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

Particle acceleration in axisymmetric pulsar current sheets

The equatorial current sheet in pulsar magnetospheres is often regarded as an ideal site for particle acceleration via relativistic reconnection. Using 2D spherical particle-in-cell simulations, we investigate particle acceleration in the axisymmetric pulsar magnetosphere as a function of the injected plasma multiplicity and magnetization. We observe a clear transition from a highly charge-separated magnetosphere for low plasma injection with little current and spin-down power, to a nearly force-free solution for high plasma multiplicity characterized by a prominent equatorial current sheet and high spin-down power. We find significant magnetic dissipation in the current sheet, up to 30% within 5 light-cylinder radii in the high-multiplicity regime. The simulations unambiguously demonstrate that the dissipated Poynting flux is efficiently channeled to the particles in the sheet, close to the Y-point within about 1-2 light cylinder radii from the star. The mean particle energy in the sheet is given by the upstream plasma magnetization at the light cylinder. The study of particle orbits shows that all energetic particles originate from the boundary layer between the open and the closed field lines. Energetic positrons always stream outward, while high-energy electrons precipitate back towards the star through the sheet and along the separatrices, which may result in auroral-like emission. Our results suggest that the current sheet and the separatrices may be the main source of high-energy radiation in young pulsars.Comment: 16 pages, 17 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

Modeling the three-dimensional pair cascade in binaries. Application to LS 5039

10 pages, 11 figures, accepted for publication in Astronomy and AstrophysicsLS 5039 is a Galactic binary system emitting high and very-high energy gamma rays. The gamma-ray flux is modulated on the orbital period and the TeV lightcurve shaped by photon-photon annihilation. The observed very-high energy modulation can be reproduced with a simple leptonic model but fails to explain the flux detected by HESS at superior conjunction, where gamma rays are fully absorbed. The contribution from an electron-positron pair cascade could be strong and prevail over the primary flux at superior conjunction. The created pairs can be isotropized by the magnetic field, resulting in a three-dimensional cascade. The aim of this article is to investigate the gamma ray radiation from this pair cascade in LS 5039. This additional component could account for HESS observations at superior conjunction in the system. A semi-analytical and a Monte Carlo method for computing three-dimensional cascade radiation are presented and applied in the context of binaries. Three-dimensional cascade radiation contributes significantly at every orbital phase in the TeV lightcurve, and dominates close to superior conjunction. The amplitude of the gamma-ray modulation is correctly reproduced for an inclination of the orbit of about 40 degrees. Primary pairs should be injected close to the compact object location, otherwise the shape of the modulation is not explained. In addition, synchrotron emission from the cascade in X-rays constrains the ambient magnetic field to below 10 G. The radiation from a three-dimensional pair cascade can account for the TeV flux detected by HESS at superior conjunction in LS 5039, but the very-high energy spectrum at low fluxes remains difficult to explain in this model

Energetic Constraints on a Rapid Gamma-Ray Flare in PKS 1222+216

We study theoretical implications of a rapid Very-High-Energy (VHE) flare detected by MAGIC in the Flat-Spectrum Radio Quasar PKS 1222+216. The minimum distance from the jet origin at which this flare could be produced is 0.5 pc. A moderate Doppler factor of the VHE source, D_{VHE} ~ 20, is allowed by all opacity constraints. The concurrent High-Energy (HE) emission observed by Fermi provides estimates of the total jet power and the jet magnetic field strength. Energetic constraints for the VHE flare are extremely tight: for an isotropic particle distribution they require a huge co-moving energy density in the emitting region and a very efficient radiative process. We disfavor hadronic processes due to their low radiative efficiency, as well as the synchrotron scenario recently proposed for the case of HE flares in the Crab Nebula, since the parameters needed to overcome the radiative losses are quite extreme. The VHE emission can be explained by the Synchrotron Self-Compton (SSC) mechanism for D_{VHE} ~ 20 or by the External Radiation Compton (ERC) mechanism involving the infrared radiation of the dusty torus for D_{VHE} ~ 50. After discussing several alternative scenarios, we propose that the extreme energy density constraint can be satisfied when the emission comes from highly anisotropic short-lived bunches of particles formed by the kinetic beaming mechanism in magnetic reconnection sites. By focusing the emitting particles into very narrow beams, this mechanism allows one to relax the causality constraint on the source size, decreasing the required energy density by 4 orders of magnitude.Comment: 12 pages, 2 figures, accepted for publication in MNRA

Tracing baculovirus AcMNPV infection using a real time method based on ANCHORTM DNA labeling technology

Many steps in the baculovirus life cycle, from initial ingestion to the subsequent infection of all larval cells, remain largely unknown; primarily because it has hitherto not been possible to follow individual genomes and their lineages. Use of ANCHORTM technology allows a high intensity fluorescent labelling of DNA. When applied to a virus genome, it is possible to follow individual particles, and the overall course of infection. This technology has been adapted to enable labelling of the baculovirus Autographa californica Multiple NucleoPolyhedroVirus genome, as a first step to its application to other baculoviruses. AcMNPV was modified by inserting the two components of ANCHORTM: a specific DNA-binding protein fused to a fluorescent reporter, and the corresponding DNA recognition sequence. The resulting modified virus was stable, infectious, and replicated correctly in Spodoptera frugiperda 9 (Sf9) cells and in vivo. Both budded viruses and occlusion bodies were clearly distinguishable, and infecting cells or larvae allowed the infection process to be monitored in living cells or tissues. The level of fluorescence in the culture medium of infected cells in vitro showed a good correlation with the number of infectious budded viruses. A cassette that can be used in other baculoviruses has been designed. Altogether our results introduce for the first time the generation of autofluorescent baculovirus and their application to follow infection dynamics directly in living cells or tissues

A single polyploidization event at the origin of the tetraploid genome of Coffea arabica is responsible for the extremely low genetic variation in wild and cultivated germplasm

The genome of the allotetraploid species Coffea arabica L. was sequenced to assemble independently the two component subgenomes (putatively deriving from C. canephora and C. eugenioides) and to perform a genome-wide analysis of the genetic diversity in cultivated coffee germplasm and in wild populations growing in the center of origin of the species. We assembled a total length of 1.536 Gbp, 444 Mb and 527 Mb of which were assigned to the canephora and eugenioides subgenomes, respectively, and predicted 46,562 gene models, 21,254 and 22,888 of which were assigned to the canephora and to the eugeniodes subgenome, respectively. Through a genome-wide SNP genotyping of 736 C. arabica accessions, we analyzed the genetic diversity in the species and its relationship with geographic distribution and historical records. We observed a weak population structure due to low-frequency derived alleles and highly negative values of Taijma's D, suggesting a recent and severe bottleneck, most likely resulting from a single event of polyploidization, not only for the cultivated germplasm but also for the entire species. This conclusion is strongly supported by forward simulations of mutation accumulation. However, PCA revealed a cline of genetic diversity reflecting a west-to-east geographical distribution from the center of origin in East Africa to the Arabian Peninsula. The extremely low levels of variation observed in the species, as a consequence of the polyploidization event, make the exploitation of diversity within the species for breeding purposes less interesting than in most crop species and stress the need for introgression of new variability from the diploid progenitors

Present Status Of Particle Acceleration In Relativistic Outflows

International audienceRelativistic outflows are ubiquitous in high-energy cosmic phenomena. Whether their reservoir of free energy at launch is in the form of bulk kinetic, magnetic or gravitational, it is ultimately channeled into energetic particles and non-thermal radiation. A key question is to understand how this transfer operates efficiently and under which conditions. Here, I review some of the recent developments in the modeling of two particle acceleration processes, namely relativistic diffusive shock acceleration and relativistic magnetic reconnection from the perspective of ab-initio particle-in-cell simulations. These results and their astrophysical implications are briefly discussed in the context of relativistic outflows

Particle acceleration and radiation in pulsars: New insights from kinetic simulations

International audiencePulsar magnetospheres are efficient particle accelerators, as evidenced by high-energy gamma-ray observations. Where and how particle acceleration occurs are difficult questions to answer because it results from a complex interplay between relativistic electrodynamics, pair creation and non-thermal radiation. The recent development of global particle-in-cell simulations allows, for the first time, to address this problem from first principles and self-consistently. Simulations indicate that relativistic reconnection in the equatorial current sheet plays a key role in particle acceleration and the emission of high-energy radiation. We discuss these results in the context of young gamma-ray pulsars