250 research outputs found
Effect of topology on the collapse transition and the instantaneous shape of a model heteropolymer
The effect of topology on the collapse transition and instantaneous shape of
an energy polydisperse polymer (a model heteropolymer) is studied by means of
computer simulations. In particular, we consider three different chain
topology, namely, linear (L), ring (R) and trefoil knot (T). The heteropolymer
is modeled by assigning each monomer an interaction parameter, ,
drawn randomly from a Gaussian distribution. Through chain size scaling the
transition temperature, , is located and compared among the chains of
different topogies. The influence of topology is reflected in the value of
and observed that in a similar fashion to that of the homopolymer counterpart.
Also studied chain size distributions, and the shape changes across the
transition temperature characterised through shape parameters based on the
eigenvalues of the gyration tensor. It is observed that, for the model
heteropolymer, in addition to chain topology the -temperature also
depends on energy polydispersity.Comment: 10 pages, 9 figure
Cosmic-ray energy spectrum and composition up to the ankle - the case for a second Galactic component
We have carried out a detailed study to understand the observed energy
spectrum and composition of cosmic rays with energies up to ~10^18 eV. Our
study shows that a single Galactic component with subsequent energy cut-offs in
the individual spectra of different elements, optimised to explain the observed
spectra below ~10^14 eV and the knee in the all-particle spectrum, cannot
explain the observed all-particle spectrum above ~2x10^16 eV. We discuss two
approaches for a second component of Galactic cosmic rays -- re-acceleration at
a Galactic wind termination shock, and supernova explosions of Wolf-Rayet
stars, and show that the latter scenario can explain almost all observed
features in the all-particle spectrum and the composition up to ~10^18 eV, when
combined with a canonical extra-galactic spectrum expected from strong radio
galaxies or a source population with similar cosmological evolution. In this
two-component Galactic model, the knee at ~ 3x10^15 eV and the second knee at
~10^17 eV in the all-particle spectrum are due to the cut-offs in the first and
second components, respectively. We also discuss several variations of the
extra-galactic component, from a minimal contribution to scenarios with a
significant component below the ankle (at ~4x10^18 eV), and find that
extra-galactic contributions in excess of regular source evolution are neither
indicated nor in conflict with the existing data. Our main result is that the
second Galactic component predicts a composition of Galactic cosmic rays at and
above the second knee that largely consists of helium or a mixture of helium
and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast
to most common assumptions. This prediction is in agreement with new
measurements from LOFAR and the Pierre Auger Observatory which indicate a
strong light component and a rather low iron fraction between ~10^17 and 10^18
eV.Comment: Added Table 4; Published in A&A, 595 (2016) A33 (Highlight paper
Polarized radio emission from extensive air showers measured with LOFAR
We present LOFAR measurements of radio emission from extensive air showers.
We find that this emission is strongly polarized, with a median degree of
polarization of nearly , and that the angle between the polarization
direction of the electric field and the Lorentz force acting on the particles,
depends on the observer location in the shower plane. This can be understood as
a superposition of the radially polarized charge-excess emission mechanism,
first proposed by Askaryan and the geomagnetic emission mechanism proposed by
Kahn and Lerche. We calculate the relative strengths of both contributions, as
quantified by the charge-excess fraction, for individual air showers. We
find that the measured charge-excess fraction is higher for air showers
arriving from closer to the zenith. Furthermore, the measured charge-excess
fraction also increases with increasing observer distance from the air shower
symmetry axis. The measured values range from for very
inclined air showers at to for almost
vertical showers at . Both dependencies are in qualitative
agreement with theoretical predictions.Comment: 22 pages, 14 figures, accepted for publication in JCA
The radio emission pattern of air showers as measured with LOFAR - a tool for the reconstruction of the energy and the shower maximum
The pattern of the radio emission of air showers is finely sampled with the
Low-Frequency ARray (LOFAR). A set of 382 measured air showers is used to test
a fast, analytic parameterization of the distribution of pulse powers. Using
this parameterization we are able to reconstruct the shower axis and give
estimators for the energy of the air shower as well as the distance to the
shower maximum.Comment: 15 pages, 10 figures, accepted for publication in JCA
An air shower array for LOFAR: LORA
LOFAR is a new form of radio telescope which can detect radio emission from
air showers induced by very high-energy cosmic rays. It can also look for radio
emission from particle cascades on the Moon induced by ultra high-energy cosmic
rays or neutrinos. To complement the radio detection, we are setting up a small
particle detector array LORA (LOfar Radboud Air shower array) within an area of
m diameter in the LOFAR core. It will help in triggering and
confirming the radio detection of air showers with the LOFAR antennas. In this
paper, we present a short overview about LORA and discuss its current status.Comment: 10 pages (using article.cls), 6 figures, accepted for the proceedings
of 22nd European Cosmic Ray Symposium, 3-6 August 2010, Finlan
Search for TeV -rays from H1426+428 during 2004-07 with the TACTIC telescope
The BL Lac object H1426+428 () is an established source of TeV
-rays and detections of these photons from this object also have
important implications for estimating the Extragalactic Background Light (EBL)
in addition to the understanding of the particle acceleration and -ray
production mechanisms in the AGN jets. We have observed this source for about
244h in 2004, 2006 and 2007 with the TACTIC -ray telescope located at
Mt. Abu, India. Detailed analysis of these data do not indicate the presence of
any statistically significant TeV -ray signal from the source
direction. Accordingly, we have placed an upper limit of
on the integrated
-ray flux at 3 significance level.Comment: 11 pages, 5 figures accepted for publication in Journal of Physics G:
Nuclear and Particle Physic
Cosmic Ray Physics with the LOFAR Radio Telescope
The LOFAR radio telescope is able to measure the radio emission from cosmic
ray induced air showers with hundreds of individual antennas. This allows for
precision testing of the emission mechanisms for the radio signal as well as
determination of the depth of shower maximum , the shower observable
most sensitive to the mass of the primary cosmic ray, to better than 20
g/cm. With a densely instrumented circular area of roughly 320 m, LOFAR
is targeting for cosmic ray astrophysics in the energy range -
eV. In this contribution we give an overview of the status, recent
results, and future plans of cosmic ray detection with the LOFAR radio
telescope.Comment: Proceedings of the 26th Extended European Cosmic Ray Symposium
(ECRS), Barnaul/Belokurikha, 201
Realtime processing of LOFAR data for the detection of nano-second pulses from the Moon
The low flux of the ultra-high energy cosmic rays (UHECR) at the highest
energies provides a challenge to answer the long standing question about their
origin and nature. Even lower fluxes of neutrinos with energies above
eV are predicted in certain Grand-Unifying-Theories (GUTs) and e.g.\ models for
super-heavy dark matter (SHDM). The significant increase in detector volume
required to detect these particles can be achieved by searching for the
nano-second radio pulses that are emitted when a particle interacts in Earth's
moon with current and future radio telescopes.
In this contribution we present the design of an online analysis and trigger
pipeline for the detection of nano-second pulses with the LOFAR radio
telescope. The most important steps of the processing pipeline are digital
focusing of the antennas towards the Moon, correction of the signal for
ionospheric dispersion, and synthesis of the time-domain signal from the
polyphased-filtered signal in frequency domain. The implementation of the
pipeline on a GPU/CPU cluster will be discussed together with the computing
performance of the prototype.Comment: Proceedings of the 22nd International Conference on Computing in High
Energy and Nuclear Physics (CHEP2016), US
Calibration of the LOFAR low-band antennas using the Galaxy and a model of the signal chain
The LOw-Frequency ARray (LOFAR) is used to make precise measurements of radio
emission from extensive air showers, yielding information about the primary
cosmic ray. Interpreting the measured data requires an absolute and
frequency-dependent calibration of the LOFAR system response. This is
particularly important for spectral analyses, because the shape of the detected
signal holds information about the shower development. We revisit the
calibration of the LOFAR antennas in the range of 30 - 80 MHz. Using the
Galactic emission and a detailed model of the LOFAR signal chain, we find an
improved calibration that provides an absolute energy scale and allows for the
study of frequency-dependent features in measured signals. With the new
calibration, systematic uncertainties of 13% are reached, and comparisons of
the spectral shape of calibrated data with simulations show promising
agreement.Comment: 23 pages, 10 figure
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