5,836 research outputs found
What caused the GeV flare of PSR B1259-63 ?
PSR B1259-63 is a gamma-ray binary system composed of a high spindown pulsar
and a massive star. Non-thermal emission up to TeV energies is observed near
periastron passage, attributed to emission from high energy e+e- pairs
accelerated at the shock with the circumstellar material from the companion
star, resulting in a small-scale pulsar wind nebula. Weak gamma-ray emission
was detected by the Fermi/LAT at the last periastron passage, unexpectedly
followed 30 days later by a strong flare, limited to the GeV band, during which
the luminosity nearly reached the spindown power of the pulsar. The origin of
this GeV flare remains mysterious. We investigate whether the flare could have
been caused by pairs, located in the vicinity of the pulsar, up-scattering
X-ray photons from the surrounding pulsar wind nebula rather than UV stellar
photons, as usually assumed. Such a model is suggested by the geometry of the
interaction region at the time of the flare. We compute the gamma-ray
lightcurve for this scenario, based on a simplified description of the
interaction region, and compare it to the observations. The GeV lightcurve
peaks well after periastron with this geometry. The pairs are inferred to have
a Lorentz factor ~500. They also produce an MeV flare with a luminosity ~1e34
erg/s prior to periastron passage. A significant drawback is the very high
energy density of target photons required for efficient GeV emission. We
propose to associate the GeV-emitting pairs with the Maxwellian expected at
shock locations corresponding to high pulsar latitudes, while the rest of the
non-thermal emission arises from pairs accelerated in the equatorial region of
the pulsar wind termination shock.Comment: 6 pages, 3 figures, accepted for publication in A&
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
Particle Acceleration in Pulsar Wind Nebulae: PIC modelling
We discuss the role of particle-in-cell (PIC) simulations in unveiling the
origin of the emitting particles in PWNe. After describing the basics of the
PIC technique, we summarize its implications for the quiescent and the flaring
emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be
emerging that, in addition to the standard scenario of particle acceleration
via the Fermi process at the termination shock of the pulsar wind, magnetic
reconnection in the wind, at the termination shock and in the Nebula plays a
major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae"
edited by D. Torres for Springer, based on the invited contributions to the
workshop held in Sant Cugat (Barcelona), June 14-17, 201
One-dimensional pair cascade emission in gamma-ray binaries
In gamma-ray binaries such as LS 5039 a large number of electron-positron
pairs are created by the annihilation of primary very high energy (VHE)
gamma-rays with photons from the massive star. The radiation from these
particles contributes to the total high energy gamma-ray flux and can initiate
a cascade, decreasing the effective gamma-ray opacity in the system. The aim of
this paper is to model the cascade emission and investigate if it can account
for the VHE gamma-ray flux detected by HESS from LS 5039 at superior
conjunction, where the primary gamma-rays are expected to be fully absorbed. A
one-dimensional cascade develops along the line-of-sight if the deflections of
pairs induced by the surrounding magnetic field can be neglected. A
semi-analytical approach can then be adopted, including the effects of the
anisotropic seed radiation field from the companion star. Cascade equations are
numerically solved, yielding the density of pairs and photons. In LS 5039, the
cascade contribution to the total flux is large and anti-correlated with the
orbital modulation of the primary VHE gamma-rays. The cascade emission
dominates close to superior conjunction but is too strong to be compatible with
HESS measurements. Positron annihilation does not produce detectable 511 keV
emission. This study provides an upper limit to cascade emission in gamma-ray
binaries at orbital phases where absorption is strong. The pairs are likely to
be deflected or isotropized by the ambient magnetic field, which will reduce
the resulting emission seen by the observer. Cascade emission remains a viable
explanation for the detected gamma-rays at superior conjunction in LS 5039.Comment: 8 pages, 7 figures, 1 table, accepted for publication in Astronomy
and Astrophysic
The relativistic jet of Cygnus X-3 in gamma rays
High energy gamma-rays have been detected from Cygnus X-3, a system composed
of a Wolf-Rayet star and a black hole or neutron star. The gamma-ray emission
is linked to the radio emission from the jet launched in the system. The flux
is modulated with the 4.8 hr orbital period, as expected if high energy
electrons are upscattering photons emitted by the Wolf-Rayet star to gamma-ray
energies. This modulation is computed assuming that high energy electrons are
located at some distance along a relativistic jet of arbitrary orientation.
Modeling shows that the jet must be inclined and that the gamma ray emitting
electrons cannot be located within the system. This is consistent with the idea
that the electrons gain energy where the jet is recollimated by the stellar
wind pressure and forms a shock. Jet precession should strongly affect the
gamma-ray modulation shape at different epochs. The power in non-thermal
electrons represents a small fraction of the Eddington luminosity only if the
inclination is low i.e. if the compact object is a black hole.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
Fidelity and level correlations in the transition from regularity to chaos
Mean fidelity amplitude and parametric energy--energy correlations are
calculated exactly for a regular system, which is subject to a chaotic random
perturbation. It turns out that in this particular case under the average both
quantities are identical. The result is compared with the susceptibility of
chaotic systems against random perturbations. Regular systems are more
susceptible against random perturbations than chaotic ones.Comment: 7 pages, 1 figur
Rates, Delays, and Completeness of General Practitioners' Responses to a Postal Versus Web-Based Survey: A Randomized Trial.
Web-based surveys have become a new and popular method for collecting data, but only a few studies have directly compared postal and Web-based surveys among physicians, and none to our knowledge among general practitioners (GPs).
Our aim is to compare two modes of survey delivery (postal and Web-based) in terms of participation rates, response times, and completeness of questionnaires in a study assessing GPs' preventive practices.
This randomized study was conducted in Western Switzerland (Geneva and Vaud) and in France (Alsace and Pays de la Loire) in 2015. A random selection of community-based GPs (1000 GPs in Switzerland and 2400 GPs in France) were randomly allocated to receive a questionnaire about preventive care activities either by post (n=700 in Switzerland, n=400 in France) or by email (n=300 in Switzerland, n=2000 in France). Reminder messages were sent once in the postal group and twice in the Web-based group. Any GPs practicing only complementary and alternative medicine were excluded from the study.
Among the 3400 contacted GPs, 764 (22.47%, 95% CI 21.07%-23.87%) returned the questionnaire. Compared to the postal group, the participation rate in the Web-based group was more than four times lower (246/2300, 10.70% vs 518/1100, 47.09%, P<.001), but median response time was much shorter (1 day vs 1-3 weeks, P<.001) and the number of GPs having fully completed the questionnaire was almost twice as high (157/246, 63.8% vs 179/518, 34.6%, P<.001).
Web-based surveys offer many advantages such as reduced response time, higher completeness of data, and large cost savings, but our findings suggest that postal surveys can be still considered for GP research. The use of mixed-mode approaches is probably a good strategy to increase GPs' participation in surveys while reducing costs
The Impact of Vacuum Gate Valves on the LHC Beam
The LHC vacuum sector valves are located in the straight sections of the LHC ring, and designed to sectorize the LHC vacuum. The valves are interlocked and should trigger a beam dump request if they close on a circulating beam. This report studies the impact on the machine if this request is not made and the valve scrapes the LHC beam halo. Cascade calculations are made using a model of IR7, with several different valve locations, to calculate the downstream energy deposition in superconducting magnet coils and the corresponding signal in beam loss monitors at the quench level. The calculations are done at 7, 5, and 3.5 TeV. It is found that when a downstream magnet reaches the quench level, the neighbouring BLMs see a signal well above the detection threshold. Furthermore, the BLM signal is consistent with the BLM applied threshold settings and a signal is seen in the time domain before the quench level is reached. Therefore the report concludes that the BLMs can see the closing valve and trigger a beam dump before the quench (or damage) level is reached
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