8,481 research outputs found
Modeling high-energy pulsar lightcurves from first principles
Current models of gamma-ray lightcurves in pulsars suffer from large
uncertainties on the precise location of particle acceleration and radiation.
Here, we present an attempt to alleviate these difficulties by solving for the
electromagnetic structure of the oblique magnetosphere, particle acceleration,
and the emission of radiation self-consistently, using 3D spherical
particle-in-cell simulations. We find that the low-energy radiation is
synchro-curvature radiation from the polar-cap regions within the light
cylinder. In contrast, the high-energy emission is synchrotron radiation that
originates exclusively from the Y-point and the equatorial current sheet where
relativistic magnetic reconnection accelerates particles. In most cases,
synthetic high-energy lightcurves contain two peaks that form when the current
sheet sweeps across the observer's line of sight. We find clear evidence of
caustics in the emission pattern from the current sheet. High-obliquity
solutions can present up to two additional secondary peaks from energetic
particles in the wind region accelerated by the reconnection-induced flow near
the current sheet. The high-energy radiative efficiency depends sensitively on
the viewing angle, and decreases with increasing pulsar inclination. The
high-energy emission is concentrated in the equatorial regions where most of
the pulsar spindown is released and dissipated. These results have important
implications for the interpretation of gamma-ray pulsar data.Comment: 14 pages, 11 figures, Accepted for publication in MNRA
Ab-initio pulsar magnetosphere: the role of general relativity
It has recently been demonstrated that self-consistent particle-in-cell
simulations of low-obliquity pulsar magnetospheres in flat spacetime show weak
particle acceleration and no pair production near the poles. We investigate the
validity of this conclusion in a more realistic spacetime geometry via
general-relativistic particle-in-cell simulations of the aligned pulsar
magnetospheres with pair formation. We find that the addition of frame-dragging
effect makes local current density along the magnetic field larger than the
Goldreich-Julian value, which leads to unscreened parallel electric fields and
the ignition of a pair cascade. When pair production is active, we observe
field oscillations in the open field bundle which could be related to pulsar
radio emission. We conclude that general relativistic effects are essential for
the existence of pulsar mechanism in low obliquity rotators.Comment: 5 pages, 4 figure, submitted to ApJLetter
Simulations of particle acceleration beyond the classical synchrotron burnoff limit in magnetic reconnection: An explanation of the Crab flares
It is generally accepted that astrophysical sources cannot emit synchrotron
radiation above 160 MeV in their rest frame. This limit is given by the balance
between the accelerating electric force and the radiation reaction force acting
on the electrons. The discovery of synchrotron gamma-ray flares in the Crab
Nebula, well above this limit, challenges this classical picture of particle
acceleration. To overcome this limit, particles must accelerate in a region of
high electric field and low magnetic field. This is possible only with a
non-ideal magnetohydrodynamic process, like magnetic reconnection. We present
the first numerical evidence of particle acceleration beyond the synchrotron
burnoff limit, using a set of 2D particle-in-cell simulations of
ultra-relativistic pair plasma reconnection. We use a new code, Zeltron, that
includes self-consistently the radiation reaction force in the equation of
motion of the particles. We demonstrate that the most energetic particles move
back and forth across the reconnection layer, following relativistic Speiser
orbits. These particles then radiate >160 MeV synchrotron radiation rapidly,
within a fraction of a full gyration, after they exit the layer. Our analysis
shows that the high-energy synchrotron flux is highly variable in time because
of the strong anisotropy and inhomogeneity of the energetic particles. We
discover a robust positive correlation between the flux and the cut-off energy
of the emitted radiation, mimicking the effect of relativistic Doppler
amplification. A strong guide field quenches the emission of >160 MeV
synchrotron radiation. Our results are consistent with the observed properties
of the Crab flares, supporting the reconnection scenario.Comment: 15 pages, 16 figures, Accepted for publication in The Astrophysical
Journa
What can Simbol-X do for gamma-ray binaries?
Gamma-ray binaries have been uncovered as a new class of Galactic objects in
the very high energy sky (> 100 GeV). The three systems known today have hard
X-ray spectra (photon index ~ 1.5), extended radio emission and a high
luminosity in gamma-rays. Recent monitoring campaigns of LSI +61 303 in X-rays
have confirmed variability in these systems and revealed a spectral hardening
with increasing flux. In a generic one-zone leptonic model, the cooling of
relativistic electrons accounts for the main spectral and temporal features
observed at high energy. Persistent hard X-ray emission is expected to extend
well beyond 10 keV. We explain how Simbol-X will constrain the existing models
in connection with Fermi Space Telescope measurements. Because of its
unprecedented sensitivity in hard X-rays, Simbol-X will also play a role in the
discovery of new gamma-ray binaries, giving new insights into the evolution of
compact binaries.Comment: 4 pages, 1 figure, Proceedings of the 2nd International Simbol-X
symposium held in Paris, 2-5 December 200
An exploratory study on internet addiction, somatic symptoms and emotional and behavioral functioning in school-aged adolescents
Objective: In the last two decades there has been a significant transformation regarding the use of new technologies. Despite growing acknowledgement concerning the different activities and functions of digital technologies, there remains a lack of understanding on how technology overuse may negatively impact both physical and psychosocial well-being. Although researchers have begun to explore the meaning and implications of excessive Internet use in non-clinical populations of children and adolescents, there is still little consistent knowledge on the topic. This study aimed to extend existing knowledge on the excessive use of the Internet among school-aged adolescents, focusing on its association with recurrent somatic symptoms, depressive risk and behavioral and emotional problems. Method: Two hundred and forty adolescents (51.9% females) aged between 10 and 15, participated in this study. Data was collected using the Children’s Somatization Inventory, the Internet Addiction Test, the Children’s Depression Inventory, the Youth Self Report and the Emotion Regulation Questionnaire. Structural Equation Model analysis was used to analyse the data. Results: Approximately 21.8% of participants reported excessive Internet use based on Young’s criteria. Higher levels of Internet use were associated with somatic and depressive symptoms as well as emotional and behavioral problems. Depressive Symptoms predicted both Internet Addiction (b = 0.304, p < 0.001) and Internalizing (b = 0.542, p <0.001) and Externalizing problems (b = 0.484, p < 0.001). Internet Addiction also significantly predicted both Internalizing (b = 0.162, p = 0.02) and Externalizing problems (b = 0.183, p = 0.02). Finally, Structural Equation Modeling showed that the indirect effect of Depressive Symptoms (via Internet Addiction) on Internalizing or Externalizing problems were significant. Conclusions: Longitudinal studies are needed to confirm these findings and to identify the mechanisms linking Internet use, somatic symptoms and adaptive functioning
A static scheduling approach to enable safety-critical OpenMP applications
Parallel computation is fundamental to satisfy the performance requirements of advanced safety-critical systems. OpenMP is a good candidate to exploit the performance opportunities of parallel platforms. However, safety-critical systems are often based on static allocation strategies, whereas current OpenMP implementations are based on dynamic schedulers. This paper proposes two OpenMP-compliant static allocation approaches: an optimal but costly approach based on an ILP formulation, and a sub-optimal but tractable approach that computes a worst-case makespan bound close to the optimal one.This work is funded by the EU projects P-SOCRATES (FP7-ICT-2013-10) and HERCULES (H2020/ICT/2015/688860), and the Spanish Ministry of Science and Innovation under contract TIN2015-65316-P.Peer ReviewedPostprint (author's final draft
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