Pair Cascades Research Articles

PROMPT HIGH-ENERGY EMISSION FROM PROTON-DOMINATED GAMMA-RAY BURSTS

The prompt emission of gamma-ray bursts (GRBs) is widely thought to be radiation from accelerated electrons, but an appreciably larger amount of energy could be carried by accelerated protons, particularly if GRBs are the sources of ultra-high-energy cosmic rays (UHECRs). We model the expected photon spectra for such "proton-dominated" GRBs in the internal shock scenario through Monte Carlo simulations, accounting for various processes related to high-energy electrons and protons. Besides proton and muon synchrotron components, emission from photomeson-induced secondary pair cascades becomes crucial, generally enhancing the GeV–TeV and/or eV–keV photons and offering a signature of UHE protons. In some cases, it can overwhelm the primary electron component and result in GRBs peaking in the 10 MeV–1 GeV range, which may be relevant to some bursts discussed in a recent re-analysis of EGRET TASC data. The dependence of the spectra on key quantities such as the bulk Lorentz factor, magnetic field, and proton-to-electron ratio is nontrivial due to the nonlinear nature of cascading and the interplay of electron- and proton-induced components. Observations by Fermi, ground-based telescopes, and other facilities should test these expectations and provide critical constraints on the proton acceleration efficiency.

TIME-DEPENDENT MODELING OF RADIATIVE PROCESSES IN HOT MAGNETIZED PLASMAS

Numerical simulations of radiative processes in magnetized compact sources such as hot accretion disks around black holes, relativistic jets in active galaxies and gamma-ray bursts are complicated because the particle and photon distributions span many orders of magnitude in energy, they also strongly depend on each other, the radiative processes behave significantly differently depending on the energy regime, and finally due to the enormous difference in the time-scales of the processes. We have developed a novel computer code for the time-dependent simulations that overcomes these problems. The processes taken into account are Compton scattering, electron-positron pair production and annihilation, Coulomb scattering as well as synchrotron emission and absorption. No approximation has been made on the corresponding rates. For the first time, we solve coupled integro-differential kinetic equations for photons and electrons/positrons without any limitations on the photon and lepton energies. A numerical scheme is proposed to guarantee energy conservation when dealing with synchrotron processes in electron and photon equations. We apply the code to model non-thermal pair cascades in the blackbody radiation field, to study the synchrotron self-absorption as particle thermalization mechanism, and to simulate time evolution of stochastically heated pairs and corresponding synchrotron self-Compton photon spectra which might be responsible for the prompt emission of gamma-ray bursts. Good agreement with previous works is found in the parameter regimes where comparison is feasible, with the differences attributable to our improved treatment of the microphysics.

A model of the TeV flare of Cygnus X-1: electron acceleration and extended pair cascades

Abstract We consider theoretical models of emission of TeV photons by Cyg X-1 during a flare discovered by the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) detector. We study acceleration of electrons to energies sufficient for TeV emission, and find the emission site is allowed to be close to the black hole. We then consider pair absorption in the photon field of the central X-ray source and a surrounding accretion disc, and find its optical depth is ≲1, allowing emission close to the black hole. On the other hand, the optical depth in the stellar field is ∼10 at ∼1 TeV. However, the optical depth drops with increasing energy, allowing a model with the initial energy of ≳3 TeV, in which photons travel far away from the star, initiating a spatially extended pair cascade. This qualitatively explains the observed TeV spectrum, though still not its exact shape.

Possibility of Prolific Pair Production with High-Power Lasers

Prolific electron-positron pair production is possible at laser intensities approaching 10;{24} W cm;{-2} at a wavelength of 1 mum. An analysis of electron trajectories and interactions at the nodes (B=0) of two counterpropagating, circularly polarized laser beams shows that a cascade of gamma rays and pairs develops. The geometry is generalized qualitatively to linear polarization and laser beams incident on a solid target.

Oscillating pulsar polar gaps

An analytical model for oscillating pair creation above the pulsar polar cap is presented in which the parallel electric field is treated as a large amplitude, superluminal, electrostatic wave. An exact formalism for such wave is derived in one dimension and applied to both the low-density regime in which the pair plasma density is much lower than the corotating charge density and the high-density regime in which the pair plasma density is much higher than the corotating charge density. In the low-density regime, which is relevant during the phase leading to a pair cascade, a parallel electric field develops resulting in a rapid acceleration of particles. The rapid acceleration leads to bursts of pair production and the system switches to the oscillatory phase, corresponding to the high-density regime, in which pairs oscillate with net drift motion in the direction of wave propagation. Oscillating pairs lead to a current that oscillates with large amplitude about the Goldreich–Julian current. The drift motion can be highly relativistic if the phase speed of large amplitude waves is moderately higher than the speed of light. Thus, the model predicts a relativistic outflow of pairs, a feature that is required for avoiding overheating of the pulsar polar cap and is also needed for the pulsar wind.

E+Pair Loading and the Origin of the Upstream Magnetic Field in GRB Shocks

We investigate here the effects of plasma instabilities driven by rapid e±-pair cascades, which arise in the environment of γ-ray burst (GRB) sources as a result of back-scattering of a seed fraction of the original spectrum. The injection of e± pairs induces strong streaming motions in the ambient medium. One therefore expects the pair-enriched medium ahead of the forward shock to be strongly sheared on length scales comparable to the radiation front thickness. Using three-dimensional particle-in-cell simulations, we show that plasma instabilities driven by these streaming e± pairs are responsible for the excitation of near-equipartition, turbulent magnetic fields. Our results reveal the importance of the electromagnetic filamentation instability in ensuring an effective coupling between e± pairs and ions, and may help explain the origin of large upstream fields in GRB shocks.

Condensed surfaces of magnetic neutron stars, thermal surface emission, and particle acceleration above pulsar polar caps

For sufficiently strong magnetic fields and/or low temperatures, the neutron star surface may be in a condensed state with little gas or plasma above it. Such surface condensation can significantly affect the thermal emission from isolated neutron stars, and may lead to the formation of a charge-depleted acceleration zone ("vacuum gap") in the magnetosphere above the stellar polar cap. Using the latest results on the cohesive property of magnetic condensed matter, we quantitatively determine the conditions for surface condensation and vacuum gap formation in magnetic neutron stars. We find that condensation can occur if the thermal energy kT of the neutron star surface is less than about 8% of its cohesive energy Q_s, and that a vacuum gap can form if the neutron star's rotation axis and magnetic moment point in opposite directions and kT is less than about 4% of Q_s. Thus, vacuum gap accelerators may exist for some neutron stars. Motivated by this result, we also study the physics of pair cascades in the vacuum gap model for photon emission by accelerating electrons and positrons due to both curvature radiation and resonant/nonresonant inverse Compton scattering. Our calculations of the condition of cascade-induced vacuum breakdown and the related pulsar death line/boundary generalize previous works to the superstrong field regime. We find that inverse Compton scatterings do not produce a sufficient number of high energy photons in the gap and thus do not lead to pair cascades for most neutron star parameters. We discuss the implications of our results for the recent observations of neutron star thermal radiation as well as for the detection/non-detection of radio emission from high-B pulsars and magnetars.

Neutron-rich gamma-ray burst flows: dynamics and particle creation in neutron-proton collisions

We consider gamma-ray burst outflows with a substantial neutron component that are either dominated by thermal energy (fireballs) or by magnetic energy. In the latter case, we focus on the recently introduced “AC” model which relies on magnetic reconnection to accelerate the flow and power the prompt emission. For both the fireball and the AC model, we investigate the dynamical importance of neutrons on the outflow. We study particle creation in inelastic neutron-proton collisions and find that in both models the resulting neutrino emission is too weak to be detectable. The inelastic collisions also produce γ-rays, which create pairs in interactions with soft photons carried with the flow. In magnetically driven outflows, the energy of these pairs is radiated away as synchrotron emission. The bulk of the emission takes place at a few hundred keV, which makes it difficult to disentangle this signal from the prompt emission. In fireballs, however, pair cascading leads to the emission of γ-rays with observer energy in the range of 2–20 GeV and a fluence well above the GLAST threshold. Therefore this emission can be a useful diagnostic of the nature of the outflow.

Absorption and scattering of light by Pt, Pd, Ag, and Au nanodisks: Absolute cross sections and branching ratios

Localized surface plasmons (LSPs) of metallic nanoparticles decay either radiatively or via an electron-hole pair cascade. In this work, the authors have experimentally and theoretically explored the branching ratio of the radiative and nonradiative LSP decay channels for nanodisks of Ag, Au, Pt, and Pd, with diameters D ranging from 38 to 530 nm and height h=20 nm, supported on a fused silica substrate. The branching ratio for the two plasmon decay channels was obtained by measuring the absorption and scattering cross sections as a function of photon energy. The former was obtained from measured extinction and scattering coefficients, using an integrating sphere detector combined with particle density measurements obtained from scanning electron microscopy images of the nanoparticles. Partly angle-resolved measurements of the scattered light allowed the authors to clearly identify contributions from dipolar and higher plasmonic modes to the extinction, scattering, and absorption cross sections. Based on these experiments they find that absorption dominates the total scattering cross section in all the examined cases for small metallic nanodisks (D<100 nm). For D>100 nm absorption still dominates for Pt and Pd nanodisks, while scattering dominates for Au and Ag. A theoretical approach, where the metal disks are approximated as oblate spheroids, is used to account for the trends in the measured cross sections. The field problem is solved in the electrostatic limit. The spheroid is treated as an induced dipole for which the dipolar polarizability is calculated based on spheroid geometry and the (bulk) dielectric response function of the metal the spheroid consists of and the dielectric medium surrounding it. One might expect this model to be inappropriate for disks with D>100 nm since effects due to the retardation of the incoming field across the metallic nanodisk and contributions from higher plasmonic modes are neglected. However, this model describes quite well the energy dependence of the dipolar resonance, the full width at half maximum, and the total extinction cross section for all four metallic systems, even when 100<D<500 nm, indicating that the combined contribution of the effects not included in the model is small for the systems studied. For this reason the authors have extended the use of the same model to study scattering/absorption branching ratios. The main conclusions include the following. (i) Both the magnitude and peak position in extinction cross section are well accounted for by the model. (ii) The branching ratio for radiative and nonradiative decay is reasonably well accounted for. (iii) The model fails to account for the correct magnitudes of the measured absorption and scattering cross sections for larger particles in the case of Ag and Au. Possible reasons for this discrepancy are discussed.

Internal absorption of gamma-rays in relativistic blobs of active galactic nuclei

We investigate the production of gamma-rays in the inverse Compton (IC) scattering process by leptons accelerated inside relativistic blobs in jets of active galactic nuclei. Leptons are injected homogeneously inside the spherical blob and initiate IC e ± pair cascade in the synchrotron radiation (produced by the same population of leptons, SSC model), provided that the optical depth for gamma-rays is larger than unity. It is shown that for likely parameters internal absorption of gamma-rays has to be important. We suggest that new type of blazars might be discovered by the future simultaneous X-ray and γ-ray observations, showing peak emissions in the hard X-rays, and in the GeV γ-rays. Moreover, the considered scenario might be also responsible for the orphan X-ray flares recently reported from BL Lac type active galaxies.

Lower Limits on Pulsar Pair Production Multiplicities from H.E.S.S. Observations of Pulsar Wind Nebulae

One of the most fundamental questions in pulsar physics is the generation of pair cascading to explain radio through γ-ray pulsed emission. In this paper we derive the pair production multiplicity κ for pulsars from the very high energy (VHE) γ-ray spectra of their pulsar wind nebulae (PWNs), as measured by H.E.S.S., since the IC-generated γ-ray spectra are free from assumptions of the associated nebular magnetic field strength. Most VHE PWNs have been resolved by H.E.S.S., and since the VHE size is already consistent with the maximal expected PWN size of the sources considered in this paper, the age of the VHE emitting electrons must at least be comparable to the age of the pulsar. This allows us to integrate the Goldreich-Julian current over the lifetime of the pulsar, resulting in time averages κ greater that 2000 and 500 for PSR B1509-58 and PSR B1823-13, respectively, whereas the H.E.S.S. data alone for Vela X indicate an anomalously low value, less than unity. The extreme brightness of Vela X in radio, however, indicates that most of the electrons from the pulsar have been reprocessed into the radio synchrotron domain, leading to an upper limit of κ < 4 × 105, if we assume the minimal field strength of 1.7 μG derived from a comparison of H.E.S.S. and ASCA/ROSAT data of Vela X. Similar (large) numbers are, however, derived for PSR B1823-13 and PSR B1509-58 if we extrapolate the H.E.S.S.-derived electron spectra into the uncooled domain toward the unseen radio nebular domain. Such divergent limits on κ can be resolved by adding forthcoming GLAST data to H.E.S.S. information.

Very high energy gamma-ray production inside the massive binary system Cyg X-1/HDE 226868

We apply the IC e$^\pm$ pair cascade model in order to investigate the possible $\gamma$-ray emission features from another compact massive binary of the microquasar type, Cyg X-1. We conclude that the observational constraints at lower energies (from MeV to GeV) suggest that the spectrum of electrons injected in the jet is likely steeper than in other TeV $\gamma$-ray microquasars. The cascade $\gamma$-ray spectrum produced by electrons with such spectrum in Cyg X-1 should be below the sensitivities of the MAGIC and VERITAS class Cherenkov telescopes. However, if the electron spectrum is flatter, then the highest TeV $\gamma$-ray fluxes are predicted at $\sim$7 hrs before and after the phase when the compact object is in front of the massive star. We suggest that Cherenkov telescopes should concentrate on these range of phases since the TeV flux can vary by a factor of $\sim$20 with the period of the Cyg X-1 binary system. Moreover, the model predicts clear anticorrelation of the GeV and TeV $\gamma$-ray emission. This feature can be tested by the future multiwavelength observations with the AGILE and GLAST telescopes in the GeV energy range and the MAGIC and VERITAS telescopes in the TeV energy range.

GeV-TeV $\vec{\gamma}$-ray light curves expected in the IC e$^\pm$ pair cascade model for massive binaries: application to LS 5039

TeV gamma-ray emission from two massive binaries of the microquasar type, LS 5039 and LS I +61$^{\rm o}$ 303, show clear variability with their orbital periods. Our purpose is to calculate the GeV and TeV $\gamma$-ray light curves from the massive binary LS 5039 which are expected in the specific Inverse Compton $e^\pm$ pair cascade model. This model successfully predicted the basic features of the high energy $\gamma$-ray emission from LS 5039 and LS I +61 303. In the calculations we apply the Monte Carlo code which follows the IC $e^\pm$ pair cascade in the anisotropic radiation of the massive star. The $\gamma$-ray light curves and spectra are obtained for different parameters of the acceleration scenario and the inclination angles of the binary system. It is found that the GeV and TeV $\gamma$-ray light curves should be anti-correlated. This feature can be tested in the near future by the simultaneous observations of LS 5039 with the AGILE and GLAST telescopes in GeV energies and the Cherenkov telescopes in the TeV energies. Considered model also predicts a broad maximum in the TeV $\gamma$-ray light curve between the phases $\sim 0.4-0.8$ consistently with the observations of LS 5039 by the HESS telescopes. Moreover, we predict additional dip in the TeV light curve for large inclination angles $\sim 60^{\rm o}$. This feature could serve as a diagnostic for independent measuring of the inclination angle of this binary system indicating also on the presence of a neutron star in LS 5039.

Inverse Compton e pair cascade model for the -ray production in massive binary LSI +61 303

We apply an inverse Compton e ± pair cascade model for γ-ray production in the massive binary system LSI +61° 303 assuming that electrons are accelerated already inside the inner part of the jet launched by the compact object. γ-ray spectra, affected by the cascade process, and lower energy spectra, from the synchrotron cooling of the highest energy electrons in the jet, are calculated as a function of the phase of this binary system. γ-ray spectra expected in such a model have different shape than those ones produced by electrons in the jet directly to observer. Moreover, the model predicts clear anticorrelation between γ-ray fluxes in the GeV (1-10 GeV) and TeV (>200 GeV) energy ranges with the peak of the TeV emission at the phase ∼0.5 (the peak half-width ranges between the phases ∼0.4-0.9, for the inclination of the binary system equal to 60°, and ∼0.4-0.1 for 30°). The fine features of TeV γ-ray emission (fluxes and spectral shapes) as a function of the phase of the binary system are consistent with recent observations reported by the MAGIC collaboration. Future simultaneous observations in the GeV energies (by the GLAST and AGILE telescopes) and in the TeV energies (by the MAGIC and VERITAS telescopes) should test other predictions of the considered model supporting or disproving the hypothesis of acceleration of electrons already in the inner part of the microquasar jets.

Propagation of very high energy -rays inside massive binaries LS 5039 and LSI +61 303

Two massive binary systems of the microquasar type, LS 5039 and LSI +61° 303, have been suggested as possible counterparts of EGRET sources. LS 5039 has also been recently detected in TeV γ-rays. Since the massive stars in these binary systems are very luminous, it is expected that high-energy γ-rays, if injected relatively close to the massive stars, should be strongly absorbed, initiating inverse Compton e ± pair cascades in the anisotropic radiation from stellar surfaces. We investigate the influence of the propagation effects on the spectral and angular features of the γ-ray spectra emerging from these two binary systems by applying the Monte Carlo method. Two different hypotheses are considered: isotropic injection of primary γ-rays with the power-law spectrum due to e.g. interaction of hadrons with the matter of the wind, and the isotropic injection of electrons, e.g. accelerated in the jet, which comptonize the radiation from the massive star. It is concluded that the propagation effects of γ-rays can be responsible for the spectral features observed from LS 5039 (e.g. the shape of the spectrum in the GeV and TeV energy ranges and their relative luminosities). The cascade processes occurring inside these binary systems significantly reduce the γ-ray opacity obtained in other works by simple calculations of the escape of γ-rays from the radiation fields of the massive stars. Both systems provide very similar conditions for the TeV γ-ray production at the periastron passage. Any TeV γ-ray flux at the apastron passage in LSI +61° 303 will be relatively stronger with respect to its GeV flux than in LS 5039. If γ-rays are produced inside these binaries not far from the massive stars, i.e. within a few stellar radii, then clear anticorrelation between the GeV and TeV emission should be observed, provided that primary γ-rays at GeV and TeV energies are produced in the same process by the same population of relativistic particles. These γ-ray propagation features can be tested in the near future by the multiwavelength campaigns engaging the AGILE and GLAST telescopes (>30 MeV) and the Cherenkov telescopes (>100 GeV, e.g. MAGIC, HESS, VERITAS and CANGAROO).

A unified accretion-ejection paradigm for black hole X-ray binaries

We present a new picture for the central regions of Black Hole X-ray Binaries. In our view, these central regions have a multi-flow configuration which consists in (1) an outer standard accretion disc down to a transition radius rJ; (2) an inner magnetized accretion disc below rJ driving (3) a non relativistic self-collimated electron-proton jet surrounding, when adequate conditions for pair creation are met; (4) a ultra relativistic electron-positron beam. This accretion-ejection paradigm provides a simple explanation to the canonical spectral states, from radio to X/γ-rays, by varying the transition radius rJ and disc accretion rate independently. Large values of rJ correspond to the Quiescent state for low and the Hard state for larger . These states are characterized by the presence of a steady electron-proton MHD jet emitted by the disc below rJ. The hard X-ray component is expected to form at the jet basis. When rI becomes smaller than the marginally stable orbit ri, the whole disc resembles a standard accretion disc with no jet, characteristic of the Soft state. Intermediate states correspond to situations where . At large , an unsteady pair cascade process is triggered within the jet axis, giving birth to flares and ejection of relativistic pair blobs. This would correspond to the luminous intermediate state, sometimes referred to as the Very High state, with its associated superluminal motions. The variation of rJ independently of is a necessary ingredient in this picture. It arises from the presence of a large scale vertical magnetic field threading the disc. Features such as possible hysteresis and the presence of quasi-periodic oscillations would naturally fit within this new framework.

A unified accretion–ejection paradigm for Black Hole X-ray binaries

We present a unified accretion–ejection picture that explains the different spectral state of Black Hole X-ray binaries (BHXrB) from radio to X/γ-rays. In this view, the central region of BHXrB has a multi-flow configuration which consists in (1) an outer standard accretion disc, (2) an inner magnetized accretion disc driving, (3) a self-collimated electron–proton MHD jet, surrounding and (4) a relativistic electron–positron beam when adequate conditions are met. This picture provides a simple and unified explanation to the various canonical spectral states of BH X-ray binaries, by varying the transition radius r J between the inner disc driving jets and the outer standard disc. In this framework, large r J correspond to Quiescent and Hard states while small r J correspond to Thermal Dominant ones. In between these two extremes, r J can reach values that switches on the pair cascade process giving birth to a relativistic electron–positron beam. This would correspond to the bright intermediate state.

GeV gamma-rays and TeV neutrinos from very massive compact binary systems: the case of WR 20a

Abstract Massive Wolf—Rayet stars in a compact binary system are characterized by very strong winds which collide, creating a shock wave. If the wind nuclei, from helium up to oxygen, accelerated at the shock can reach large enough energies, they suffer disintegration in collisions with soft thermal radiation from the massive stars injecting relativistic protons and neutrons. Protons collide with the matter of the wind and a fraction of neutrons collide with the massive stars producing γ-rays and neutrinos via decay of pions. We calculate γ-ray fluxes from inverse Compton e± pair cascades initiated by primary γ-rays and leptons, which are produced by protons, and the neutrino fluxes, produced by protons and neutrons, for the example compact massive binary WR 20a. From normalization of these cascade γ-ray spectra to the fluxes of the EGRET sources 2EG J1021 − 5835 and 2EG J1049 − 5847 observed in the direction of WR 20a, we conclude that this massive binary can be detected by the IceCube type of neutrino detector. The estimated muon neutrino event rate inside the 1-km2 detector during 1 year is between a few and a few tens, depending on the spectrum of accelerated nuclei.

High energy neutrinos from fast spinning magnetars

Fast spinning magnetars are discussed as strong sources of high energy neutrinos. Pulsars may be born with a short rotation period of milliseconds with the magnetic field amplified through dynamo processes up to ∼10 15–10 16 G. As such millisecond magnetars (MSMs) have an enormous spin-down power ∼10 50 erg s −1, they can be potentially a strong, extragalactic high-energy neutrino source. Specifically, acceleration of ions and subsequent photomeson production within the MSM magnetosphere are considered. As in normal pulsars, particle acceleration leads to electron–positron pair cascades that constrains the acceleration efficiency. The limit on the neutrino power as a fraction of the spin-down power is calculated. It is shown that neutrinos produced in the inner magnetosphere have characteristic energy about a few ×100 GeV due to the constraint of cooling of charged pions through inverse Compton scattering. TeV neutrinos may be produced in the outer magnetosphere where ions can be accelerated to much higher energies and the pion cooling is less severe than in the inner magnetosphere. High energy neutrinos can also be produced from interactions between ultra-high energy protons accelerated in the magnetosphere and a diffuse thermal radiation from the ejecta or from the interaction region between the MSM wind and remnant shell. The detectability of neutrinos in the early spin-down phase by the current available and planned neutrino detectors is discussed.

An active differential broad-band phase splitter for quadrature-modulator applications

Area-efficient single-ended-to-differential phase-splitter (balun) integration is difficult for frequencies below 6 GHz. Few broad-band solutions are available, and those given in the literature often introduce quite high amplitude and/or phase errors to differential signaling. In this paper, a cascade of differential pairs is shown always to outperform any single-stage solution, and the measurement data for a two-amplifier prototype are supplied to support the theory. The prototype circuit was realized on 0.8-mum SiGe as a 2.5-V RF integrated circuit measuring 1.2times1.2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> including pads. The measured gain error is better than -1 dB up to 4.6 GHz, while phase difference is between 179.7deg-180.9deg in 0.6-4.1 GHz. The application of the realized phase splitter in a broad-band quadrature modulator is discussed