COMPTEL observations of AGNs

During the first part of the COMPTON Gamma Ray Observatory sky survey, COMPTEL has detected the quasars 3C273 and 3C279 and the radio galaxy Centaurus A. This paper summarizes the preliminary findings and gives an upper limit on the MeV flux of the Seyfert galaxy NGC4151.     

COMPTEL results on the 1.809 MeV gamma-ray line from the Galactic-center region

The COMPTEL experiment on the Compton Gamma-Ray Observatory is designed to image celestial gamma radiation in the energy range from 0.75–30 MeV within a field of view of 1 steradian. It can locate stronger point sources with an accuracy better than 0.5° and is capable of mapping diffuse emission as well. The Galactic-center region was observed by COMPTEL for several 2-week periods in 1991/1992. These observations show evidence for 1.8 MeV line emission along the Galactic disk (attributed to radioactive ²⁶Al), extending over at least 40 degrees in longitude.     

Energy content of accelerated electrons and ions in intense solar flares

The energy content of nonthermal particles in solar flares is shared between accelerated electrons and ions. It isimportant for understanding the particle acceleration mechanism in solar flares. Yohkoh observed a few intense flares which produced both strong gamma-ray lines and electron bremsstrahlung continuum. We analyze energy spectra of X-class solar flares on October 27, 1991(X6.1), November 6, 1997 (X9.4), July 14, 2000 (X5.7) and November 24, 2000 (X2.3). The accelerated electron and proton spectra are derived from a spectral analysis of their high-energy photon emission and the energy contents in >1 MeV electrons and >10 MeV protons are estimated to be 6×l0²⁸ – 4×10³⁰ and 2×10²⁸ – 5×10²⁹ erg, respectively. We study the flare to flare variation in the energy content of >1 MeV electrons and >10 MeV protons for the four Yohkoh gamma-ray flares. Ratios of >1 MeV electron energy content to >10 MeV proton energy content are roughly within an order of magnitude.     

Sigma: A new gamma-ray space observatory with high angular resolution for 1985 and beyond

The SIGMA mission is designed to obtain images of the sky in the hard X-ray/low energy gamma-ray domain 30 keV - 2 MeV, with an angular accuracy a few arc min., and a sensitivity for point sources down to a level of a few UFU. The principal scientific objectives of the SIGMA mission are described, as well as the anticipated performance of the instrument.     

X-ray and gamma-ray observations of a white-light flare

HEAO-1 observed hard radiations (X- and gamma-rays) from a major solar flare on 11 July 1978. The observations showed gamma-ray line and continuum emission extending to the highest energy observed. The lines are identified with the 2.2 MeV line of deuterium formation and the 4.4 MeV line of inelastic scattering on ¹²C, both previously observed in the flares of August 1972 [1]. The 11 July flare was identified as a white-light flare by observations at Debrecen [2]. It thus provides the first opportunity for a detailed examination of white-light flare theories that depend upon proton heating of the photosphere. The line strength over a four-minute integration at 2.2 MeV was 1.00 ± 0.29 ph(cm² sec)⁻¹, and the gamma-ray emission (excluding the 2.2 MeV line which was appreciably delayed) lagged by less than 20 sec approximately after the hard X-ray and microwave fluxes. We conclude that the “second-stage” acceleration of high-energy solar particles must commence promptly after the impulsive phase.     

Pulsar studies with GRO-COMPTEL

Pulsar measurements performed by the experiment COMPTEL, aboard the Compton Gamma Ray Observatory, are described. The main results refer to the Crab and Vela pulsars whose pulse shape characteristics are given in some detail and light curves are compared with those above 50 MeV, as observed by the COS-B satellite. No other gamma-ray pulsars have been detected to date by COMPTEL, the upper limit on the pulsed signal from Geminga being compatible with indications by other experiments.     

A model of solar energetic particles for use in calculating LET spectra developed from ONR-604 data.

A model of solar energetic particles (SEP) has been developed and is applied to solar flares during the 1990/1991 CRRES mission using data measured by the University of Chicago instrument, ONR-604. The model includes the time-dependent behavior, heavy-ion content, energy spectrum and fluence, and can accurately represent the observed SEP events in the energy range between 40 to 500 MeV/nucleon. Results are presented for the March and June, 1991 flare periods.     

High precision gamma-ray spectrometer PGS for Russian interplanetary mission to Mars

The high precision gamma-ray spectrometer (PGS) is scheduled to be launched on the Russian MARS mission in 1996, and to go into an elliptical polar orbit around Mars. The PGS consists of two high-purity germanium detectors, associated electronics, and a passive cooler and will be deployed from one of the solar panels. The PGS will measure nuclear gamma-ray emissions from the Martian surface, cosmic gamma-ray bursts, and the high-energy component of solar flares in the broad energy range from 50 keV to 8 MeV in 4096 energy channels. The first results are presented of development, integration and qualification of the instrument, both for the passive cooler and for the detector with spectrometric electronics.     

The low energy gamma-ray experiments on GRO in perspective

The NASA Gamma-Ray Observatory, GRO, will carry two instruments for low energy gamma-ray astronomy. The ‘Oriented Scintillation Spectrometer Experiment - OSSE’ represents the latest step in the evolution of collimated detectors. A large detection area, simultaneous source and background observation and rigorous control over systematic errors yield significant improvements in sensitivity over earlier instruments. The ‘Imaging Compton Telescope - COMPTEL’ brings the proven concept of the Compton telescope to the state of the art level. Position sensitive scintillation detectors make it possible to generate sky images with a resolution of about 2° over a f.o.v. of about 1 sr. The complementary nature of these two experiments promises a first in-depth exploration of the sky in a wavelength range which covers the transition from the X-ray sky to the apparently unrelated high energy gamma-ray sky. Possible directions of further evolution of these experiments will be discussed.     

Low and medium energy gamma-ray astronomy — present status and future aspects

During the last few years quite some progress has been achieved in the field of low and medium energy gamma-ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti-center region have been detected, which require a high interstellar electron flux in the 100 MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebula are still the only galactic gamma-ray sources which definitely have been detected, some recently determined upper limits to the gamma-ray fluxes of other radio pulsars are close to the theoretically expected values. Active galaxies seem to have a maximum of luminosity in the range between several 100 keV and a few MeV and, therefore, are of special interest. First observational results have been reported on the Seyfert galaxies NGC 4151 and MCG 8-11-11, and the radio galaxy CenA. The nature of the diffuse cosmic gamma-ray component at low gamma-ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin.Considering the present status of gamma ray astronomy the study of galactic sources like radio pulsars and the unidentified high energy gamma-ray sources, the Milky Way as a whole, active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band observations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-ray astronomy, however, is always open for surprises.     

COMPTEL observations of gamma-ray bursts

The COMPTEL experiment on GRO images 0.7 – 30 MeV celestial gamma-radiation that falls within its 1 steradian field of view. During the first fifteen months in orbit, preliminary localizations from BATSE triggers indicated that about 1 in 6 cosmic events could have fallen within COMPTEL's field of view. We summarize work on the brightest of these gamma-ray bursts and present new position constraints for GRB 911118 and GRB 920622.     

Multi-component synchrotron self-compton emission from NGC 1275

COS-B gamma-ray data (70–5000 MeV) in the latitude range 10°< |b| <90° are compared with the expected emission from cosmic-ray interactions with interstellar gas. An additional component is found to be necessary to explain the latitude dependence of the emission. Two possible origins for this component are discussed: a gamma-ray halo around the Galaxy and a local emission region.     

Results of solar observations by the CORONAS-F payload

The CORONAS-F mission experiments and results have been reviewed. The observations with the DIFOS multi-channel photometer in a broad spectral range from 350 to 1500 nm have revealed the dependence of the relative amplitudes of p-modes of the global solar oscillations on the wavelength that agrees perfectly well with the earlier data obtained in a narrower spectral ranges. The SPIRIT EUV observations have enabled the study of various manifestations of solar activity and high-temperature events on the Sun. The data from the X-ray spectrometer RESIK, gamma spectrometer HELICON, flare spectrometer IRIS, amplitude–temporal spectrometer AVS-F, and X-ray spectrometer RPS-1 have been used to analyze the X- and gamma-ray emission from solar flares and for diagnostics of the flaring plasma. The absolute and relative content of various elements (such as potassium, argon, and sulfur) of solar plasma in flares has been determined for the first time with the X-ray spectrometer RESIK. The Solar Cosmic Ray Complex monitored the solar flare effects in the Earth’s environment. The UV emission variations recorded during solar flares in the vicinity of the 120-nm wavelength have been analyzed and the amplitude of relative variations has been determined.     

Solar-flare neutrons and gamma rays

Calculations of neutron and gamma-ray production in solar flares are reviewed and compared with neutron and gamma-ray data from the 21 June 1980 and 3 June 1982 flares, as well as gamma-ray data from other flares. The implied charged-particle numbers and spectra are compared with interplanetary observations.     

Centaurus A: Multiwavelength observations of the nearest active galaxy from radio to gamma-rays

Centaurus A (Cen A, NGC 5128) is the nearest active galaxy and, notably, the viewing angle with respect to the jet axis is very large (> 70°). A first contemporaneous OSSE, COMPTEL, and EGRET spectrum obtained in October 1991 covers an energy range from 50 keV up to 1 GeV. This γ-ray broad-band spectrum was taken when Cen A was in an intermediate emission state as defined by the BATSE X-ray light-curve. The first simultaneous multiwavelength spectrum from radio to γ-rays was measured in July 1995 when Cen A was in a low emission state (the prevailing state for the last 7 years). The different spatial and temporal resolution in the different frequency regimes produces problems in the construction and interpretation of the multiwavelength spectra. These are addressed in this paper. The detection of emission > 1 MeV makes the inclusion of such high-energy emission into models for the spectral energy distribution mandatory.     

Energetic particles during the first two years of SOHO science mission

Energetic particle intensities observed by ERNE instrument onboard SOHO spacecraft during the first two years of SOHO science mission have been analysed and compared to observations of IMP 8 satellite around two earlier sunspot minima. During an eight month period around the latest sunspot minimum, which occurred in October 1996, energetic particle intensities stayed at a lower level than during any equivalent period around solar minima of July 1976 and June 1986. During the period from March to October, 1996, there was not a single day, when the daily averaged intensity of 1.6–3 MeV protons exceeded the level of 1 proton/(cm² sr s MeV). Also monthly counts of grouped solar flares and mean monthly sunspot number were lower than during minima of 1976 and 1986.     

The high precision gamma-ray spectrometer for lunar polar orbiter SELENE

The high precision gamma-ray spectrometer (GRS) is scheduled to be launched on the lunar polar orbiter of the SELENE mission in 2007. The GRS consists of a large Ge crystal as a main detector and massive bismuth germanate crystals as an anticoincidence detector. A Stirling cryocooler was adopted in cooling the Ge detector. The flight model of SELENE GRS has been completed and an energy resolution of 3.0 keV (FWHM) at 1.332 MeV has been achieved. The spectrometer aims to observe nuclear line gamma rays emitted from the lunar surface in a wide energy range from 100 keV to 12 MeV for one year and more to obtain chemical composition on the entire lunar surface. The gamma-ray data enable us to study lunar geoscience problems including crust and mantle composition, and volatile reservoirs at polar regions.     

What gamma-ray deexcitation lines reveal about solar-flares

Gamma-ray emission from solar flares reveals information about the nature of the accelerated particles and about the physical conditions of the medium through which the accelerated particles are transported. In this paper, we present the gamma-ray line-production and loop transport models used in our calculations of high-energy emission. We discuss the calculated interaction time history, the depth distribution, the interacting-particle angular distribution, and fluence ratios of the narrow gamma-ray lines. We show the relationship between the γ-ray observables and the parameters of the transport and line-production models. For illustration, we use calculations of 4.44 MeV ¹²C nuclear deexcitation line-production. Applications of the calculations to flare observations by both SMM and RHESSI are also presented.     

The radial distribution of gamma rays and cosmic rays in the outer Galaxy

The radial distribution of the high-energy (70 MeV-5 GeV) gamma-ray emissivity in the outer Milky Way is derived. The kinematics of HI are used to construct column-density maps in three galacto-centric distance ranges in the outer Galaxy. These maps are used in combination with COS-B gamma-ray data to determine gamma-ray emissivities in these distance ranges. A steep negative gradient of the emissivity for the 70 MeV-150 MeV energy range is found in the outer Galaxy. The emissivity for the 300 MeV-5 GeV range is found to be approximately constant (within 20%) and equal to the local value out to large (20 kpc) galacto-centric distances. These results imply a hardening of the gamma-ray spectrum with increrasing distance and for R > 16 kpc the spectrum is shown to be consistent with a π°-decay spectrum with the intensity expected from the local measurement of the cosmic-ray nuclei spectrum. The energy-dependent decrease is interpreted as a steep gradient in the cosmic-ray electron density and a near constancy of the nuclear component. The galactic origin of electrons with energies up to several hundreds of MeV is confirmed, while for cosmic-ray nuclei with energies of a few GeV either confinement in a large galactic halo or an extragalactic origin is suggested by the data.     

The local neutron flux at low earth-orbiting altitudes

The COMPTEL instrument onboard the Compton Gamma Ray Observatory (CGRO) has been used to measure the variation of the atmospheric neutron flux below 5 MeV as a function of vertical cutoff rigidity and spacecraft orientation at an altitude of 450 km. The instrumental 2.2 MeV background line, resulting from thermal neutron capture on hydrogen, was used for the measurement. The dependence of the 2.2 MeV rate on rigidity and geocentre zenith can be described by an analytic function: the line rate decreases linearly with geocentre zenith, and decreases exponentially with the vertical cutoff rigidity. The flux varies on average by about a factor of 3.7 between the extremes in rigidity, and by a factor of 1.7 between the extremes of spacecraft orientation with respect to the Earth. We believe that mass shielding is more important in attenuating the atmospheric albedo than as a source of secondary neutrons. The COMF'TEL instrument is well suited for a long-duration study of the dependence of the neutron flux on the vertical cutoff rigidity and the solar cycle.