2,445 research outputs found
High-energy radiation from the relativistic jet of Cygnus X-3
Cygnus X-3 is an accreting high-mass X-ray binary composed of a Wolf-Rayet
star and an unknown compact object, possibly a black hole. The gamma-ray space
telescope Fermi found definitive evidence that high-energy emission is produced
in this system. We propose a scenario to explain the GeV gamma-ray emission in
Cygnus X-3. In this model, energetic electron-positron pairs are accelerated at
a specific location in the relativistic jet, possibly related to a
recollimation shock, and upscatter the stellar photons to high energies. The
comparison with Fermi observations shows that the jet should be inclined close
to the line of sight and pairs should not be located within the system.
Energetically speaking, a massive compact object is favored. We report also on
our investigations of the gamma-ray absorption of GeV photons with the
radiation emitted by a standard accretion disk in Cygnus X-3. This study shows
that the gamma-ray source should not lie too close to the compact object.Comment: 4 pages, 3 figures, Proceedings of the SF2A conference held in
Marseille, 21-24 June 201
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
Extreme particle acceleration in magnetic reconnection layers. Application to the gamma-ray flares in the Crab Nebula
The gamma-ray space telescopes AGILE and Fermi detected short and bright
synchrotron gamma-ray flares at photon energies above 100 MeV in the Crab
Nebula. This discovery suggests that electron-positron pairs in the nebula are
accelerated to PeV energies in a milliGauss magnetic field, which is difficult
to explain with classical models of particle acceleration and pulsar wind
nebulae. We investigate whether particle acceleration in a magnetic
reconnection layer can account for the puzzling properties of the flares. We
numerically integrate relativistic test-particle orbits in the vicinity of the
layer, including the radiation reaction force, and using analytical expressions
for the large-scale electromagnetic fields. As they get accelerated by the
reconnection electric field, the particles are focused deep inside the current
layer where the magnetic field is small. The electrons suffer less from
synchrotron losses and are accelerated to extremely high energies. Population
studies show that, at the end of the layer, the particle distribution piles up
at the maximum energy given by the electric potential drop and is focused into
a thin fan beam. Applying this model to the Crab Nebula, we find that the
emerging synchrotron emission spectrum peaks above 100 MeV and is close to the
spectral shape of a single electron. The flare inverse Compton emission is
negligible and no detectable emission is expected at other wavelengths. This
mechanism provides a plausible explanation for the gamma-ray flares in the Crab
Nebula and could be at work in other astrophysical objects such as relativistic
jets in active galactic nuclei.Comment: 16 pages, 16 figures, Accepted for publication in The Astrophysical
Journa
Plasmoid identification and statistics in two-dimensional Harris sheet and GRMHD simulations
Magnetic reconnection is a ubiquitous phenomenon for magnetized plasmas and
leads to the rapid reconfiguration of magnetic field lines. During reconnection
events, plasma is heated and accelerated until the magnetic field lines enclose
and capture the plasma within a circular configuration. These plasmoids could
therefore observationally manifest themselves as hot spots that are associated
with flaring behavior in supermassive black hole systems, such as Sagittarius
A. We have developed a novel algorithm for identifying plasmoid
structures, which incorporates watershed and custom closed contouring steps.
From the identified plasmoids, we determine the plasma characteristics and
energetics in magnetohydrodynamical simulations. The algorithm's performance is
showcased for a high-resolution suite of axisymmetric ideal and resistive
magnetohydrodynamical simulations of turbulent accretion discs surrounding a
supermassive black hole. For validation purposes, we also evaluate several
Harris current sheets that are well-investigated in the literature.
Interestingly, we recover the characteristic power-law distribution of plasmoid
sizes for both the black hole and Harris sheet simulations. This indicates that
while the dynamics are vastly different, with different dominant plasma
instabilities, the plasmoid creation behavior is similar. Plasmoid occurrence
rates for resistive general relativistic magnetohydrodynamical simulations are
significantly higher than for the ideal counterpart. Moreover, the largest
identified plasmoids are consistent with sizes typically assumed for
semi-analytical interpretation of observations. We recover a positive
correlation between the plasmoid formation rate and a decrease in
black-hole-horizon-penetrating magnetic flux. The developed algorithm has
enabled an extensive quantitative analysis of plasmoid formation in black hole
accretion simulations.Comment: 23 pages, 15 figures, submitted to MNRA
Characterization update of HIV-1 M subtypes diversity and proposal for subtypes A and D sub-subtypes reclassification.
BACKGROUND: The large and constantly evolving HIV-1 pandemic has led to an increasingly complex diversity. Because of some taxonomic difficulties among the most diverse HIV-1 subtypes, and taking advantage of the large amount of sequence data generated in the recent years, we investigated novel lineage patterns among the main HIV-1 subtypes. RESULTS: All HIV full-length genomes available in public databases were analysed (n = 2017). Maximum likelihood phylogenies and pairwise genetic distance were obtained. Clustering patterns and mean distributions of genetic distances were compared within and across the current groups, subtypes and sub-subtypes of HIV-1 to detect and analyse any divergent lineages within previously defined HIV lineages. The level of genetic similarity observed between most HIV clades was deeply consistent with the current classification. However, both subtypes A and D showed evidence of further intra-subtype diversification not fully described by the nomenclature system at the time and could be divided into several distinct sub-subtypes. CONCLUSIONS: With this work, we propose an updated nomenclature of sub-types A and D better reflecting their current genetic diversity and evolutionary patterns. Allowing a more accurate nomenclature and classification system is a necessary step for easier subtyping of HIV strains and a better detection or follow-up of viral epidemiology shifts
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