Biography:

In the past V. Schönfelder has collaborated on articles with A. Connors and G. Weidenspointner. One of their most recent publications is Low and medium energy gamma-ray astronomy — present status and future aspects. Which was published in journal Advances in Space Research.

More information about V. Schönfelder research including statistics on their citations can be found on their Copernicus Academic profile page.

V. Schönfelder's Articles: (6)

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

AbstractDuring 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

AbstractThe 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.

Invited lectureAn overview about initial results from the Compton observatory GRO

AbstractGamma-ray astronomy provides a diagnostic tool to study high energy processes in the Universe. At present, NASA's Compton Observatory is performing the first complete sky survey in gamma-ray astronomy. The observatory was launched on April 5, 1991 by the Space Shuttle Atlantis into a near-earth orbit. An overview about the first highlight results from this mission is given. Special topics of presentation are: pulsars (especially the Crab and Vela pulsars), the diffuse galactic gamma-ray emission from interstellar space, the nuclei of active galaxies (especially quasars), the 511 keV and 1.8 MeV gamma-ray line emissions from the central region of the galaxy, the puzzle of cosmic gamma-ray bursts and finally, the Sun during solar flare activities.

The local neutron flux at low earth-orbiting altitudes

AbstractThe 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.

Comptel measurements of the omnidirectional high-energy neutron flux in near-earth orbit

AbstractOn four occasions, twice in 1991 (near solar maximum) and twice in 1994 (near solar minimum), one COMPTEL D1 detector module was used as an omnidirectional detector to measure the high-energy (>12.8 MeV) neutron flux near an altitude of 450 km. The Dl modules are cylindrical, with radius 13.8 cm and depth 8 cm, and are filled with liquid scintillator (NE213A). The combined flux measurements can be fit reasonably well by a product of the Mt. Washington neutron monitor rate, a linear function in the spacecraft geocenter zenith angle, and an exponential function of the vertical geomagnetic cutoff rigidity in which the coefficient of the rigidity is a linear function of the neutron monitor rate. When pointed at the nadir, the flux is consistent with that expected from the atmospheric neutron albedo alone. When pointed at the zenith the flux is reduced by a factor of about 0.54. Thus the production of secondary neutrons in the massive (16000 kg) Compton Gamma-Ray Observatory spacecraft is negligible. Rather, the mass of the spacecraft provides shielding from the earth albedo.

High-energy measurements of the 1991 november 15 solar flare

AbstractWe report λ-ray observations of the 1991 November 15 solar flare. Although the event was not as well measured as some with the Compton Gamma Ray Observatory (CGRO) because the axis of the Compton spacecraft was far from the solar direction, the data are comprehensive and span a wide range of energies with good sensitivity and resolution. Of particular interest are the observations that the energetic proton spectrum was harder during the impulsive phase, the transport of protons in the corona was diffusive rather than adiabatic, the abundance of 3He may be higher than is normally believed and there is not enough energy in the energetic electrons to power the white light emission, although proton heating is still possible.

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