Possible Contribution of Mature γ-ray Pulsars to Cosmic-ray Positrons

We restudy the possible contribution of mature gamma-ray pulsars to cosmic ray positrons based on the new version of outer gap model. In this model, the inclination angle and average properties of the outer gap are taken into account, and more mature pulsars can have the outer gap and emit high energy photons. Half of the primary particles in the outer gaps will flow back toward the star surface and emit synchrotron photons, which can produce electron/positron pairs by the cascade of pair production. Some of these pairs will escape from the light cylinder and be accelerated to relativistic energies in the pulsar wind driven by low-frequency electromagnetic waves. Using a Monte Carlo method, we obtain a sample of mature gamma-ray pulsars and then calculate the production of the positrons from these pulsars. The observed excess of cosmic positrons can be well explained by this model.     

Possible Contribution of Mature γ-ray Pulsars to Cosmic-ray Positrons

We restudy the possible contribution of mature gamma-ray pulsars to cosmic ray positrons based on the new version of outer gap model. In this model, the inclination angle and average properties of the outer gap are taken into account, and more mature pulsars can have the outer gap and emit high energy photons. Half of the primary particles in the outer gaps will flow back toward the star surface and emit synchrotron photons, which can produce electron/positron pairs by the cascade of pair production. Some of these pairs will escape from the light cylinder and be accelerated to relativistic energies in the pulsar wind driven by low-frequency electromagnetic waves. Using a Monte Carlo method, we obtain a sample of mature gamma-ray pulsars and then calculate the production of the positrons from these pulsars. The observed excess of cosmic positrons can be well explained by this model.     

Cosmic-ray positrons: are there primary sources?

Galactic cosmic rays consist of primary and secondary particles. Primary cosmic rays are thought to be energized by first order Fermi acceleration processes at supernova shock fronts within our Galaxy. The cosmic rays that eventually reach the Earth from this source are mainly protons and atomic nuclei, but also include electrons. Secondary cosmic rays are created in collisions of primary particles with the diffuse interstellar gas. They are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.     

Possible Contribution of Mature γ-ray Pulsars to Cosmic-ray Positrons

We restudy the possible contribution of mature gamma-ray pulsars to cosmic ray positrons based on the new version of outer gap model. In this model, the inclination angle and average properties of the outer gap are taken into account, and more mature pulsars can have the outer gap and emit high energy photons. Half of the primary particles in the outer gaps will flow back toward the star surface and emit synchrotron photons, which can produce electron/positron pairs by the cascade of pair production. Some of these pairs will escape from the light cylinder and be accelerated to relativistic energies in the pulsar wind driven by low-frequency electromagnetic waves. Using a Monte Carlo method, we obtain a sample of mature gamma-ray pulsars and then calculate the production of the positrons from these pulsars. The observed excess of cosmic positrons can be well explained by this model.     

Electron-Positron Plasmas Created by Ultra-Intense Laser Pulses Interacting with Solid Targets

We discuss the necessary requirements to create dense electron-positron plasmas in the laboratory and the possibility of using them to investigate certain aspects of various astrophysical phenomena, such as gamma ray burst engines. Earth-based electron-positron plasmas are created during the interaction of ultra-intense laser pulses impinging on a solid density target. The fact that positrons can be generated during this interaction has already been demonstrated by Cowan et al. (2000). However, several questions concerning the number, energy, and dynamics of these positrons have yet to be answered. Through insight gathered from PIC simulations, we postulate that the e⁺e⁻ plasma leaves the creation region in dense jets, with relativistic energies. In order to estimate the number density of the positrons created, we begin by first experimentally measuring the hot electron temperatures and densities of such interactions using a compact electron spectrometer. Once the electron distribution is known, the positron creation rate, Γ, can be estimated. This same experimental diagnostic can also, with minor modification, measure the energy distribution of positrons. Initial estimates are that, with proper target and laser configurations, we could potentially create one of the densest arraignments of positrons ever assembled on earth. This experimental configuration would only last for a few femtoseconds, but would eventually evolve into astrophysically relevant pure electron-positron jets, possibly relevant to e⁺e⁻ outflow from black holes.     

Cosmic ray propagation in a closed galaxy

A simple model of cosmic ray propagation is proposed from which the major experimental results can be derived: The model reproduces the observed nuclear abundances and accounts for the observed changes of nuclear composition with energy, the high degree of isotropy of cosmic ray flux at all energies, and the high degree of its constancy throughout the history of the Solar System. It is consistent with the observed size distribution of extensive airshowers, the intensity and energy distribution of the electron component, and the diffuse emission of γ-rays and radio waves. The model is characterized by the two basic assumptions: (1) that cosmic rays have been injected at an unchanging rate by sources located in the galactic spiral arms and (2) that a large-scale magnetic field retains all particles in our galaxy, where they interact with interstellar gas, so that all complex nuclei are finally fragmented and their energy dissipated in meson production and electro-magnetic interactions.     

Solar Impulsive Hard X-Ray Emission and Two-Stage Electron Acceleration

Heating and acceleration of electrons in solar impulsive hard X-ray (HXR) flares are studied according to the two-stage acceleration model developed by Zhang for solar 3He-rich events. It is shown that electrostatic H-cyclotron waves can be excited at a parallel phase velocity less than about the electron thermal velocity and thus can significantly heat the electrons (up to 40 MK) through Landau resonance. The preheated electrons with velocities above a threshold are further accelerated to high energies in the flare-acceleration process. The flare-produced electron spectrum is obtained and shown to be thermal at low energies and power law at high energies. In the non-thermal energy range, the spectrum can be double power law if the spectral power index is energy dependent or related. The electron energy spectrum obtained by this study agrees quantitatively with the result derived from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) HXR observations in the flare of 2002 July 23. The total flux and energy flux of electrons accelerated in the solar flare also agree with the measurements.     

GEM-based TPC with CCD imaging for directional dark matter detection

The most mature directional dark matter experiments at present all utilize low-pressure gas Time Projection Chamber (TPC) technologies. We discuss some of the challenges for this technology, for which balancing the goal of achieving the best sensitivity with that of cost effective scale-up requires optimization over a large parameter space. Critical for this are the precision measurements of the fundamental properties of both electron and nuclear recoil tracks down to the lowest detectable energies. Such measurements are necessary to provide a benchmark for background discrimination and directional sensitivity that could be used for future optimization studies for directional dark matter experiments. In this paper we describe a small, high resolution, high signal-to-noise GEM-based TPC with a 2D CCD readout designed for this goal. The performance of the detector was characterized using alpha particles, X-rays, gamma-rays, and neutrons, enabling detailed measurements of electron and nuclear recoil tracks. Stable effective gas gains of greater than 1 × 10⁵ were obtained in 100 Torr of pure CF₄ by a cascade of three standard CERN GEMs each with a 140 µm pitch. The high signal-to-noise and sub-millimeter spatial resolution of the GEM amplification and CCD readout, together with low diffusion, allow for excellent background discrimination between electron and nuclear recoils down below ∼10 keVee (∼23 keVr fluorine recoil). Even lower thresholds, necessary for the detection of low mass WIMPs for example, might be achieved by lowering the pressure and utilizing full 3D track reconstruction. These and other paths for improvements are discussed, as are possible fundamental limitations imposed by the physics of energy loss.     

On the origin of the 511-keV emission in the Galactic Centre

Diffuse 511-keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 yr. The latest high-resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of ∼70 luminous, low-mass X-ray binaries detected in the soft gamma-ray band. The X-ray band, however, is generally a more sensitive probe of X-ray binary populations. Recent X-ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X-ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair-dominated jets generated by this population of fainter accretion-powered X-ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the Wilkinson Microwave Anisotropy Probe 'haze') and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511-keV emission with the brightest X-ray binaries.     

A systematic investigation of several phenomena associated with contaminated Langmuir probes

The first part of this paper uses a contamination layer model to discuss the effects of electrode contamination upon electron temperatures estimated from Langmuir probe measurements. The model assumes that the contamination layer is equivalent to a parallel capacitor and resistor. It predicts two main features associated with electrode contamination. One of them, the so-called “frequency dependence,” concerns the sweep-rate of the probe voltage and is well understood. The other is that the effect of a contaminated electrode is decreased as the density of the ambient plasma is decreased and this will be called “density dependence.” We present several experimental results which verify the above predictions.This paper also presents some other experimental results which may be useful in improving the accuracy of Langmuir probe measurements.Finally the effects of electrode contamination upon electron density estimates and energy distribution measurements are briefly discussed.     

Modeling ionization and recombination from low energy nuclear recoils in liquid argon

Coherent elastic neutrino-nucleus scattering (CENNS) is an as-yet undetected, flavor-independent neutrino interaction predicted by the Standard Model. Detection of CENNS could offer benefits for detection of supernova and solar neutrinos in astrophysics, or for detection of antineutrinos for nuclear reactor monitoring and nuclear nonproliferation. One challenge with detecting CENNS is the low energy deposition associated with a typical CENNS nuclear recoil. In addition, nuclear recoils result in lower ionization yields than those produced by electron recoils of the same energy. While a measurement of the nuclear recoil ionization yield in liquid argon in the keV energy range has been recently reported, a corresponding model for low-energy ionization yield in liquid argon does not exist. For this reason, a Monte Carlo simulation has been developed to predict the ionization yield at sub-10 keV energies. The model consists of two distinct components: (1) simulation of the atomic collision cascade with production of ionization, and (2) the thermalization and drift of ionization electrons in an applied electric field including local recombination. As an application of our results we report updated estimates of detectable ionization in liquid argon from CENNS at a nuclear reactor.     

Evidence that Synchrotron Emission from Nonthermal Electrons Produces the Increasing Submillimeter Spectral Component in Solar Flares

We investigate the origin of the increasing spectra observed at submillimeter wavelengths detected in the flare on 2 November 2003 starting at 17:17 UT. This flare, classified as an X8.3 and 2B event, was simultaneously detected by RHESSI and the Solar Submillimeter Telescope (SST) at 212 and 405 GHz. Comparison of the time profiles at various wavelengths shows that the submillimeter emission resembles that of the high-energy X rays observed by RHESSI whereas the microwaves observed by the Owens Valley Solar Array (OVSA) resemble that of ∼50 keV X rays. Moreover, the centroid position of the submillimeter radiation is seen to originate within the same flaring loops of the ultraviolet and X-ray sources. Nevertheless, the submillimeter spectra are distinct from the usual microwave spectra, appearing to be a distinct spectral component with peak frequency in the THz range. Three possibilities to explain this increasing radio spectra are discussed: (1) gyrosynchrotron radiation from accelerated electrons, (2) bremsstrahlung from thermal electrons, and (3) gyrosynchrotron emission from the positrons produced by pion or radioactive decay after nuclear interactions. The latter possibility is ruled out on the grounds that to explain the submillimeter observations requires 3000 to 2×10⁵ more positrons than what is inferred from X-ray and γ-ray observations. It is possible to model the emission as thermal; however, such sources would produce too much flux in the ultraviolet and soft X-ray wavelengths. Nevertheless we are able to explain both spectral components at microwave and submillimeter wavelengths by gyrosynchrotron emission from the same population of accelerated electrons that emit hard X rays and γ rays. We find that the same 5×10³⁵ electrons inferred from RHESSI observations are responsible for the compact submillimeter source (0.5 arcsec in radius) in a region of 4500 G low in the atmosphere, and for the traditional microwave spectral component by a more extended source (50 arcsec) in a 480 G magnetic field located higher up in the loops. The extreme values in magnetic field and source size required to account for the submillimeter emission can be relaxed if anisotropy and transport of the electrons are taken into account.     

Relativistic electrons from solar flares

Observations of interplanetary relativistic electrons from several solar-flare events monitored through 1964 to mid-1967 are presented. These are the first direct spectral measurements and time histories, made outside the magnetosphere, of solar-flare electrons having relativistic velocities. The 3- to 12-MeV electrons detected have kinetic energies about two orders of magnitude higher than those solar electrons previously studied in space, and measurements of both the time histories and energy spectra for a number of events in the present solar cycle were carried out. These measurements of interplanetary electrons are also directly compared with solar X-ray data and with measurements of related interplanetary solar protons.The time histories of at least four electron events show fits to the typical diffusion picture. A demonstrated similarity between the electron and the medium-energy proton fits for the event of 7 July, in particular, indicates that at these electron energies, but over several orders of magnitude of rigidity, whatever diffusion does take place is very nearly on a velocity, rather than a rigidity or an energy, basis. Diffusion-fit time histories varied as a function of T ₀ also indicate that the electrons in certain flare events originate at times near the X-ray and microwave burst, establishing their likely identity as the same electrons which cause the impulsive radiations. Also, the energy spectra and total numbers of the interplanetary electrons, compared with those of the flare-site electrons calculated from X-ray and microwave measurements, indicate that probably a small fraction of flare electrons escape into interplanetary space.     

Gamma-Ray Line Versus Continuum Emission of Electron-Dominated Episodes During Solar Flares

Electron-dominated episodes or events during solar flares are characterized by a flattening of the electron bremsstrahlung continuum above about 1 MeV. This flattening leads to a dominance of the continuum at MeV energies over nuclear emissions. We analyzed events recorded by the gamma-ray spectrometer on SMM with the aim to determine the nuclear contribution in the energy range between 4 and 8 MeV. We find that for comparable continuum fluences it is less by about an order of magnitude than for other flares. The spectral index of the best-fit power law of the > 1 MeV continuum with a median at –1.84 turns out to be independent of the heliocentric angle of the events, implying that the degree of anisotropy of the radiating particles was low. It is of interest to note that a value of –1.5 seems to be a limit. The spectral index of the continuum between 0.3 and 1 MeV does not differ significantly from that of other flares. Only measurements with detectors sensitive up to at least 10 MeV can, therefore, sort out electron dominated episodes during solar flares.     

Pulsar Slot Gaps and Unidentified Egret Sources

A new picture of pulsar high-energy emission is proposed that is different from both the traditional polar cap and outer gap models, but combines elements of each. The slot gap model is based on electron acceleration along the edge of the open field region from the neutron star surface to near the light cylinder and thus could form a physical basis for the two-pole caustic model of Dyks and Rudak (2003). Along the last open field line, the pair formation front rises to very high altitude forming a slot gap, where the accelerating electric field is unscreened by pairs. The resulting radiation features both hollow cones from the lower-altitude pair cascades, seen at small viewing angles, as well as caustic emission on the trailing-edge field lines at high altitude, seen from both poles at large viewing angle. The combination of the small solid angle of slot gap emission (≪ 1 sr) with a high probability of viewing the emission predicts that more gamma-ray pulsars could be detected at larger distances. In this picture, many of the positional coincidences of radio pulsars with unidentified EGRET sources become plausible as real associations, as the flux predicted by the slot gap model for many of the pulsars would provide the observed EGRET source flux. The expected probability of seeing radio-quiet gamma-ray pulsars in this model will also be discussed.     

Red-shifted annihilation lines in thermal synchrotron model of gamma-ray bursts

The origins of the gravitationally-red-shifted annihilation lines of gamma-ray bursts in thermal synchrotron-radiation model are discussed in this paper. It is shown that the positrons produced by high-energy photons which are obtained by extrapolation from the continua through strong magnetic field could not be the main source of the annihilation lines, while that escaping from the hot thin region might account the emission features.     

Incoherent Scatter Radar Studies of Daytime Plasma Lines

First results from wideband (electron phase energies of 5–51 eV), high-resolution (0.1 eV) spectral measurements of photoelectron–enhanced plasma lines made with the 430 MHz radar at Arecibo Observatory are presented. In the F region, photoelectrons produced by solar EUV line emissions (He II and Mg IX) give rise to plasma line spectral peaks/valleys. These and other structures occur within an enhancement zone extending from electron phase energies of 14–27 eV in both the bottomside and topside ionosphere. However, photoelectron–thermal electron Coulomb energy losses can lead to a broadened spectral structure with no resolved peaks in the topside ionosphere. The plasma line energy spectra obtained in the enhancement zone exhibit a unique relation in that phase energy is dependent on pitch angle; this relation does not exist in any other part of the energy spectrum. Moreover, large fluctuations in the difference frequency between the upshifted and downshifted plasma lines are evident in the 14–27 eV energy interval. At high phase energies near 51 eV the absolute intensities of photoelectron-excited Langmuir waves are much larger than those predicted by existing theory. The new measurements call for a revision/improvement of plasma line theory in several key areas.     

Strict Limits on the Ionizing Luminosity in NGC 1068 from Jet-Axis Molecular Clouds

The radio jet axis of NGC 1068 is characterised by energetic activity from x-ray to radio wavelengths. Detailed kinematic and polarization studies have shown that this activity is confined to bipolar cones centered on the AGN which intersect the plane of the disk. Thus, molecular clouds at 1 kpc distance along this axis are an important probe of the nuclear ionizing luminosity and spectrum. Extended 10.8μm emission coincident with the clouds is reasonably understood by dust heated to high temperatures by the nuclear radiation field. This model predicts that the nuclear spectrum is quasar-like (power law + blue excess) with a luminosity 2-5 times higher than inferred by Pier et al. (1994). Consequently, there is little or no polyaromatic hydrocarbon (PAH) emission associated with the radio-axis molecular clouds. We review this model in the light of new observations. A multi-waveband collage is included to illustrate the possible orientations of the double cones to our line of sight and the galaxian plane. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calibration of a CsI(Tl) crystal with nuclear recoils and pulse shape measurements for dark matter detection

The quenching factor of cesium and iodine nuclei recoiling in a CsI(Tl) scintillator is measured by scattering of 3 to 6 MeV neutrons. This factor increases when recoil energy decreases, from 7% at 150 keV to 15% at 25 keV. This relatively high efficiency could be useful in experiments dealing with very low recoil energies like the WIMP direct detection. These values are well explained by the Birks model. Pulse shape discrimination between electron and nuclei recoils is also investigated. Results are sufficiently good to allow a significant statistical rejection of radioactive background. This rejection capability is shown to be better than for NaI(Tl), at the same electron equivalent energy.     

Equilibrium spectra of secondary cosmic-ray positrons in the galaxy and the spectrum of cosmic gamma-rays resulting from their annihilation

A general formula is derived for calculating the -ray spectrum resulting from the annihilation of cosmic-ray positrons. This formula is used to calculate annihilation--ray spectra from various equilibrium spectra of secondary galactic positrons. These spectra are then compared with the -ray spectra produced by other astrophysical processes.Particular attention is paid to the form of the -ray spectrum resulting from the annihilation of positrons having kinetic energies below 5 keV. It is found that for mean leakage times out of the galaxy of less than 400 million years, most of the positrons annihilating near rest come from the -decay of unstable nuclei produced in cosmic-ray p-C¹², p-N¹⁴, and p-O¹⁶ interactions, rather than from pi-meson decay. It is further found that the large majority of these positrons will annihilate from an S state of positronium and that 3/4 of these will produce a three-photon annihilation continuum rather than the two-photon line spectrum at 0.51 MeV. The results of numerical calculations of the -ray fluxes from these processes are given. It is concluded that annihilation -rays from the galactic halo may remain forever masked by a metagalactic continuum. However, an 0.51 MeV line from the disk may well be detectable. It is most reasonable to assume that this line is formed predominantly by the annihilation of the CNO -decay positrons. Under this assumption, the intensity of the line becomes a sensitive measure of the galactic cosmic-ray flux below 1000 MeV/nucleon.