasma was observed as
backreaction. However, all published simulations used periodic boundary
conditions, which do not account for mass conservation in decelerating flows
and only allow the temporal development to be studied. Here we report results
of fully kinetic Particle-In-Cell simulations with open boundaries that permit
inflow of plasma on one side of the simulation box and outflow at the other
end, hence allowing an investigation of both the temporal and the spatial
development of the instability. Magnetic-field amplification proceeds as in
studies with periodic boundaries and, observed here for the first time, the
reduction of relative drifts causes the formation of a shock-like compression
structure at which a fraction of the plasma ions are reflected. Turbulent
electric field generated by the nonresonant instability inelastically scatters
cosmic rays, modifying and anisotropizing their energy distribution. Spatial CR
scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads
to significant nonadiabatic heating of the background plasma maintaining bulk
equipartition between ions and electrons. The highest temperatures are reached
at sites of large-amplitude electrostatic fields. Ion spectra show
supra-thermal tails resulting from stochastic scattering in the turbulent
electric field. Together, these modifications in the plasma flow will affect
the properties of the shock and particle acceleration there.Comment: Accepted for publication in MNRAS. 16 pages, 15 figure
Reporting on the Minsk II Agreement: the Effect of Russian Narratives in French and German Media. IES Policy Brief Issue 2016/17 • October 2016
This Policy Brief is the second in a two-part feature that examines Russia’s ability to influence French and German narratives on the Minsk II agreement and Ukraine’s evolving position in the international system. While the first Policy Brief analysed the gap between Ukrainian-Russian interpretations of the Minsk II agreement, this Brief traces how these narratives are contextualised in French and German media landscapes. The research concludes that while there is a consistent presence of Russian narratives in public discourse in these countries, they had limited impact on their public opinion. The German/French news coverage of the Minsk II agreement as well as the role of Russia and Ukraine in its implementation differs from Russian-sponsored news. The Brief analyzes the wider diplomatic relations between Germany, France and Russia, particularly focusing on the deteriorating relations before the Ukraine crisis
Electron Pre-Acceleration at Nonrelativistic High-Mach-Number Perpendicular Shocks
We perform particle-in-cell simulations of perpendicular nonrelativistic
collisionless shocks to study electron heating and pre-acceleration for
parameters that permit extrapolation to the conditions at young supernova
remnants. Our high-resolution large-scale numerical experiments sample a
representative portion of the shock surface and demonstrate that the efficiency
of electron injection is strongly modulated with the phase of the shock
reformation. For plasmas with low and moderate temperature (plasma beta
and ), we explore the
nonlinear shock structure and electron pre-acceleration for various
orientations of the large-scale magnetic field with respect to the simulation
plane while keeping it at to the shock normal. Ion reflection off
the shock leads to the formation of magnetic filaments in the shock ramp,
resulting from Weibel-type instabilities, and electrostatic Buneman modes in
the shock foot. In all cases under study, the latter provides first-stage
electron energization through the shock-surfing acceleration (SSA) mechanism.
The subsequent energization strongly depends on the field orientation and
proceeds through adiabatic or second-order Fermi acceleration processes for
configurations with the out-of-plane and in-plane field components,
respectively. For strictly out-of-plane field the fraction of supra-thermal
electrons is much higher than for other configurations, because only in this
case the Buneman modes are fully captured by the 2D simulation grid. Shocks in
plasma with moderate provide more efficient pre-acceleration.
The relevance of our results to the physics of fully three-dimensional systems
is discussed
- …