893 research outputs found
Search for Galactic Cosmic Ray Sources with H.E.S.S
Supernova remnants (SNRs) are the prime candidates for the acceleration of
the Galactic Cosmic Rays. Tracers for interactions of Cosmic Rays with ambient
material are gamma rays at TeV energies, which can be observed with ground
based Cherenkov telescopes like H.E.S.S. In the recent years H.E.S.S. has
detected several SNRs and interactions of SNRs with molecular clouds. Here the
current results of these observations are presented and possible leptonic and
hadronic scenarios are discussed. It is shown that it is likely that SNRs are
the sources of Galactic Cosmic Rays.Comment: 4 pages, 2 tables, proceedings of the 23rd Rencontres de Blois, 201
Synchronization and entrainment of coupled circadian oscillators
Circadian rhythms in mammals are controlled by the neurons located in the
suprachiasmatic nucleus of the hypothalamus. In physiological conditions, the
system of neurons is very efficiently entrained by the 24-hour light-dark
cycle. Most of the studies carried out so far emphasize the crucial role of the
periodicity imposed by the light dark cycle in neuronal synchronization.
Nevertheless, heterogeneity as a natural and permanent ingredient of these
cellular interactions is seemingly to play a major role in these biochemical
processes. In this paper we use a model that considers the neurons of the
suprachiasmatic nucleus as chemically-coupled modified Goodwin oscillators, and
introduce non-negligible heterogeneity in the periods of all neurons in the
form of quenched noise. The system response to the light-dark cycle periodicity
is studied as a function of the interneuronal coupling strength, external
forcing amplitude and neuronal heterogeneity. Our results indicate that the
right amount of heterogeneity helps the extended system to respond globally in
a more coherent way to the external forcing. Our proposed mechanism for
neuronal synchronization under external periodic forcing is based on
heterogeneity-induced oscillators death, damped oscillators being more
entrainable by the external forcing than the self-oscillating neurons with
different periods.Comment: 17 pages, 7 figure
Embedded star clusters as sources of high-energy cosmic rays: Modelling and constraints
Massive stars are mainly found in stellar associations. These massive star
clusters occur in the heart of giant molecular clouds. The strong stellar wind
activity in these objects generates large bubbles and induces collective
effects that could accelerate particles up to high energy and produce gamma
rays. The best way to input an acceleration origin to the stellar wind
interaction in massive stellar cluster is to observe young massive star
clusters in which no supernova explosion has occurred yet.
This work aims to constrain the part of stellar wind mechanical energy that
is converted into energetic particles using the sensitivity of the ongoing
Fermi/LAT instrument. This work further provides detailed predictions of
expected gamma-ray fluxes in the view of the on-set of the next generation of
imaging atmospheric Cherenkov telescopes.
A one-zone model where energetic particles are accelerated by repeated
interactions with strong supersonic shocks occurring in massive star clusters
was developed. The particle escape from the star cluster and subsequent
interaction with the surrounding dense material and magnetic fields of the HII
region was computed. We applied this model to a selection of eight embedded
star clusters constricted by existing observations. We evaluated the gamma-ray
signal from each object, combining both leptonic and hadronic contributions. We
searched for these emissions in the Fermi/LAT observations in the energy range
from 3 to 300 GeV and compared them to the sensitivity of the Cherenkov
Telescope Array.
No significant gamma-ray emission from these star clusters has been found.
Less than 10% of stellar wind luminosities are supplied to the relativistic
particles. Some clusters even show acceleration efficiency of less than 1%. The
CTA would be able to detect gamma-ray emission from several clusters in the
case of an acceleration efficiency of close to 1%.Comment: accepted for publication in Astronomy&Astrophysic
Order parameter expansion study of synchronous firing induced by quenched noise in the active rotator model
We use a recently developed order parameter expansion method to study the
transition to synchronous firing occuring in a system of coupled active
rotators under the exclusive presence of quenched noise. The method predicts
correctly the existence of a transition from a rest state to a regime of
synchronous firing and another transition out of it as the intensity of the
quenched noise increases and leads to analytical expressions for the critical
noise intensities in the large coupling regime. It also predicts the order of
the transitions for different probability distribution functions of the
quenched variables. We use numerical simulations and finite size scaling theory
to estimate the critical exponents of the transitions and found values which
are consistent with those reported in other scalar systems in the exclusive
presence of additive static disorder
Advantages of Hopping on a Zig-zag Course
We investigate self-moving particles which prefer to hop with a certain
turning angle equally distributed to the right or left. We assume this turning
angle distribution to be given by a double Gaussian distribution. Based on the
model of Active Brownian particles and we calculate the diffusion coefficient
in dependence on the mean and the dispersion of the turning angles. It is shown
that bounded distribution of food in patches will be optimally consumed by the
objects if they hop preferably with a given angle and not straight forwardly.Comment: 10 pages, 3 figures, accepted to be published in Physica
H.E.S.S. deeper observations on SNR RX J0852.0-4622
Supernova Remnants (SNRs) are believed to be acceleration sites of Galactic
cosmic rays. Therefore, deep studies of these objects are instrumental for an
understanding of the high energy processes in our Galaxy. RX J0852.0-4622, also
known as Vela Junior, is one of the few (4) shell-type SNRs resolved at Very
High Energies (VHE; E > 100 GeV). It is one of the largest known VHE sources (~
1.0 deg radius) and its flux level is comparable to the flux level of the Crab
Nebula in the same energy band. These characteristics allow for a detailed
analysis, shedding further light on the high-energy processes taking place in
the remnant. In this document we present further details on the spatial and
spectral morphology derived with an extended data set. The analysis of the
spectral morphology of the remnant is compatible with a constant power-law
photon index of 2.11 +/- 0.05_stat +/- 0.20_syst from the whole SNR in the
energy range from 0.5 TeV to 7 TeV. The analysis of the spatial morphology
shows an enhanced emission towards the direction of the pulsar PSR J0855-4644,
however as the pulsar is lying on the rim of the SNR, it is difficult to
disentangle both contributions. Therefore, assuming a point source, the upper
limit on the flux of the pulsar wind nebula (PWN) between 1 TeV and 10 TeV, is
estimated to be ~ 2% of the Crab Nebula flux in the same energy range
X- and gamma-ray studies of HESS J1731-347 coincident with a newly discovered SNR
In the survey of the Galactic plane conducted with H.E.S.S., many VHE
gamma-ray sources were discovered for which no clear counterpart at other
wavelengths could be identified. HESS J1731-347 initially belonged to this
source class. Recently however, the new shell-type supernova remnant (SNR)
G353.6-0.7 was discovered in radio data, positionally coinciding with the VHE
source. We will present new X-ray observations that cover a fraction of the VHE
source, revealing nonthermal emission that most likely can be interpreted as
synchrotron emission from high-energy electrons. This, along with a larger
H.E.S.S. data set which comprises more than twice the observation time used in
the discovery paper, allows us to test whether the VHE source may indeed be
attributed to shell-type emission from that new SNR. If true, this would make
HESS J1731-347 a new object in the small but growing class of non-thermal
shell-type supernova remnants with VHE emission.Comment: 4 pages, 5 figures, to appear in proceedings of the 31st ICRC, Lodz,
Polan
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