Compton Polarimetry in Gamma-Ray Astronomy

The analysis of compact astronomical objects has generally dealt with the physical properties of the source within a two-parameter space, which is defined by the spectral characteristics and time variability. This approach often leads to the situation whereby two or more very different models can explain the observations successfully. Polarimetric observations have the diagnostic potential to discriminate between the different compact source models and can offer a unique insight into the geometrical nature of the emission zones. To date, however, no polarization observation in the gamma-ray energy domain has been successfully performed, due to the difficulties in making polarimetric measurements in this high-energy region of the spectrum. In this paper the polarized gamma-ray emission mechanisms are reviewed with the emphasis on their detectable characteristics. Potential astronomical sites in which these emission mechanisms may be at work are discussed. Observational results obtained in other wavebands and theoretical predications made for some of the most likely astronomical sources of polarization are reviewed. Compton polarimetry has long been used in the field of nuclear gamma-ray spectroscopy in the laboratory. The operational principle behind all generations of nuclear gamma-ray polarimeters has been to measure the asymmetry in the azimuthal distribution of the scattered photons. However none of the polarimeters designed for laboratory experiments will be sensitive enough to observe even the strongest astronomical source. In the past few years there have been a number of innovative developments aimed at the construction of astronomical gamma-ray polarimeters, either as dedicated experiments or in missions with polarimetric capability. The designs of all the polarimeters are based on either discrete or continuous position sensitive detector planes. In this paper the data analysis techniques associated with this type of polarimeter are discussed as well as methods of removing some of the systematic effects introduced by a non-ideal detector response function and observation conditions. Laboratory tests of these new polarimetric techniques are reviewed. They demonstrate the feasibility of building a suitably sensitive astronomical gamma-ray polarimeter. Optimization of the design of pixellated detector array based polarimeters is also addressed. The INTEGRAL mission, which is to be launched by ESA in the year 2001, is the most likely telescope to perform the first successful gamma-ray polarization observation. The polarimetric characteristics of the two main instruments on board INTEGRAL are evaluated and their sensitivities to a wide range of potentially polarized gamma-ray sources are estimated.     

Determining the Absolute Abundances of Natural Radioactive Elements on the Lunar Surface by the Kaguya Gamma-ray Spectrometer

The Kaguya gamma-ray spectrometer (KGRS) has great potential to precisely determine the absolute abundances of natural radioactive elements K, Th and U on the lunar surface because of its excellent spectroscopic performance. In order to achieve the best performance of the KGRS, it is important to know the spatial response function (SRF) that describes the directional sensitivity of the KGRS. The SRF is derived by a series of Monte Carlo simulations of gamma-ray transport in the sensor of the KGRS using the full-fledged simulation model of the KGRS, and is studied in detail. In this paper, the method for deriving absolute abundance of natural radioactive elements based on the SRF is described for the analysis of KGRS data, which is also applicable to any gamma-ray remote sensings. In the preliminary analysis of KGRS data, we determined the absolute abundances of K and Th on the lunar surface without using any previous knowledge of chemical information gained from Apollo samples, lunar meteorites and/or previous lunar remote sensings. The results are compared with the previous measurements and the difference and the correspondence are discussed. Future detailed analysis of KGRS data will provide new and more precise maps of K, Th and U on the lunar surface.     

The GRASP telescope

The GRASP mission Gamma-Ray Astronomy with Spectroscopy and Positioning addresses the scientific goals of fine spectroscopy with imaging and accurate positioning of gamma-ray sources, an unexplored area within gamma-ray astronomy. The assessment of GRASP as a future space astronomy mission in the mid-1990s has led to the design of the instrument outlined in this article. Thus GRASP is a third generation gamma-ray telescope and is designed to operate as a high quality spectral imager in the mid-1990s, when, following the GRO, SIGMA, and GAMMA-1 missions, there will be requirement for a more sophisticated instrument to maintain the momentum of advance in gamma-ray astronomy. The telescope will be capable of locating point sources with a precision of typically 1 arc min, whilst making a fine spectral analysis (E/E 1000) of any gamma-ray line features. The high sensitivity of this instrument and the long (> 2 year) lifetime of the mission will enable a large number ( 1000) of astronomical objects to be studied. The GRASP mission has the potential to move gamma-ray astronomy from an era of basic exploration to one in which detailed and novel measurements can be used to gain a better understanding of many astrophysical problems.     

Background in space-borne low-energy γ-ray telescopes

The scope of observational astronomy in the gamma-ray region of the spectrum is vast. The intimate relationship of these energetic photons with their parent particles and fields provides a direct probe of the high-energy physics phenomena which take place throughout the Universe. As an added bonus the gamma-ray domain contains a wealth of diagnostic information within discrete emission lines, which are derived from a variety of processes including nuclear de-excitation, cyclotron emission, and matter-antimatter annihilation. Consequently observational gamma-ray astronomy addresses directly some of the most fundamental problems in both physics and astrophysics. However, low-energy gamma-rays are the most penetrating photons encountered in nature, and, whilst this factor provides a deep probe of cosmic objects, it ensures that gamma-ray telescopes are massive, both in terms of the stopping power required in the detector systems as well as their shields. Furthermore, the intimate relationship of gamma-rays with nuclear de-excitations ensures that the telescope itself becomes a bright source of background noise, a factor which is aggravated by the necessity that gamma-ray telescopes are obliged to operate in regions pervaded by intense particle fluxes. The background noise experienced in gamma-ray telescopes is, therefore, both high and extremely complex in its origin, and due to the high-energy content of individual photons, their numbers which arrive from distant cosmic sources are necessarily low, even for those objects which radiate the bulk of their power at gamma-ray wavelengths. Current gamma-ray telescopes are thus obliged to operate under conditions of intrinsically low signal-to-noise ratio and it is vital that techniques are developed which reduce the background noise level to more acceptable levels, thus improving the sensitivity. To achieve such a goal, a thorough understanding of the sources of background noise is first required before effective measures can be taken for its reduction.In this paper the sources of background noise are reviewed with the aim to obtain a quantitative analysis of individual contributions, as derived from the various classes of irradiative particle fluxes. The estimated contributions from the individual sources are combined in order to evaluate the total background level of a given telescope in a specific radiation environment, which for practical considerations generally relates to the orbit choice and detailed design of the telescope. The published background noise spectra of a number of past missions are compared to the computed values so as to provide an assessment of the validity of the overall calculations. The level of agreement achieved indicates that a good understanding of the sources of background noise exists. Finally some possibilities for the improvement of the sensitivity of future gammaray telescopes, in terms of the reduction of the background noise, are discussed.     

Impacts of solar and auroral storms on power line systems

A sensitive search for pulsars inside a sample of gamma-ray source error boxes has been carried out using the Arecibo and Parkes radiotelescopes. The paper describes the motivation of this search and the characteristics of the experiments used. As a preliminary result, new pulsars have been discovered and some of them are possibly candidates to be the counterparts of the gamma-ray sources.     

Observational studies of gamma-rays and neutrons from solar flares

Gamma-ray observations from HINOTORI satellite and possible neutron observations from the Tokyo neutron monitor are reviewed. Time histories of gamma-ray and X-ray emissions for both typical impulsive and gradual flares are discussed in connection with the particle acceleration time. The gamma-ray spectral hardening observed around 400 keV is explained from superimposition of two different electron bremsstrahlung spectra. Proton-energy spectra derived from the gamma-ray observations are compared with the solar energetic particle spectra in interplanetary space. The weak correlation between the gamma-ray fluence and the proton flux is discussed in connection with the particle trapping and escaping in the flare region. The limb darkening of the 2.22 MeV line resulting from neutron-proton capture is interpreted in terms of the attenuation by the Compton scattering in the photosphere. Possible solar neutron events recorded by the Tokyo neutron monitor are presented and the correlation between the gamma-ray fluence and the neutron fluence are described.     

Comparison of far-infrared radiation with gamma radiation

The gamma-ray emissivity for the narrow component (FWHM = 2°) at the 0.3–5 GeV range is derived as a function of the galactocentric distance. The narrow component might result from the interaction between cosmic rays and H₂ gas. The mass of gas in the Galactic Center is not large enough to produce the gamma-ray peak, but enough to produce the far-infrared peak. The relation of far-infrared dip and near-infrared hump near l = 356° to gamma-ray hump is discussed.     

High-Energy Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related Phenomena

It is now well established that both thunderclouds and lightning routinely emit x-rays and gamma-rays. These emissions appear over wide timescales, ranging from sub-microsecond bursts of x-rays associated with lightning leaders, to sub-millisecond bursts of gamma-rays seen in space called terrestrial gamma-ray flashes, to minute long glows from thunderclouds seen on the ground and in or near the cloud by aircraft and balloons. In particular, terrestrial gamma-ray flashes (TGFs), which are thought to be emitted by thunderclouds, are so bright that they sometimes saturate detectors on spacecraft hundreds of kilometers away. These TGFs also generate energetic secondary electrons and positrons that are detected by spacecraft in the inner magnetosphere. It is generally believed that these x-ray and gamma-ray emissions are generated, via bremsstrahlung, by energetic runaway electrons that are accelerated by electric fields in the atmosphere. In this paper, we review this newly emerging field of High-Energy Atmospheric Physics, including the production of runaway electrons, the production and propagation of energetic radiation, and the effects of both on atmospheric electrodynamics.     

Large-scale structure of the Galaxy and high-energy gamma-ray observations

Detailed information on the high-energy gamma-ray emission from our Galaxy has become available through the two dedicated satellite missions SAS-2 and COS-B. The consistency of the two datasets is discussed; while a satisfying general agreement is observed, a few distinct discrepancies point to possible time variations within the compact source component of the total galactic emission. The bulk of emission appears very well correlated to the column density of the total interstellar gas, as traced by radio observations of Hi and CO. The gamma-ray observations exclude the possibility that H₂ dominates in the inner Galaxy, its mass should not exceed the mass existing in the form of Hi. Neither a significant galactocentric gradient of the (high-energy) cosmic-ray flux density is suggested inside the solar circle (outside a decrease is needed), nor a linear coupling between the cosmic rays and the gas is indicated by the gamma-ray data. The systematic variation with longitude of the spectrum of the gamma-ray emission points to an increased flux of cosmic-ray electrons in the 100 MeV to 1 GeV energy range in regions where dense clouds are concentrated. The variation could as well be due to the largely unresolved population of compact gamma-ray objects.     

γ测井仪探管角响应对测量结果的影响

介绍了γ测井仪探管角响应的概念、检定方法,分析了引起角响应的原因及其对测量结果不确定度的贡献.     

Cosmic gamma-ray bursts

The observational characteristics of gamma-ray bursts are reviewed, concerning their spectra as well as their temporal structure and spatial distribution. From this data, it is suggested that the sources belong to a thick halo population (scale height > 3 kpc), and that the mean recurrence time for one source is greater than 5 yr. The implications of these results are discussed, concerning the future experimental perspectives of detection of gamma-ray bursts, and also the constraints on theoretical models.     

Nuclear processes and accelerated particles in solar flares

Nuclear processes and particle acceleration in solar flares are reviewed. The theory of gamma-ray and neutron production is discussed and results of calculations are compared to gamma-ray, neutron, and charged-particle observations from solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies, electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site, are presented. The information on elemental and isotopic abundances derived from gamma-ray observations is compared to abundances obtained from escaping accelerated particles and other sources.NAS/NRC Resident Research Associate.     

Some problems of very high-energy gamma-ray astronomy

The latest achievements in very high-energy (VHE) gamma-ray astronomy are discussed. Types of candidate objects for the sources of very high-energy gamma-quanta are considered, and pulsars, as the most probable ones, are anticipated. The objectives of VHE gamma-ray astronomy are presented, outlining the pressing need for complex observations of individual objects.     

Some problems of gamma-astronomy

The modern state of gamma-ray astronomy is reviewed, the paper being mainly devoted to the theoretical models that describe generation of gamma-ray emission under astrophysical conditions. Basic information on the processes of generation and absorption of gamma-rays, as well as the results of observations for various gamma-ray photon energies are reported.In the region of soft gamma-ray emission (i.e., for energies less than tens of MeV), where emission in gamma-ray lines dominates, we also discuss the nature of gamma-ray bursts, the origin of gamma-ray emission from the galactic centre, etc.Discrete sources and, in particular, the mysterious source Cyg X-3 are discussed in the region of very high (E > 10¹² eV) and ultra-high (E > 10¹⁵ eV) energy gamma-ray emission.A larger portion of the review is devoted to the analysis of cosmic-ray origin on the basis of the available gamma-ray data in the region from several tens of MeV to several GeV. The peculiarity of this energy range is, in particular, in the fact that the diffuse galactic emission was observed mainly there. We also discuss the problem of determination of the cosmic-ray density gradient from the gamma-ray data.The origin of high-latitude gamma-ray emission, the problem of galactic gamma-ray halo, etc., are discussed.The theoretical models explaining the nature of unidentified gamma-ray sources, as well as the results of measurements and theoretical estimations of a gamma-ray flux from SN1987A are analysed.List of Notations m electron mass, m = 9.108 × 10^–28 g, - M proton mass, M = 1.672 × 10^–24 g, - e electron charge, e = 4.803 × 10^–10 CGS - c velocity of light, c = 2.9979 × 10¹⁰ cm s^–1, - k Boltzmann constant, k = 1.380 × 10^–16 erg grad^–1, - e electron - p proton - gamma-ray photon - p antiproton - ^{0 0}-meson - -lepton - e ⁺ positron - r, , x radio-frequency, gamma-ray, and X-ray emission bands - E total energy of a particle - E _k kinetic energy - p particle momentum - spectral index for particles - spectral index for emission - n particle density (concentration) - H magnetic field strength - T temperature - _ph energy of low-energy photons - emission frequency - r _H Larmor radius of relativistic particles - k wave number - , z cylindric coordinates, in this case the coordinate (radius) along the galactic disk, z perpendicular to the galactic disk - M solar mass, M = 1.99 × 10³³ g.     

Solar flares as sources of energetic particles

In this paper a review is presented of the present status of our knowledge of solar flare phenomena with special emphasis on the production of suprathermal particles and their solar effects. Of these energetic particles electrons play an important role since they produce the X-ray and radiobursts observed during many flares. Also, during their slowing down to thermal energies they contribute to the heating of localized regions in the solar atmosphere, through energy exchange with the ambient electrons. Observable radiations of energetic protons, and other nuclei, are produced through nuclear interactions leading to the emissions of gamma-ray lines. Detectable fluxes of these gamma-ray lines are produced only in the most powerful flares. Also the nuclei that enter into deeper layers of the solar atmosphere transfer most of their kinetic energy to the ambient plasma.     

INTEGRAL: Science Highlights and Future Prospects

ESA??s hard X-ray and soft gamma-ray observatory INTEGRAL is covering the 3 keV to 10 MeV energy band, with excellent sensitivity during long and uninterrupted observations of a large field of view (??100 square degrees), with ms time resolution and keV energy resolution. It links the energy band of pointed soft X-ray missions such as XMM-Newton with that of high-energy gamma-ray space missions such as Fermi and ground based TeV observatories. Key results obtained so far include the first sky map in the light of the 511 keV annihilation emission, the discovery of a new class of high mass X-ray binaries and detection of polarization in cosmic high energy radiation. For the foreseeable future, INTEGRAL will remain the only observatory allowing the study of nucleosynthesis in our Galaxy, including the long overdue next nearby supernova, through high-resolution gamma-ray line spectroscopy. Science results to date and expected for the coming mission years span a wide range of high-energy astrophysics, including studies of the distribution of positrons in the Galaxy; reflection of gamma-rays off clouds in the interstellar medium near the Galactic Centre; studies of black holes and neutron stars particularly in high- mass systems; gamma-ray polarization measurements for X-ray binaries and gamma-ray bursts, and sensitive detection capabilities for obscured active galaxies with more than 1000 expected to be found until 2014. This paper summarizes scientific highlights obtained since INTEGRAL??s launch in 2002, and outlines prospects for the INTEGRAL mission.     

用大面积源检定核应急机载/车载伽玛能谱仪

人工核素大面积源标准是核工业航测遥感中心等单位经过十余年研究建立的新型计量保障标准,使用该标准,可对监测核应急伽玛辐射环境的机载/车载伽玛能谱仪进行检定.使用大面积源标准检定,可以有效地弥补因机载/车载伽玛能谱仪系统的角响应影响因素复杂而难于模拟计算的缺陷,为监测核应急伽玛辐射环境的机载/车载伽玛能谱仪提供了量值传递的保障.本文介绍了大面积源检定的原理和技术方法.     

The use of gamma rays to trace the local interstellar hydrogen

COS-B data in the latitude range 11° <¦b¦ < 19° show the gamma-ray intensity to be closely correlated with the total line-of-sight absorption derived from galaxy counts. It is found that to a good approximation the gamma-ray intensity is proportional to the total gas column density, and on this basis a map of the angular distribution of local molecular hydrogen at intermediate latitudes is presented. Comparing this with a simular map produced from galaxy counts, several structures appear in both maps in regions related to Gould's Belt and elsewhere. Some regions in the Southern celestial hemisphere not accessible in galaxy counts show high molecular hydrogen column densities.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.     

GLAST

A proposal for a new high energy gamma-ray space based detector using silicon strip technology is discussed. The detector is called GLAST for Gamma-ray Large Area Space Telescope. Results from Monte Carlo simulations are presented. The projected physics capabilities of GLAST are discussed in comparison to EGRET, currently in orbit on CGRO. Connections to the ground based gamma-ray observation program are explored.