Solar gamma rays

The theory of gamma-ray production in solar flares is treated in detail. Both lines and continuum are produced. The strongest line predicted at 2.225 MeV with a width of less than 100 eV and detected at 2.24±0.02 MeV, is due to neutron capture by protons in the photosphere. Its intensity is dependent on the photospheric ³He abundance. The neutrons are produced in nuclear reactions of flare accelerated particles which also produce positrons and prompt nuclear deexcitation lines. The strongest prompt lines are at 4.43 MeV from ¹²C and at 6.2 from ¹⁶O and ¹⁵N. These lines result from both direct excitation and spallation. The widths of individual prompt lines are determined by nuclear kinematics. The width of the 4.43 MeV line is 100 keV and that of the 6.2 MeV feature is 300 keV. Both these lines have been observed from a solar flare. Other potentially observable lines are predicted at 0.845 and 1.24 MeV from ⁵⁶Fe, at 1.63 MeV principally from ¹⁴N and ²⁰Ne, at 1.78 MeV from ²⁸Si, at 5.3 MeV from ¹⁵O and ¹⁵N, and at 7.12 MeV from ¹⁶O. The widths of the iron lines are only a few keV, while those of the other lines are about 100 keV. The only other observed line is at 0.511 MeV from positron annihilation. The width of this line is determined by the temperature, and its temporal variation depends on the density of the ambient medium in the annihilation region. Positrons can also annihilate from the ³ S state of positronium to produce a 3-photon continuum below 0.511 MeV. In addition, the lines of ⁷Li and ⁷Be at 0.478 keV and 0.431 keV, which have kinematical widths of 30 keV, blend into a strong feature just below the 0.511 MeV line.From the comparison of the observed and calculated intensities of the line at 4.4 MeV to that of the 2.2 MeV line it is possible to obtain information on the spectrum of accelerated nuclei in flares. Moreover, from the absolute intensities of these lines the total number of accelerated nuclei at the Sun and their heating of the flare region can be estimated. We find that about 10³³ protons of energies greater than 30 MeV were produced in the 1972, August 4 flare.The gamma-ray continuum, produced by electron bremsstrahlung, allows the determination of the spectrum and number of accelerated electrons in the MeV region. From the comparison of the line and continuum intensities we find a proton-to-electron ratio of about 10 to 10² at the same energy for the 1972, August 4 flare. For the same flare the protons above 2.5 MeV which are responsible for the gamma-ray emission produce a few percent of the heat generated by the electrons which make the hard X-rays above 20 keV.NAS-NRC Resident Research Associate.Research supported by the National Science Foundation under Grant GP 31620.     

Overview of TeV gamma ray observations

TeV emission has been observed with high signal-to-noise from: two pulsar-driven supernova remnants, the Crab Nebula and PSR B1706-44 and a blazar class AGN, Markarian 421. Other sources, observed without the benefit of major background reduction techniques, are some of the compact object binaries. These discoveries plus the discoveries of EGRET of more than 130 sources at lower energies point to a growing number of scientific questions capable of being addressed by this field (particularly for distant sources via intergalactic absorption processes). Rapid further development will come as new instruments employing a variety of background reduction techniques come on-line.     

Target prescreening based on a quadratic gamma discriminator

This work presents the development, analysis and validation of a new target discrimination module for synthetic aperture radar (SAR) imagery based on an extension of gamma functions to 2-D. Using the two parameter constant false-alarm rate (CFAR) stencil as a prototype, a new stencil based on 2-D gamma functions is used to estimate the intensity of the pixel under test and its surroundings. A quadratic discriminant function is created from these estimates, which is optimally adapted with least squares in a training set of representative clutter and target chips. This discriminator is called the quadratic gamma discriminator (QGD). The combination of the CFAR and the QGD was tested in realistic SAR environments and the results show a large improvement of the false alarm rate with respect to the two-parameter CFAR, both with high resolution (1 ft) fully polarimetric SAR and with one polarization, 1 m SAR data     

Topological defects and highest energy cosmic and gamma rays

In this paper we review the hypothesis that a substantial part of the cosmic ray flux observed above about 10¹⁹ eV may be produced by decaying or annihilating topological defects left over from phase transitions in the early universe at grand unification energy scales ( 10¹⁶ GeV). Possible signatures of cosmic ray producing defect models are discussed which could be tested experimentally in the near future. We thereby focus on model independent universal spectral properties of the predicted particle fluxes.     

Present status of very high energy gamma ray observations

Recent results from observations of the southern sky objects are summarized. The unpulsed, persistent very high energy (VHE) emission from the gamma ray pulsars, the Crab and PSR1706-44, is discussed. A process of energetic electrons ejection may take place from a variety of other objects such as from X-ray binaries, similarly to the pulsars. Such an effect may be seen also in pair halos around extragalactic VHE gamma ray emitters, the observational study of which is still in a preliminary stage in the southern hemisphere.     

Cyg X-3 not seen at high-energy gamma rays

The ESA satellite COS-B viewed the Cyg-X region 7 times between November 1975 and March 1982. A search for periodic gamma-ray emission (E > 70 MeV) from Cyg X-3 at the characteristic 4.8 h period did not reveal the source. Combining all observations, the 2 upperlimit (E > 70 MeV) on the flux for the phase interval in which X-ray emission has been detected is 1.0 × 10^-6 ph cm^-2 s^-1 and for the phase intervals in which ultra-high-energy (E 500 GeV) gamma-ray emission has been reported 1.0 × 10^-7 ph cm^-2 s^-1. This is about one and two orders of magnitude, repectively, below the flux reported earlier by the SAS-2 team. A comparison of the spatial gamma-ray distribution in the Cyg-X region measured by SAS-2 and COS-B with the total-interstellar-gas distribution leads to the conclusion that in both cases, COS-B and SAS-2, no source has been detected at the position of Cyg X-3 in addition to the diffuse gamma-ray emission expected from the total-gas distribution.The Caravane Collaboration for the COS-B satellite: Laboratory for Space Research Leiden, Leiden, The Netherlands Istituto di Fisica Cosmica del CNR, Milano, Italy Istituto di Fisica Cosmica e Informatica del CNR, Palermo, Italy Max Planck Institut für Physik und Astrophysik, Institut für Extraterrestrische Physik, Garching-bei-München, Germany Service d'Astrophysique, Centre d'Etudes Nucléaires de Saclay, France Space Science Department of the European Space Agency, ESTEC, Noordwijk, The Netherlands.     

镧掺杂锶铁氧体/聚苯胺纳米复合材料的制备及电磁性能

为了比较不同形态的镧掺杂纳米锶铁氧体复合材料电磁性能的优劣,分别以溶胶-凝胶法制备了稀土镧掺杂纳米锶铁氧体( SrLa0.4Fe11.6O19)颗粒、原位复合技术制备了镧掺杂锶铁氧体/聚苯胺(SrLa0.4Fe11.6 O19/PANI)纳米复合颗粒,静电自组装法制备了镧掺杂锶铁氧体/聚苯乙烯磺酸钠( SrLa0.4F...     

Pulsars as gamma ray sources: Nebular shocks and magnetospheric gaps

I summarize the results of recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is primarily electrons and positrons with an admixture of heavy ions. Shocks which contain heavy ions that are a minority constituent by number but which carry most of the energy density in the upstream medium put 20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) E ^-2, where N(E)dE is the number of particles with energy between E and E+dE. Synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front, provides the mechanism of thermalization and non-thermal particle acceleration. The maximum energy achievable by the pairs is _± m _± c ² = m _i c ² ₁/Z _i, where ₁ is the Lorentz factor of the upstream flow and Z _i is the atomic number of the ions. The shock's spatial structure contains a series of overshoots in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value. These overshoots provide a new interpretation of the structure of the inner regions of the Crab Nebula, in particular of the wisps, surface brightness enhancements near the pulsar. The wisps appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar's wind in the Crab Nebula is spatially resolved, and allows one to measure ₁ 4 × 10⁶, the upstream magnetic field B ₁ to be 3 × 10^-5 Gauss, as well as to show that the total ion flow is 3 × 10³⁴ elementary charges/sec, in good agreement with the total current flow predicted by the early Goldreich and Julian (1969) model. The total pair outflow is shown to be about 5 × 10³⁷ pairs per second, in good agreement with the particle flux required to explain the nebular X—ray source.The energetics of particle acceleration within the magnetospheres of rotation powered pulsars and the consequences for pulsed gamma ray emission are also briefly discussed. The gamma ray luminosity above 100 MeV is shown to scale in proportion to _R ^1/2 , as is in accord with some of the simplest ideas about polar cap models. Models based on acceleration in the outer magnetosphere are also briefly discussed.