Benoit Cerutti

Princeton University, Princeton, New Jersey, United States

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Publications (34)78.31 Total impact

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    ABSTRACT: The Crab Nebula was formed after the collapse of a massive star about a thousand years ago, leaving behind a pulsar that inflates a bubble of ultra-relativistic electron-positron pairs permeated with magnetic field. The observation of brief but bright flares of energetic gamma rays suggests that pairs are accelerated to PeV energies within a few days; such rapid acceleration cannot be driven by shocks. Here, it is argued that the flares may be the smoking gun of magnetic dissipation in the Nebula. Using 2D and 3D particle-in-cell simulations, it is shown that the observations are consistent with relativistic magnetic reconnection, where pairs are subject to strong radiative cooling. The Crab flares may highlight the importance of relativistic magnetic reconnection in astrophysical sources.
    01/2014; 21(5).
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    ABSTRACT: The discovery of rapid synchrotron gamma-ray flares above 100 MeV from the Crab Nebula has attracted new interest in alternative particle acceleration mechanisms in pulsar wind nebulae. Diffuse shock-acceleration fails to explain the flares because particle acceleration and emission occur during a single or even sub-Larmor timescale. In this regime, the synchrotron energy losses induce a drag force on the particle motion that balances the electric acceleration and prevents the emission of synchrotron radiation above 160 MeV. Previous analytical studies and 2D particle-in-cell (PIC) simulations indicate that relativistic reconnection is a viable mechanism to circumvent the above difficulties. The reconnection electric field localized at X-points linearly accelerates particles with little radiative energy losses. In this paper, we check whether this mechanism survives in 3D, using a set of large PIC simulations with radiation reaction force and with a guide field. In agreement with earlier works, we find that the relativistic drift kink instability deforms and then disrupts the layer, resulting in significant plasma heating but few non-thermal particles. A moderate guide field stabilizes the layer and enables particle acceleration. We report that 3D magnetic reconnection can accelerate particles above the standard radiation reaction limit, although the effect is less pronounced than in 2D with no guide field. We confirm that the highest energy particles form compact bunches within magnetic flux ropes, and a beam tightly confined within the reconnection layer, which could result in the observed Crab flares when, by chance, the beam crosses our line of sight.
    11/2013; 782(2).
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    G. Dubus, B. Cerutti
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    ABSTRACT: 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.
    Astronomy and Astrophysics 08/2013; · 5.08 Impact Factor
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    ABSTRACT: Magnetic reconnection converts magnetic field energy into particle kinetic energy, accelerating particles to sufficient energies to emit gamma-ray synchrotron radiation in astrophysical contexts, possibly including pulsar wind nebulae, Gamma-Ray Bursts, and blazar jets. A balance between acceleration (by the electric field E) and synchrotron braking (while orbiting a B-field line) limits particle energy so that synchrotron processes cannot emit photons above about 160 MeV, unless E > B. However, short, intense gamma-ray flares of much higher energies have recently been observed in the Crab nebula. This work demonstrates, using 2D simulations, that reconnection in relativistic electron-positron pair plasmas can accelerate particles in Speiser orbits around the magnetic null (where E > B) such that the particles can emit synchrotron photons above the 160 MeV limit. Furthermore, reconnection bunches particles and focuses them into beams; high-energy synchrotron radiation is also strongly beamed, and the sweeping of the beam across the observer's line of sight can explain the fast time variability of the flares.
    04/2013;
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    ABSTRACT: 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.
    The Astrophysical Journal 02/2013; 770(2). · 6.73 Impact Factor
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    ABSTRACT: We report on the first study of energetic particles and radiation angular distributions generated in relativistic collisionless pair plasma reconnection, using 2.5-dimensional particle-incell simulations. We have discovered that the energetic particles are focused within a small solid angle, and bunched into compact regions inside magnetic islands. In addition, we find that the synchrotron radiation emitted by these particles, as seen by an external observer, is tightly beamed and variable on time scales much shorter than the light-crossing time of the system. This energy dependent "kinetic beaming" differs fundamentally from the achromatic Doppler beaming usually ascribed to relativistic jets. Our findings can account for the puzzling discoveries of bright, short flares seen in high-energy gamma rays, especially from the Crab Nebula and from blazars.
    12/2012;
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    ABSTRACT: Magnetic reconnection is one of a few astrophysical mechanisms that can accelerate particles to energies sufficient to emit observable high-energy radiation. This work reports on 2D simulations of reconnection in relativistic electron-positron pair plasmas, which may power gamma-ray emission from pulsar wind nebulae (PWNe), Gamma-Ray Bursts (GRBs), and blazar jets. The most important new discovery is the strong, energy-dependent angular anisotropy and spatial inhomogeneity of accelerated particles: high-energy particles are bunched in space and focused into beams mostly confined to the reconnection layer midplane. Another important advance is the calculation of the associated radiative signatures (spectra and light curves) seen by a distant observer. The synchrotron and inverse Compton radiation from the high-energy particles is likewise focused in narrow beams. The beams sweep back and forth within the midplane, so that an observer sees intense bursts (only) when a beam crosses the line of sight. The resulting rapid variability, on timescales much shorter than the light-crossing time of the reconnection region, could explain the short, intense gamma-ray flares observed in blazar jets and PWNe, including the GeV flares recently discovered in the Crab nebula.
    10/2012;
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    ABSTRACT: We report on the first study of the angular distribution of energetic particles and radiation generated in relativistic collisionless electron-positron pair plasma reconnection, using two-dimensional particle-in-cell simulations. We discover a strong anisotropy of the particles accelerated by reconnection and the associated strong beaming of their radiation. The focusing of particles and radiation increases with their energy; in this sense, this "kinetic beaming" effect differs fundamentally from the relativistic Doppler beaming usually invoked in high-energy astrophysics, in which all photons are focused and boosted achromatically. We also present, for the first time, the modeling of the synchrotron emission as seen by an external observer during the reconnection process. The expected lightcurves comprise several bright symmetric sub-flares emitted by the energetic beam of particles sweeping across the line of sight intermittently, and exhibit super-fast time variability as short as about one tenth of the system light-crossing time. The concentration of the energetic particles into compact regions inside magnetic islands and particle anisotropy explain the rapid variability. This radiative signature of reconnection can account for the brightness and variability of the gamma-ray flares in the Crab Nebula and in blazars.
    The Astrophysical Journal Letters 05/2012; 754(2). · 6.35 Impact Factor
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    ABSTRACT: 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.
    Monthly Notices of the Royal Astronomical Society 02/2012; 425(4). · 5.52 Impact Factor
  • Benoit Cerutti, D. A. Uzdensky, M. C. Begelman
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    ABSTRACT: The discovery by Agile and Fermi of intense day-long synchrotron gamma-ray flares above 100 MeV in the Crab Nebula challenges classical models of pulsar wind nebulae and particle acceleration. We argue that the flares are powered by magnetic reconnection in the nebula. Using relativistic test-particle simulations, we show that particles are naturally focused into a thin fan beam, deep inside the reconnection layer where the magnetic field is small. The particles then suffer less from synchrotron losses and pile up at the maximum energy given by the electric potential drop in the layer. Applying this model to the Crab Nebula, we find that the emerging synchrotron emission spectrum above 100 MeV is consistent with the September 2010 flare observations. No detectable emission is expected at other wavelengths. This scenario provides a viable explanation for the Crab Nebula gamma-ray flares.
    01/2012;
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    ABSTRACT: We study models of the gamma-ray emission of Cyg X-3 observed by Fermi. We calculate the average X-ray spectrum during the gamma-ray active periods. Then, we calculate spectra from Compton scattering of a photon beam into a given direction by isotropic relativistic electrons with a power-law distribution, both based on the Klein-Nishina cross section and in the Thomson limit. Applying the results to scattering of stellar blackbody radiation in the inner jet of Cyg X-3, we find that a low-energy break in the electron distribution at a Lorentz factor of ~ 300--1000 is required by the shape of the observed X-ray/gamma-ray spectrum in order to avoid overproducing the observed X-ray flux. The electrons giving rise to the observed \g-rays are efficiently cooled by Compton scattering, and the power-law index of the acceleration process is ~ 2.5--3. The bulk Lorentz factor of the jet and the kinetic power before the dissipation region depend on the fraction of the dissipation power supplied to the electrons; if it is ~ 1/2, the Lorentz factor is ~ 2.5, and the kinetic power is ~ 10^38 erg/s, which represents a firm lower limit on the jet power, and is comparable to the bolometric luminosity of Cyg X-3. Most of the power supplied to the electrons is radiated. The broad band spectrum constrains the synchrotron and self-Compton emission from the gamma-ray emitting electrons, which requires the magnetic field to be relatively weak, with the magnetic energy density < a few times 10^-3 of that in the electrons. The actual value of the magnetic field strength can be inferred from a future simultaneous measurement of the IR and gamma-ray fluxes.
    Monthly Notices of the Royal Astronomical Society 11/2011; · 5.52 Impact Factor
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    ABSTRACT: Recent discovery of gamma-ray flares in the Crab Nebula challenges traditional relativistic particle acceleration models. These flares are presumably produced by PeV electrons radiating >100 MeV synchrotron photons in a milli-gauss magnetic field. In traditional models, where the accelerating electric field is smaller than the magnetic field, synchrotron radiation cannot exceed 100 MeV because radiative losses balance the acceleration rate. We propose that linear electric acceleration in a magnetic reconnection layer can resolve this difficulty. The gyroradii of PeV electrons are so large that their motion is insensitive to small-scale turbulent structures and is controlled only by large-scale fields. As these particles are accelerated by the reconnection electric field, their relativistic Speiser-like orbits collapse deep into the layer and get focused into a tight beam. Furthermore, since perpendicular magnetic field is small inside the layer, the radiation reaction there is suppressed, so the particles can reach higher energies and emit synchrotron radiation in excess of the 100 MeV limit, resolving the Crab gamma-ray flare paradox.
    11/2011;
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    ABSTRACT: 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.
    The Astrophysical Journal 10/2011; 746(2). · 6.73 Impact Factor
  • Benoit Cerutti, D. A. Uzdensky, M. C. Begelman
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    ABSTRACT: The Fermi and Agile gamma-ray space telescopes recently discovered short and powerful gamma-ray flares in the Crab Nebula. These events presumably originate from a tiny region of the nebula where electrons are accelerated to PeV energies and radiate >100 MeV synchrotron radiation in a milli-Gauss magnetic field. In classical models of particle acceleration, where the accelerating electric field is smaller than the magnetic field, the synchrotron radiation cannot exceed 100 MeV because radiative losses balance the acceleration rate. We propose that particles are efficiently accelerated to PeV energies in a magnetic reconnection layer. We find that ultrarelativistic electron orbits are trapped and collapse rapidly deep into the current layer where the magnetic field becomes small. After a few days of acceleration by the reconnection electric field, electrons are accelerated to PeV energies and are focused into a tight, narrow beam. This mechanism provides a viable explanation to the gamma-ray flares in the Crab Nebula and could be at work in other astrophysical objects such as relativistic jets in AGN.
    09/2011;
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    ABSTRACT: The recent discovery of day-long gamma-ray flares in the Crab Nebula, presumed to be synchrotron emission by PeV (10^{15} eV) electrons in milligauss magnetic fields, presents a strong challenge to particle acceleration models. The observed photon energies exceed the upper limit (~100 MeV) obtained by balancing the acceleration rate and synchrotron radiation losses under standard conditions where the electric field is smaller than the magnetic field. We argue that a linear electric accelerator, operating at magnetic reconnection sites, is able to circumvent this difficulty. Sufficiently energetic electrons have gyroradii so large that their motion is insensitive to small-scale turbulent structures in the reconnection layer and is controlled only by large-scale fields. We show that such particles are guided into the reconnection layer by the reversing magnetic field as they are accelerated by the reconnection electric field. As these electrons become confined within the current sheet, they experience a decreasing perpendicular magnetic field that may drop below the accelerating electric field. This enables them to reach higher energies before suffering radiation losses and hence to emit synchrotron radiation in excess of the 100 MeV limit, providing a natural resolution to the Crab gamma-ray flare paradox.
    The Astrophysical Journal Letters 05/2011; 737(2). · 6.35 Impact Factor
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    ABSTRACT: The microquasar Cygnus X-3 was detected at high energies by the gamma-ray space telescopes AGILE and Fermi. The gamma-ray emission is transient, modulated with the orbital period and seems related to major radio flares, i.e. to the relativistic jet. The GeV gamma-ray flux can be substantially attenuated by internal absorption with the ambient X-rays. In this study, we examine quantitatively the effect of pair production in Cygnus X-3 and put constraints on the location of the gamma-ray source. Cygnus X-3 exhibits complex temporal and spectral patterns in X-rays. During gamma-ray flares, the X-ray emission can be approximated by a bright disk black body component and a non-thermal tail extending in hard X-rays, possibly related to a corona above the disk. We calculate numerically the exact optical depth for gamma rays above a standard accretion disk. Emission and absorption in the corona are also investigated. GeV gamma rays are significantly absorbed by soft X-rays emitted from the inner parts of the accretion disk. The absorption pattern is complex and anisotropic. Isotropization of X-rays due to Thomson scattering in the companion star wind tends to increase the gamma-ray opacity. Gamma rays from the corona suffer from strong absorption by photons from the disk and cannot explain the observed high-energy emission, unless the corona is unrealistically extended. The lack of absorption feature in the GeV emission indicates that high-energy gamma rays should be located at a minimum distance ~10^8-10^10 cm from the compact object. The gamma-ray emission is unlikely to have a coronal origin.
    Astronomy and Astrophysics 03/2011; 529. · 5.08 Impact Factor
  • Benoît Cerutti, Guillaume Dubus, Gilles Henri
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    ABSTRACT: Gamma-ray binaries are composed of a massive star and possibly a young energetic pulsar. The stellar wind may confine the pulsar wind in a collimated relativistic outflow. Recent X-ray observations by Suzaku and INTEGRAL of LS 5039 show a steady and periodic X-ray emission correlated with the TeV modulation. The X-ray flux is maximum and minimum at both conjunctions, suggesting that the modulation is due to the peculiar orientation of the binary system with respect to the observer. The X-ray modulation could be explained by the relativistic motion of the flow in the pulsar wind. We investigate the Doppler-boosting effect in synchrotron radiation and inverse Compton scattering in gamma-ray binaries. This model can explain the X-ray modulation in LS 5039 and possibly explain the puzzling phasing of the TeV maximum emission in LS I +61?303.
    01/2011;
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    Benoit Cerutti, Guillaume Dubus, Gilles Henri
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    ABSTRACT: 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 2010
    07/2010;
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    Guillaume Dubus, Benoît Cerutti
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    ABSTRACT: γ-ray binaries are systems that emit most of their radiative power above 1 MeV. They are associated with O or Be stars in orbit with a compact object, possibly a young pulsar. Much like colliding wind binaries, the pulsar generates a relativistic wind that interacts with the stellar wind. The result is non-thermal emission from radio to very high energy γ-rays. The wind, radiation and magnetic field of the massive star play a major role in the dynamics and radiative output of the system. They are particularly important to understand the high energy physics at work. Inversely, γ-ray binaries offer novel probes of stellar winds and insights into the fate of O/B binaries.
    Proceedings of the International Astronomical Union 06/2010; 6:581 - 586.
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    ABSTRACT: LS 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. Comment: 10 pages, 11 figures, accepted for publication in Astronomy and Astrophysics
    Astronomy and Astrophysics 06/2010; · 5.08 Impact Factor

Publication Stats

151 Citations
78.31 Total Impact Points

Institutions

  • 2013–2014
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
  • 2011–2013
    • University of Colorado at Boulder
      • Department of Physics
      Boulder, Colorado, United States
  • 2010–2012
    • University of Colorado
      • Department of Physics
      Denver, Colorado, United States
  • 2007–2010
    • University Joseph Fourier - Grenoble 1
      Grenoble, Rhône-Alpes, France
  • 2009
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France