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We provide in this work Stark broadening data (widths and shifts) for 120 ultraviolet spectral lines of Ni  v ion. Our calculations are performed using the semiclassical perturbation method. For energy levels and oscillator strength calculations, we use the multiconfiguration Hatree–Fock approach. Stark shifts and widths are calculated for collisions with electrons and with the positive ions: H+, He+, and He++, allowing us to take into account the important perturbers in stellar atmospheres. We compare our Stark widths with estimations obtained using the Cowley formula. Our electron impact Stark widths are also fitted with temperature using a logarithmic formula. Finally, our obtained Stark widths are used to investigate the influence of Stark broadening in the atmospheric conditions of hot DA white dwarfs. Despite the importance of ultraviolet lines of Ni  v ion for modelling white dwarf atmospheres and also for investigations of variation of fundamental constants with gravitational potential, we did not find Stark broadening data previously calculated or measured for Ni  v ion. The objective of this work is to give the missed data. [ABSTRACT FROM AUTHOR]

N 157B located in the Large Magellanic Cloud is the first pulsar wind nebula detected outside of the Galaxy in γ-rays. In this paper, we analyse the emission above 30 GeV from N 157B using ∼14.3 yr of Fermi -LAT data. The γ-ray spectrum between 30 and 500 GeV is well described by a single power-law function with a photon index of 1.83 ± 0.26, and its integral photon flux is (4.10 ± 0.83) × 10−11 photons cm−2 s−1. We adopt a one-zone leptonic model to investigate whether the multiband non-thermal emission of the target source can be generated by synchrotron radiation and inverse-Compton scattering of the electrons/positrons. Assuming the electrons/positrons in the nebula have a broken power-law spectrum with two breaks, the model can reproduce the observed fluxes in the radio, X-ray, and γ-ray bands. This result indicates that the γ-ray emission from N 157B can be explained by the leptonic process of electrons/positrons via inverse-Compton scattering. [ABSTRACT FROM AUTHOR]

Geometry and dynamical structure of emission regions in accreting pulsars are shaped by the interplay between gravity, radiation, and strong magnetic field, which significantly affects the opacities of a plasma and radiative pressure under such extreme conditions. Quantitative consideration of magnetic plasma opacities is therefore an essential ingredient of any self-consistent modelling of emission region structure of X-ray pulsars (XRPs). We present results of computations of the Rosseland and Planck mean opacities of a strongly magnetized plasma with a simple chemical composition, namely the solar hydrogen/helium mix. We consider all relevant specific opacities of the magnetized plasma including vacuum polarization effect and contribution of electron–positron pairs where the pair number density is computed in the thermodynamic equilibrium approximation. The magnetic Planck mean opacity determines the radiative cooling of an optically thin strongly magnetized plasma. It is by factor of three smaller than non-magnetic Planck opacity at |$k_{\rm B}T \lt 0.1\, E_{\rm cyc}$| and increases by a factor of 102–104 at |$k_{\rm B}T \gt 0.3\, E_{\rm cyc}$| due to cyclotron thermal processes. We propose a simple approximate expression which has sufficient accuracy for the magnetic Planck opacity description. We provide the Rosseland opacity in a tabular form computed in the temperature range 1–300 keV, magnetic field range 3 × 1010–1015 G, and a broad range of plasma densities. We demonstrate that the scattering on the electron–positron pairs increases the Rosseland opacity drastically at temperatures > 50 keV in the case of mass densities typical for accretion channel in XRPs. [ABSTRACT FROM AUTHOR]

Coherent radio emission from pulsars originates from excited plasma waves in an ultra-relativistic and strongly magnetized electron–positron pair plasma streaming along the open magnetic field lines of the pulsar. Traditional coherent radio emission models have relied on instabilities in this pair plasma. Recently, alternative models have been suggested. These models appeal to direct coupling of the external electromagnetic field to the superluminal O-mode (lt2 mode) during the time-dependent pair cascade process at the polar gap. The objective of this work is to provide generic constraints on plasma models based on lt2 mode using realistic pulsar parameters. We find that the very short time-scale associated with pair cascades does not allow lt2 mode to be excited at radio frequencies and the impulsive energy transfer can only increase the kinetic spread ('temperature') of the pair plasma particles. Moreover, under homogeneous plasma conditions, plasma waves on both branches of O mode (i.e. superluminal lt2 and subluminal lt1) cannot escape the plasma. In the strongly magnetized pair plasma, only the extraordinary mode (t mode) can escape freely. We show that any generic fictitious mechanisms do not result in the wave electric field of t mode to have predominant orientation either parallel or perpendicular to the magnetic field plane as observed. Such fictitious mechanisms will inevitably lead to depolarization of signals and cannot account for the highly polarized single pulses observed in pulsars. We suggest coherent curvature radiation as a promising candidate for pulsar radio emission mechanism. [ABSTRACT FROM AUTHOR]

Diffuse radio emission at the centre of galaxy clusters has been observed both in merging clusters on scales of Mpc, called giant radio haloes, and in relaxed systems with a cool-core on smaller scales, named mini haloes. Giant radio haloes and mini haloes are thought to be distinct classes of sources. However, recent observations have revealed the presence of diffuse radio emission on Mpc scales in clusters that do not show strong dynamical activity. RX J1720.1+2638 is a cool-core cluster, presenting both a bright central mini halo and a fainter diffuse, steep-spectrum emission extending beyond the cluster core that resembles giant radio halo emission. In this paper, we present new observations performed with the LOw Frequency ARray Low Band Antennas (LBA) at 54 MHz. These observations, combined with data at higher frequencies, allow us to constrain the spectral properties of the radio emission. The large-scale emission presents an ultrasteep spectrum with |$\alpha _{54}^{144}\sim 3.2$|⁠. The radio emission inside and outside the cluster core have strictly different properties, as there is a net change in spectral index and they follow different radio-X-ray surface brightness correlations. We argue that the large-scale diffuse emission is generated by particles re-acceleration after a minor merger. While for the central mini halo, we suggest that it could be generated by secondary electrons and positrons from hadronic interactions of relativistic nuclei with the dense cool-core gas, as an alternative to re-acceleration models. [ABSTRACT FROM AUTHOR]

A search for neutrinos produced in coincidence with gamma-ray bursts (GRBs) was conducted with the Super-Kamiokande (SK) detector. Between December 2008 and March 2017, the Gamma-ray Coordinates Network recorded 2208 GRBs that occurred during normal SK operation. Several time windows around each GRB were used to search for coincident neutrino events. No statistically significant signal in excess of the estimated backgrounds was detected. The |$\bar\nu_e$| fluence in the range from 8 MeV to 100 MeV in positron total energy for |$\bar\nu_e+p\rightarrow e^{+}+n$| was found to be less than |$\rm 5.07\times10^5$| cm |$^{-2}$| per GRB at a 90% confidence level. For all GRBs, upper bounds were obtained on the fluence as a function of neutrino energy. Additionally, for GRBs at known distances, upper limits were set for the neutrino energy emission at the GRB. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) constitute an essential family of compounds in interstellar (ISM) and circumstellar (CSM) media. Recently, formation routes for the corresponding polycyclic aromatic phosphorus heterocycles (PAPHs) in astrophysical environments have been proposed. In order to contribute to a better understanding of the phosphorus chemistry in the ISM, infrared (IR) spectra and selected properties of PAPHs were computed at the density functional theory level for neutral, cationic, and anionic species. Our results reveal that several protonated PAPHs do not have planar backbones, and all species have permanent dipole moments between 2D and 4D. Closed-shell PAPHs have similar ionization potentials compared to the parent PAHs, below the Lyman threshold limit. In addition, all PAPHs show positive electron affinities higher than those of PAHs. Protonation preferably occurs on the heteroatom but with lower proton affinities than those of the corresponding nitrogen analogues (polycyclic aromatic nitrogen heterocycles). In general, neutral species have similar IR spectra profile with the most intense bands around 800 cm−1 (12.5 μ m) related to C−H wagging. Charge and protonation affect the IR spectra mainly by decreasing the intensities of these modes and increasing the ones between 1000 (10.0 μ m) and 1800 cm−1 (5.6 μ m). The P−H stretching appears in a different spectral region, between 2300 (4.3 μ m) and 2700 cm−1 (3.7 μ m). Our results are discussed in the context of distinct sources where PAHs and phosphorus are detected. PAPHs, in particular the coronene derivatives, can contribute to the unidentified infrared emission band at 6.2 μ m. [ABSTRACT FROM AUTHOR]

We have studied the evolution of ion acoustic solitary waves (IASWs) in pulsar wind. The pulsar wind is modelled by considering a weakly relativistic unmagnetized collisionless plasma comprised of relativistic ions and superthermal electrons and positrons. Through fluid simulations, we have demonstrated that the localized ion density perturbations generated in the polar wind plasma can evolve the relativistic IASW pulses. It is found that the concentration of positrons, relativistic factor, superthermality of electrons, and positrons have a significant influence on the dynamical evolution of IASW pulses. Our results may provide insight to understand the evolution of IASW pulses and their role in astrophysical plasmas, especially in the relativistic pulsar winds with supernova outflow, which is responsible for the production of superthermal particles and relativistic ions. [ABSTRACT FROM AUTHOR]

Recent multifrequency measurements of pulse widths W 50 for the long-period pulsar J0250+5854 by Agar et al. provide a unique insight into the emission process owing to its small polar-cap radius. The frequency dependence of W 50 can be simply understood as a consequence of the emitting plasma remaining under acceleration during the interval of radio emission. This is possible in a plasma of ions and protons but not in one of high-multiplicity electron–positron pairs. Extension of the model to the pulse profiles of the general pulsar population is considered briefly. [ABSTRACT FROM AUTHOR]

The energetic composition of radio lobes in the FR II galaxies – estimated by comparing their radio luminosities with the powers required to inflate cavities in the external medium – seems to exclude the possibility of their energetic domination by protons. Furthermore, if the jets were dominated by the kinetic energy of cold protons, it would be difficult to efficiently accelerate leptons in the jets' terminal shocks. Assuming that the relative energy contents of leptons, protons, and magnetic fields are preserved across the shocks, the above implies that the large-scale jets should also be energetically dominated by leptons: P e,j ≳ P p,j. On the other hand, previous studies of small-scale jets in blazars and radio cores suggest a pair content (number of electrons and positrons per proton) of the order of n e/ n p ∼ 20. Assuming further that the particle composition of jets does not evolve beyond the blazar scales, we show that this implies an average random Lorentz factor of leptons in large-scale jets of |$\bar{\gamma }_{\rm e,j} \gtrsim 70(1+\chi _{\rm p})(20n_{\rm p}/n_{\rm e})$|⁠ , and that the protons should be mildly relativistic with χp ≡ (ϵp + p p)/ρp c 2 ≲ 2, p p the pressure of protons, ϵp the internal energy density of protons, and ρp c 2 the rest-mass energy density of protons. We derive the necessary conditions for loading the inner jets by electron–positron pairs and proton–electron plasma, and provide arguments that heating of leptons in jets is dominated by magnetic reconnection. [ABSTRACT FROM AUTHOR]

It is known that the concept of limiting polarization introduced 70 yr ago by K. G. Budden has the capacity to explain the magnitude of circular polarization seen in normal pulsars with light-cylinder radii of the order of 109–10 cm under the assumption of a high-multiplicity electron–positron plasma. However, a review of limiting polarization under the same assumption in millisecond pulsars indicates that it is inapplicable there because the region of limiting polarization lies far outside the light cylinder. This paper, using the ion–proton model, evaluates circular polarization both generally and specifically for J2144−3933, and gives a fairly detailed understanding of the observations in normal pulsars including the change of sign as a function of frequency seen in J0908−4913. But it also fails to explain circular polarization in millisecond pulsars owing to the smaller particle number densities and birefringence of the magnetosphere in these objects. However, the review of limiting polarization finds that, within the ion–proton model, this distinct process can describe their circular polarization. It is argued that certain features of millisecond pulsar Stokes profiles are clearly consistent with limiting polarization. [ABSTRACT FROM AUTHOR]

OBJECTIVES Open in new tab Download slide Open in new tab Download slide For patients with locally advanced oesophageal cancer, improved complete pathological response after neoadjuvant chemoradiation (nCRT) and the detrimental effects on the quality of life related to oesophagectomy have led to the need for a reliable method to select patients who have achieved complete pathological response and do not need surgery. The reliability of 18F-fluorodeoxyglucose positron emission–computed tomography (PET-CT) for predicting the pathological response after nCRT was evaluated. METHODS Patients with locally advanced oesophageal cancer who were treated with nCRT and oesophagectomy from July 2010 to February 2017 were analysed. On the post-nCRT PET-CT, a complete metabolic response was defined as all tumourous lesions demonstrating maximum standardized uptake value (SUVmax) ≤2.5. To minimize the effect of radiation-induced oesophagitis, complete metabolic response was also defined as no viable lesion distinguishable from the background with diffuse uptake. The sensitivity, specificity, positive predictive value and negative predictive value were analysed for SUVmax, [X]ΔSUVmax and %ΔSUVmax. RESULTS A total of 158 patients with oesophageal squamous cell carcinoma were analysed. The rate of complete pathological response was 27.8%, and that of complete metabolic response was 7.6%. The sensitivity, specificity, positive predictive value and negative predictive value based on SUVmax ≤2.5 and visual normalization were 95%, 14%, 74% and 50%, respectively. Analysis for [X]ΔSUVmax and %ΔSUVmax using the optimal cut-off values determined by the receiver operating characteristic curves did not show an improved predictive efficacy. CONCLUSIONS PET-CT is not a reliable tool for predicting pathological response. Patients diagnosed with resectable oesophageal cancer who underwent neoadjuvant therapy should not be exempt from surgery based on PET-CT results. [ABSTRACT FROM AUTHOR]

Extinct radio pulsars, in which stationary, self-sustaining generation of a relativistic electron–positron plasma becomes impossible when rotation brakes down, can be sources of a subrelativistic flux of positrons and electrons. We assume that the observed excess of positrons in the bulge and the disc of the Galaxy is associated with these old neutron stars. The production of pairs in their magnetospheres occurs due to one-photon absorption of gamma quanta of the Galactic and extragalactic backgrounds. The cascade process of plasma production leads to the flux of positrons escaping from the open magnetosphere ≃3 × 1034 s−1. The total flux of positrons from all old Galactic neutron stars with rotational periods 1.5 < P < 35 s is ≃3 × 1043 s−1. The energy of positrons is less than ≃10 MeV. The estimated characteristics satisfy the requirements for the positron source responsible for the 511-keV Galactic annihilation line. [ABSTRACT FROM AUTHOR]

We present the results of two-dimensional particle-in-cell (PIC) simulations of relativistic magnetic reconnection (RMR) in electron–positron plasma, including the dynamical influence of the synchrotron radiation process, and integrating the observable emission signatures. The simulations are initiated with a single Harris current layer with a central gap that triggers the RMR process. We achieve a steady-state reconnection with unrestricted outflows by means of open boundary conditions. The radiative cooling efficiency is regulated by the choice of initial plasma temperature Θ. We explore different values of Θ and of the background magnetization σ0. Throughout the simulations, plasmoids are generated in the central region of the layer, and they evolve at different rates, achieving a wide range of sizes. The gaps between plasmoids are filled by smooth relativistic outflows called minijets, whose contribution to the observed radiation is very limited due to their low-particle densities. Small-sized plasmoids are rapidly accelerated; however, they have lower contributions to the observed emission, despite stronger relativistic beaming. Large-sized plasmoids are slow but produce most of the observed synchrotron emission, with major part of their radiation produced within the central cores, the density of which is enhanced by radiative cooling. Synchrotron light curves show rapid bright flares that can be identified as originating from mergers between small/fast plasmoids and large/slow targets moving in the same direction. In the high-magnetization case, the accelerated particles form a broken power-law energy distribution with a soft tail produced by particles accelerated in the minijets. [ABSTRACT FROM AUTHOR]

Deep observation of the High Energy Stereoscopic System (HESS) on the most extended pulsar wind nebula HESS J1825−137 reveals an enhanced energy-dependent morphology, providing useful information on the particle transport mechanism in the nebula. We find that the energy-dependent morphology is consistent with a diffusion-dominated transport of electrons/positrons. It provides an alternative possible interpretation for the unusually large spatial extent (i.e. |${\gtrsim} 100$|  pc) of the nebula, which could then be attributed to the diffusion of escaping electrons/positrons from a compact plerion. The influence of various model parameters on the energy-dependent extent of the nebula is studied in the diffusion-dominated scenario. We also show that the energy-dependent morphology of the nebula may also be used to study the spin-down history of the pulsar. [ABSTRACT FROM AUTHOR]

We present a model for fast radio bursts (FRBs) where a large-amplitude Alfvén wave packet is launched by a disturbance near the surface of a magnetar, and a substantial fraction of the wave energy is converted to coherent radio waves at a distance of a few tens of neutron star radii. The wave amplitude at the magnetar surface should be about 1011 G in order to produce an FRB of isotropic luminosity 1044 erg s−1. An electric current along the static magnetic field is required by Alfvén waves with non-zero component of transverse wave vector. The current is supplied by counter-streaming electron–positron pairs, which have to move at nearly the speed of light at larger radii as the plasma density decreases with distance from the magnetar surface. The counter-streaming pairs are subject to two-stream instability, which leads to formation of particle bunches of size of the order of c /ωp, where ωp is the plasma frequency. A strong electric field develops along the static magnetic field when the wave packet arrives at a radius where electron–positron density is insufficient to supply the current required by the wave. The electric field accelerates particle bunches along the curved magnetic field lines, and that produces the coherent FRB radiation. We provide a number of predictions of this model. [ABSTRACT FROM AUTHOR]

The finite time, τdep, over which positrons from β+ decays of 56Co deposit energy in type Ia supernovae ejecta lead, in case the positrons are trapped, to a slower decay of the bolometric luminosity compared to an exponential decline. Significant light-curve flattening is obtained when the ejecta density drops below the value for which τdep equals the 56Co lifetime. We provide a simple method to accurately describe this 'delayed deposition' effect, which is straightforward to use for analysis of observed light curves. We find that the ejecta heating is dominated by delayed deposition typically from 600 to 1200 d, and only later by longer lived isotopes 57Co and 55Fe decay (assuming solar abundance). For the relatively narrow 56Ni velocity distributions of commonly studied explosion models, the modification of the light curve depends mainly on the 56Ni mass-weighted average density, 〈ρ〉 t 3. Accurate late-time bolometric light curves, which may be obtained with JWST far-infrared (far-IR) measurements, will thus enable to discriminate between explosion models by determining 〈ρ〉 t 3 (and the 57Co and 55Fe abundances). The flattening of light curves inferred from recent observations, which is uncertain due to the lack of far-IR data, is readily explained by delayed deposition in models with |$\langle \rho \rangle t^{3} \approx 0.2\, \mathrm{M}_{\odot }\, (10^{4}\, \textrm{km}\, \textrm{s}^{-1})^{-3}$|⁠ , and does not imply supersolar 57Co and 55Fe abundances. [ABSTRACT FROM AUTHOR]

The dissipation of turbulent magnetic fields is an appealing scenario to explain the origin of non-thermal particles in high-energy astrophysical sources. However, it has been suggested that the particle distribution may effectively thermalize when the radiative (synchrotron and/or Inverse Compton) losses are severe. Inspired by recent particle-in-cell simulations of relativistic turbulence, which show that electrons are impulsively heated in intermittent current sheets by a strong electric field aligned with the local magnetic field, we instead argue that in plasmas where the particle number density is dominated by the pairs (electron–positron and electron–positron–ion plasmas): (i) as an effect of fast cooling and of different injection times, the electron energy distribution is d n e/dγ ∝ γ−2 for γ ≲ γheat (the Lorentz factor γheat being close to the equipartition value), while the distribution steepens at higher energies; (ii) since the time-scales for the turbulent fields to decay and for the photons to escape are of the same order, the magnetic and the radiation energy densities in the dissipation region are comparable; (iii) if the mass energy of the plasma is dominated by the ion component, the pairs with a Lorentz factor smaller than a critical one (of the order of the proton-to-electron mass ratio) become isotropic, while the pitch angle remains small otherwise. The outlined scenario is consistent with the typical conditions required to reproduce the spectral energy distribution of blazars, and allows one to estimate the magnetization of the emission site. Finally, we show that turbulence within the Crab Nebula may power the observed gamma-ray flares if the pulsar wind is nearly charge separated at high latitudes. [ABSTRACT FROM AUTHOR]

Recent Very Long Baseline Interferometry observations of the relativistic jet in the M87 radio galaxy at 43 GHz show gradual relativistic acceleration of the plasma and suggest a linear dependence of Lorentz factor on jet radius at scales up to 8 marcsec (0.65 pc) from the core (2.5 marcsec in projection). General analysis of integrals of motion being unaltered along the jet and reflecting fundamental conservation laws shows that the above dependence implies a polytropic equation of state with index 4/3. The inferred value of the polytropic index appears independent of the exact nature of forces sustaining the transverse balance of the jet and indicates exact conservation of the longitudinal electric current and hence the existence of a stable internal electromagnetic structure at the scales under consideration. At this index the flow is hot and corresponds to relativistic thermodynamic motion of particles. Considerable weakening of the acceleration efficiency after 8 marcsec with the jet form being unchanged can be related to the plasma cooling, when the polytropic index becomes 5/3. Such a sharp change in the index without intermediate delay at 1.44 during cooling favours the existence of an electron–positron plasma and requires at least partial participation of the Blandford–Znajek mechanism in the launching of the M87 jet. [ABSTRACT FROM AUTHOR]

Geminga pulsar is surrounded by a multiTeV γ-ray halo radiated by the high-energy electrons and positrons accelerated by the central pulsar wind nebula (PWN). The angular profile of the γ-ray emission reported by High-Altitude Water Cherenkov Observatory indicates an anomalously slow diffusion for the cosmic-ray electrons and positrons in the halo region around Geminga. In the paper we study the possible mechanism for the origin of the slow diffusion. At first, we consider the self-generated Alfvén waves due to the streaming instability of the electrons and positrons released by Geminga. However, even considering a very optimistic scenario for the wave growth, we find this mechanism does not work to account for the extremely slow diffusion at the present day, if taking the proper motion of Geminga pulsar into account. The reason is straightforward as the PWN is too weak to generate enough high-energy electrons and positrons to stimulate strong turbulence at the late time. We then propose an assumption that the strong turbulence is generated by the shock wave of the parent supernova remnant (SNR) of Geminga. Geminga may still be inside the SNR, and we find that the SNR can provide enough energy to generate the slow-diffusion circumstance. The TeV haloes around PSR B0656+14, Vela X, and PSR J1826-1334 may also be explained under this assumption. [ABSTRACT FROM AUTHOR]

To manage the equivalent doses for radiation workers, exposure inhomogeneity is an important factor in the decision-making process related to protection measures and additional monitoring. Our previous study proposed the methodology to evaluate the inhomogeneity of exposure quantitatively. In this study, we applied proposed method to five different types of actual exposure situations encountered in the nuclear industry. Two of them were conventionally characterized as homogeneous exposure, and the other three were conventionally characterized as inhomogeneous exposure. The evaluation of homogeneity exposure was conducted using Monte Carlo calculations with two simplified models, which were then verified with phantom experiments. Consequently, all of the evaluations reproduced the experimental results, implying that our proposed method would be applicable for actual work conditions in the nuclear industry. Furthermore, the two presumed homogeneous exposure situations were found to be rather inhomogeneous because of the contribution of positrons and the limited source region. The results also show that the worker's posture has an impact on the inhomogeneity rather than the energy of incident radiation in nuclear works. The investigation also implies that obtaining the information on the most probable posture of the exposed worker, as well as the existence of the weekly penetrating radiation such as β± ray as a main source of exposure would be the key for more precise estimation. [ABSTRACT FROM AUTHOR]

With experimental results of AMS on the spectra of cosmic ray (CR) e −, e +, e − + e + and positron fraction, as well as new measurements of CR e − + e + flux by HESS, one can better understand the CR lepton (e − and e +) spectra and the puzzling electron–positron excess above ∼10 GeV. In this article, spectra of CR e − and e + are fitted with a physically motivated simple model, and their injection spectra are obtained with a 1D propagation model including the diffusion and energy loss processes. Our results show that the electron–positron excess can be attributed to uniformly distributed sources that continuously inject into the galactic disc electron–positron with a power-law spectrum cutting off near 1 TeV and a triple power-law model is needed to fit the primary CR electron spectrum. The lower energy spectral break can be attributed to propagation effects giving rise to a broken power-law injection spectrum of primary CR electrons with a spectral hardening above ∼40 GeV. [ABSTRACT FROM AUTHOR]

Nearby pulsars have been suggested as sources of ∼TeV e+/e− cosmic ray (CR) excess on the Earth. The High-Altitude Water Cherenkov Observatory (HAWC) detected extended TeV emission regions in the direction of two nearby middle-aged pulsars, Geminga and PSR B0656+14. By modelling the TeV emission as inverse Compton emission from electron–positron pairs diffusing in the interstellar medium (ISM), the HAWC collaboration derives a diffusion coefficient much smaller than the standard value in the vicinity of the two pulsars, which make them unlikely the origin of the positron excess. We propose that the observed γ-ray emission originate from the relic pulsar wind nebula. A two zone diffusion model with a slow diffusion in the nebula and a fast diffusion in the ISM can explain the HAWC surface-brightness profile and the positron excess simultaneously. Inefficient diffusion in the γ-ray emission region surrounding a middle-aged pulsar maybe a common phenomenon that can be tested by future observation. The implied diffusion coefficient in the ISM is smaller than the one suggested by the standard CR propagation model, but it is fully consistent with the predictions of the spiral arm model. [ABSTRACT FROM AUTHOR]

In the past few decades, observations indicated that an unexplained high production rate of positrons (the strong 511 keV line) exists in the Milky Way centre. By using the fact that a large amount of high-density gas used to exist near the Milky Way centre million years ago, we model the rate of positrons produced due to dark matter annihilation. We consider the effect of adiabatic contraction of dark matter density due to the supermassive black hole at the Milky Way centre and perform a detailed calculation to constrain the possible annihilation channel and dark matter mass range. We find that only three annihilation channels (μ+μ−, 4e, and 4μ) can provide the required positron production rate and satisfy the stringent constraint of gamma-ray observations. In particular, the constrained mass range for the μ+μ− channel is m ≈ 80−100 GeV, which is close to the mass range obtained for the dark matter interpretation of the GeV gamma-ray and positron excess. In other words, the proposed scenario can simultaneously provide the required positron production rate to explain the 511 keV emission, the positron excess, and the GeV gamma-ray excess in our Milky Way, and it is compatible with the density spike due to adiabatic growth model of the supermassive black hole. [ABSTRACT FROM AUTHOR]

In this article, we report a new signature of dark matter annihilation based on the radio continuum data of NGC 1569 galaxy detected in the past few decades. After eliminating the thermal contribution of the radio signal, an abrupt change in the spectral index is shown in the radio spectrum. Previously, this signature was interpreted as an evidence of convective outflow of cosmic ray. However, we show that the cosmic ray contribution is not enough to account for the observed radio flux.We then discover that if dark matter annihilates via the 4-e channel with the thermal relic cross-section, the electrons and positrons produced would emit a strong radio flux which can provide an excellent agreement with the observed signature. The best-fitting dark matter mass is 25 GeV. [ABSTRACT FROM AUTHOR]

We investigate the acceleration of electrons and positrons by magnetic-field-aligned electric fields in the polar funnel of an accreting black hole (BH). Applying the pulsar outer-gap theory to BH magnetospheres, we find that such a lepton accelerator arises in the immediate vicinity of the event horizon due to frame-dragging, and that their gamma-ray luminosity increases with decreasing accretion rate. Furthermore, we demonstrate that the gamma-ray flux is enhanced along the rotation axis by more than an order of magnitude if the BH spin increases from a = 0.90M to a = 0.9999M. As a result, if a ten-solar-mass, almost-maximally rotating BH is located within 3 kpc, when its accretion rate is between 0.005 and 0.01 per cent of the Eddington rate, its high-energy flare becomes detectable with the Fermi/Large Area Telescope, provided that the flare lasts longer than 1.2 months and that we view the source nearly along the rotation axis. In addition, its very high energy flux is marginally detectable with the Cherenkov Telescope Array, provided that the flare lasts longer than a night and that our viewing angle is about 45 deg with respect to the rotation axis. [ABSTRACT FROM AUTHOR]

We perform the NuSTAR and Swift/XRT joint energy spectral fitting of simultaneous observations from the broad-line Seyfert 1.5 galaxy NGC 5273. When fitted with the combination of an exponential cut-off power law and a reflection model, a high-energy cut-off is detected at 143+96 -40 keV with 2σsignificance. Existence of such cut-off is also consistent with the observed Comptonizing electron temperature when fitted with a Comptonization model independently. We observe amoderate hard X-ray variability of the source over the time-scale of∼12 yr using INTEGRAL/ISGRI observations in the energy range of 20-100 keV. When the hard-band count rate (6-20 keV) is plotted against the soft-band count rate (3-6 keV), a hard offset is observed. Our results indicate that the cut-off energy may not correlate with the coronal X-ray luminosity in a simple manner. Similarities in parameters that describe coronal properties indicate that the coronal structure of NGC 5273 may be similar to that of the broad-line radio galaxy 3C 390.3 and another galaxy MCG-5-23-16, where the coronal plasma is dominated by electrons, rather than electron-positron pairs. Therefore, the coronal cooling is equally efficient to the heating mechanism keeping the cut-off energy at low even at the low accretion rate. [ABSTRACT FROM AUTHOR]

A compilation of fluence-to-absorbed dose conversion coefficients for the dose to the lens of the eye is presented. The compilation consists of both previously published data and newly calculated values: photon data (5 keV-50 MeV for both kerma approximation and full electron transport), electron data (10 keV-50 MeV), and positron data (1 keV-50 MeV) - neutron data will be published separately. Values are given for angles of incidence from 0° up to 90° in steps of 15° and for rotational irradiation. The data presented can be downloaded from this article's website and they are ready for use by Report Committee (RC) 26. This committee has been set up by the International Commission on Radiation Units and Measurements (ICRU) and is working on a 'proposal for a redefinition of the operational quantities for external radiation exposure'. [ABSTRACT FROM AUTHOR]

We examine the plasma composition of relativistic jets in four Fanaroff-Riley type II (FRII) radio galaxies by analysing the total cocoon pressure in terms of partial pressures of thermal and non-thermal electrons/positrons and protons. The total cocoon pressure is determined by cocoon dynamics via comparison of theoretical model with the observed cocoon shape. By inserting the observed number density of non-thermal electrons/positrons and the upper limit of thermal electron/positron number density into the equation of state, the number density of protons is constrained. We apply this method to four FRII radio galaxies (Cygnus A, 3C 219, 3C 223 and 3C 284), for which the total cocoon pressures have been already evaluated.We find that the positron-free plasma comprising protons and electrons is ruled out, when we consider plausible particle distribution functions. In other words, the mixture of positrons is required for all four FRII radio galaxies; the number density ratio of electrons/positrons to protons is larger than 2. Thus, we find that the plasma composition is independent of the jet power and the size of cocoons. We also investigate the additional contribution of thermal electrons/positrons and protons on the cocoon dynamics. When thermal electrons/positrons are absent, the cocoon is supported by the electron/proton plasma pressure, while both electron/positron pressure supported and electron/proton plasma pressure supported cocoons are allowed if the number density of thermal electrons/positrons is about 10 times larger than that of non-thermal ones. [ABSTRACT FROM AUTHOR]

The light curves of Type Ia supernovae (SNe Ia) are powered by the radioactive decay of 56Ni to 56Co at early times, and the decay of 56Co to56Fe from â60 d after explosion. We examine the evolution of the [Coâ€III] λ5893 emission complex during the nebular phase for SNe Ia with multiple nebular spectra and show that the line flux follows the square of the mass of 56Co as a function of time. This result indicates both efficient local energy deposition from positrons produced in 56Co decay and long-term stability of the ionization state of the nebula. We compile SN Ia nebular spectra from the literature and present 21 new late-phase spectra of 7 SNe Ia, including SN 2014J. From these we measure the flux in the [CoIII] λ5893 line and remove its well-behaved time dependence to infer the initial mass of 56Ni (MNi) produced in the explosion. We then examine 56Ni yields for different SN Ia ejected masses (Mej â€" calculated using the relation between light-curve width and ejected mass) and find that the56Ni masses of SNe Ia fall into two regimes: for narrow light curves (low stretch s ~ 0.7â€"0.9), MNi is clustered near MNi â 0.4M⊙ and shows a shallow increase as Mej increases from ~1 to 1.4M⊙; at high stretch, Mejclusters at the Chandrasekhar mass (1.4M⊙) while MNi spans a broad range from 0.6 to 1.2M⊙. This could constitute evidence for two distinct SN Ia explosion mechanisms. [ABSTRACT FROM AUTHOR]

The positron flux measured near Earth shows a rise with energy beyond 30 GeV. We show that this rise might be compatible with the production of positrons in pγ interactions in the jets of microquasars. [ABSTRACT FROM AUTHOR]

Equipartition arguments provide an easy way to find a characteristic scale for the magnetic field from radio emission by assuming that the energy densities in cosmic rays and magnetic fields are the same. Yet most of the cosmic ray content in star-forming galaxies is in protons, which are invisible in radio emission. Therefore, the argument needs assumptions about the proton spectrum, typically that of a constant proton/electron ratio. In some environments, particularly starburst galaxies, the reasoning behind these assumptions does not necessarily hold: secondary pionic positrons and electrons may be responsible for most of the radio emission, and strong energy losses can alter the proton/electron ratio. We derive an equipartition expression that should work in a hadronic loss-dominated environment like starburst galaxies. Surprisingly, despite the radically different assumptions from the classical equipartition formula, numerically the results for starburst magnetic fields are similar. We explain this fortuitous coincidence using the energetics of secondary production and energy loss times. We show that these processes cause the proton/electron ratio to be ~100 for GHz-emitting electrons in starbursts. [ABSTRACT FROM AUTHOR]

The ability to accurately reconstruct original spatial positions of field-evaporated ions emitted from a surface is fundamental to the success of atom probe tomography. As such, a clear understanding of the evolution of specimen shape and the resultant ions' trajectories during field evaporation plays an important role in improving reconstruction accuracy. To further this understanding, field-evaporation simulations of a bilayer specimen composed of two materials having an evaporation field difference of 20% were performed. The simulated field-evaporation patterns qualitatively compare favorably with experimental data, which provides confidence in the accuracy of specimen shapes predicted by the simulation. Correlations of known original atom positions with detector hit positions as a function of lateral detector position and evaporated depth were derived from the simulation. These correlations are contrasted with the current state-of-the-art reconstruction method thus outlining limitations of the current methodology. A pair of transformations are defined that take into account field-evaporated specimen shapes, and the resulting radial magnifications, to relate recorded ion positions in detector space to reconstructed atomic positions in specimen space. This novel process, when applied to simulated data, results in approximately a factor of 2 improvement in accuracy for reconstructions of interfaces with unequal fields (most general interfaces). This method is not constrained by the fundamental assumption of a hemispherical specimen shape. [ABSTRACT FROM PUBLISHER]

ABSTRACT Recent γ-ray observations by Fermi Gamma-Ray Space Telescope suggest that the γ-ray millisecond pulsar (MSP) population is separated into two subclasses with respect to pair multiplicity. Here, we calculate the cosmic-ray electron/positron spectra from MSPs. Based on the assumption of equipartition in the pulsar-wind region, the typical energy of electrons/positrons ejected by a MSP with pair multiplicity of the order of unity is ∼50 TeV. In this case, we find that a large peak in the 10-50 TeV energy range would be observed in the cosmic-ray electron/positron spectrum. Even if the fraction of pair-starved MSPs is 10 per cent, a large peak would be detectable in future observations. We also calculate the contribution from MSPs with high pair multiplicity to the electron/positron spectrum. We suggest that if the multiplicity of the dominant MSP population is ∼103, electrons/positrons from this population may contribute to the observed excess from the background electron/positron flux and positron fraction. [ABSTRACT FROM AUTHOR]

Leucoaraiosis is associated with motor symptoms in otherwise normal older adults. Comorbid leucoaraiosis is predicted to contribute also to motor features in Parkinson’s disease but previous studies of white matter changes in Parkinson’s disease show variable results. No prior studies have compared directly the effects of both leucoaraiosis and the degree of nigrostriatal dopaminergic denervation on motor features. We investigated the effect of leucoaraiosis severity on motor impairment independent of the degree of nigrostriatal dopaminergic denervation in Parkinson’s disease. Seventy-three subjects with Parkinson’s disease (Hoehn and Yahr stages 1–3) underwent brain magnetic resonance and [11C]dihydrotetrabenazine vesicular monoamine transporter type 2 positron emission tomography imaging. Automated assessment of supratentorial fluid-attenuated inversion recovery magnetic resonance hyperintense white matter voxels was performed using cerebellar white matter as the intensity reference. White matter signal hyperintensity burden was log-transformed and normalized for brain volume. Unified Parkinson’s Disease Rating Scale total and subscore ratings were assessed to determine motor impairment. Subjects receiving dopaminergic medications were examined in the clinically defined ‘OFF’ state. Multivariate regression analysis with measures of white matter signal hyperintensity burden and nigrostriatal denervation as independent variables demonstrated a significant overall model for total motor Unified Parkinson's Disease Rating Scale scores (F = 11.4, P < 0.0001) with significant regression effects for both white matter signal hyperintensity burden (t = 2.0, β = 0.22, P = 0.045) and striatal monoaminergic binding (t = −3.5, β = −0.38, P = 0.0008). Axial motor impairment demonstrated a robust association with white matter signal hyperintensity burden (t = 4.0, β = 0.43, P = 0.0001) compared with striatal monoaminergic binding (t = −2.1, β = 0.22, P = 0.043). White matter signal hyperintensity burden regression effects for bradykinesia had borderline significance. No significant white matter signal hyperintensity burden effects were found for rigidity or tremor subscores. White matter signal hyperintensity burden was significantly higher in the subgroup with postural instability and gait difficulties compared with the tremor-predominant subgroup despite no significant differences in age or duration of disease. These findings indicate that increased white matter signal hyperintensity burden is associated with worse motor performance independent of the degree of nigrostriatal dopaminergic denervation in Parkinson’s disease. Comorbid white matter disease is a greater determinant of axial motor impairment than nigrostriatal dopaminergic denervation. [ABSTRACT FROM AUTHOR]

BSTRACT [ABSTRACT FROM AUTHOR]

The rise in the cosmic-ray positron fraction measured by the PAMELA satellite is likely due to the presence of astrophysical sources of positrons, for example, pulsars, on kpc scales around the Earth. Nevertheless, assessing the properties of these sources from the positron data requires a good knowledge of the secondary positron component generated by the interaction of cosmic rays with the interstellar gas. In this paper, we investigate the impact of the spectral hardening in the cosmic-ray proton and helium fluxes recently reported by the ATIC2 and CREAM balloon experiments on the predictions of the secondary positron flux. We show that the effect is not negligible, leading to an increase in the secondary positron flux by up to 60 per cent above 100 GeV. We provide fitting formulae that allow a straightforward utilization of our results, which can help in deriving constraints on one's favourite primary positron source, for example, pulsars or dark matter. [ABSTRACT FROM AUTHOR]

This paper continues a previous study of neutron stars with positive polar-cap corotational charge density in which free emission of ions maintains the surface electric field boundary condition . The composition of the accelerated plasma on any subset of open magnetic flux lines above the polar cap alternates between two states: protons or positrons and ions, of which the proton state cannot support electron-positron pair creation at higher altitudes. The two states coexist at any instant of time above different moving elements of area on the polar cap and provide a physically consistent basis for a description of pulse nulls and subpulse drift. In the latter case, it is shown that the band separation is determined not by the drift velocity, as is generally assumed, but by the diffusion time for protons produced in reverse-electron showers to reach the region of the atmosphere from which they are accelerated. An initial comparison is made with the survey of subpulse drift published by Weltevrede et al. [ABSTRACT FROM AUTHOR]

We present a critical analysis of the observational constraints on, and of the theoretical modelling of, aspects of cosmic ray (CR) generation and propagation in the Galaxy, which are relevant for the interpretation of recent positron and antiproton measurements. We give simple, analytic, model-independent expressions for the secondary flux, and an upper limit for the secondary flux, obtained by neglecting radiative losses, up to GeV. These expressions are completely determined by the rigidity-dependent grammage, which is measured from stable CR secondaries up to GeV nucleon−1, and by nuclear cross-sections measured in the laboratory. and measurements, available up to GeV, are consistent with these estimates, implying that there is no need for new, non-secondary, or sources. The radiative loss suppression factor of the flux depends on the propagation in the Galaxy, which is not understood theoretically. A rough, model-independent estimate of can be obtained at a single energy, GeV, from unstable secondary decay and is found to be consistent with measurements, including the positron fraction measured by Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics ( PAMELA). We show that specific detailed models, that agree with compositional CR data, agree with our simple expressions for the and flux, and that the claims that the positron fraction measured by PAMELA requires new primary sources are based on assumptions, that are not supported by observations. If PAMELA results are correct, they suggest that is slightly increasing with energy, which provides an interesting constraint on CR propagation models. We argue that measurements of the to ratio are more useful for challenging secondary production models than the fraction. [ABSTRACT FROM AUTHOR]

The origin and properties of the source of positrons annihilating in the Galactic Centre (GC) are still a mystery. One of the criteria that may discriminate between different mechanisms of positron production there is the injection energy of positrons. Beacom and Yüksel suggested a method to estimate this energy from the ratio of the 511-keV line to the MeV in-flight annihilation fluxes. From COMPTEL data, they derived that the maximum injection energy of positrons should be about several MeV. This significantly decreased the class of models of the positron origin in the GC, assuming that positrons lose their energy through Coulomb collisions only. However, observations show that the strength of the magnetic field in the GC is much higher than in other parts of the Galaxy; in the GC, it may range from 100 μG to several mG. In these conditions, the synchrotron losses of positrons are significant and this extends the range of acceptable values of the injection energy of positrons. We show that if positron injection in the GC is non-stationary and the magnetic field is higher than 0.4 mG, both radio and gamma-ray restrictions permit the energy to be higher than several GeV. [ABSTRACT FROM AUTHOR]

Positron emission tomography combined with computed tomography (PET/CT) is a quantitative technique suitable for diagnostics and uptake measurements. The quantitative results can be used for the purpose of the calculating absorbed dose to patients undergoing nuclear medicine investigations. Hence, the accuracy of the quantification of the activity content in organs or tissues is of great importance. When using a planar gamma camera and single photon emission computed tomography (SPECT) images, the activity content in organs and tumours has to be determined by the user, using the number of counts in the organs and the efficiency of the camera. However, when using a Philips Gemini TF PET/CT system, the activity concentration in a region of interest (ROI) is given by the system. The reliability of activity concentration values given by the Philips Gemini TF PET/CT system was studied using a Jaszczak phantom containing hot spheres of different sizes; the influence of the ROI size and the impact of organ size, that is the partial volume effect, was investigated with three different lesion-to-background ratios in the phantom. The use of a small ROI size (40 % of the large ROI size, which covered the entire sphere) showed a 15 % improvement in the recovery of the true activity. Small lesion sizes result in large underestimations of the activity concentration values. [ABSTRACT FROM PUBLISHER]

The neutrino–antineutrino annihilation into electron–positron pairs near the surface of compact general relativistic stars could play an important role in supernova explosions, neutron star collapse, or for close neutron star binaries near their last stable orbit. General relativistic effects increase the energy deposition rates due to the annihilation process. We investigate the deposition of energy and momentum due to the annihilations of neutrinos and antineutrinos in the equatorial plane of the rapidly rotating neutron and quark stars, respectively. We analyse the influence of general relativistic effects, and obtain the general relativistic corrections to the energy and momentum deposition rates for arbitrary stationary and axisymmetric space–times. We obtain the energy and momentum deposition rates for several classes of rapidly rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the Massachusetts Institute of Technology bag model equation of state and in the colour-flavour-locked phase, respectively. Compared to the Newtonian calculations, rotation and general relativistic effects increase the total annihilation rate measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino–antineutrino annihilation. [ABSTRACT FROM AUTHOR]

Benign notochordal cell tumours have recently been described as intraosseous benign lesions of notochordal cell origin. The lesions are found in vertebral bodies in 20% of autopsy studies and are a potential precursor of chordoma. We report a rare case of lumbar vertebral chordoma that was thought to arise from a benign intraosseous notochordal cell tumour and which showed significant osteosclerotic change. Radiologically, the lumbar vertebral mass lesion showed hyperintensity on T2 weighted images, with scanty enhancement on post-contrast T1 weighted MR images. High uptake corresponding to the mass was noted on fluorine- 18-fluorodeoxyglucose positron emission tomography. Bone biopsy revealed proliferation of the physaliphorous cells between thickened bone trabeculae; no nuclear mitosis was observed. Although the mass was diagnosed clinically as spinal chordoma, histopathology contained both benign notochordal cell tumour and conventional chordoma. After heavy particle (11C)-charged radiation therapy was applied to the lesion with a sufficient radiation field margin, the tumour volume significantly decreased and there was improvement in the patient's symptoms. On follow-up radiological studies, the tumour had markedly regressed and there was no tumour regrowth or distant metastasis. In this case report, benign notochordal cell tumour and conventional chordoma are histopathologically identified in the L1 vertebral body, which contains osteosclerotic and osteolytic areas. It is suggested that the benign notochordal cell tumour coexists with a conventional chordoma and that this histopathological finding supports a hypothetical relationship between benign notochordal cell tumour and chordoma. [ABSTRACT FROM AUTHOR]

An intercomparison of ring dosemeters has been organised with the aim of assessing the technical capabilities of available extremity dosemeters and focusing on their performance at clinical workplaces with potentially high extremity doses. Twenty-four services from 16 countries participated in the intercomparison. The dosemeters were exposed to reference photon (137Cs) and beta (147Pm, 85Kr and 90Sr/90Y) fields together with fields representing realistic exposure situations in interventional radiology (direct and scattered radiation) and nuclear medicine (99mTc and 18F). It has been found that most dosemeters provided satisfactory measurements of Hp(0.07) for photon radiation, both in reference and realistic fields. However, only four dosemeters fulfilled the established requirements for all radiation qualities. The main difficulties were found for the measurement of low-energy beta radiation. Finally, the results also showed a general under-response of detectors to 18F, which was attributed to the difficulties of the dosimetric systems to measure the positron contribution to the dose. [ABSTRACT FROM AUTHOR]

This paper presents the effects of electron–positron pair production on the linear growth of the resistive hose instability of a filamentary beam that could lead to snake-like distortion. For both the rectangular radial density profile and the diffuse profile reflecting the Bennett-type equilibrium for a self-collimating flow, the modified eigenvalue equations are derived from a Vlasov–Maxwell equation. While for the simple rectangular profile, current perturbation is localized at the sharp radial edge, for the realistic Bennett profile with an obscure edge, it is non-locally distributed over the entire beam, removing catastrophic wave–particle resonance. The pair production effects likely decrease the betatron frequency, and expand the beam radius to increase the resistive decay time of the perturbed current; these also lead to a reduction of the growth rate. It is shown that, for the Bennett profile case, the characteristic growth distance for a preferential mode can exceed the observational length-scale of astrophysical jets. This might provide the key to the problem of the stabilized transport of the astrophysical jets including extragalactic jets up to scales. [ABSTRACT FROM AUTHOR]

It has been proposed that the excess in cosmic ray positrons at approximately 8 GeV, observed on both flights of the High-Energy Antimatter Telescope ( HEAT) balloon experiment, may be associated with the annihilation of dark matter within the Milky Way halo. In this paper, we demonstrate how the self-annihilation of neutralino dark matter within local substructure can account for this excess, and we estimate the annihilation cross-section for several benchmark minimal supersymmetric (MSSM) models. We also demonstrate the changes in the permitted parameter space as the effects of tidal disruption become increasingly severe. [ABSTRACT FROM AUTHOR]

The objective of this study is to provide a set of proximity functions for electrons from 100 eV to 10 MeV. Numerical results of differential proximity functions are given graphically. The complete data set is available electronically upon request from the authors. The results can serve as a convenient database for anyone performing microdosimetric calculations in radiation fields of electrons. For mixed fields of electrons, the proximity functions can easily be derived from the proximity functions of monoenergetic electrons presented here. [ABSTRACT FROM PUBLISHER]

The dominant emission from bare strange stars is thought to be electron–positron pairs, produced through spontaneous pair creation (SPC) in a surface layer of electrons tied to the star by a superstrong electric field. The positrons escape freely, but the electrons are directed towards the star and quickly fill all available states, such that their degeneracy suppresses further SPC. An electron must be reflected and gain energy in order to escape, along with the positron. Each escaping electron leaves a hole that is immediately filled by another electron through SPC. We discuss the collisional processes that produce escaping electrons. When the Landau quantization of the motion perpendicular to the magnetic field is taken into account, electron–electron collisions can lead to an escaping electron only through a multistage process involving higher Landau levels. Although the available estimates of the collision rate are deficient in several ways, it appears that the rate is too low for electron–electron collisions to be effective. A simple kinetic model for electron–quark collisions leads to an estimate of the rate of pair production that is analogous to thermionic emission, but the work function is poorly determined. [ABSTRACT FROM AUTHOR]