COS-B views on the diffuse galactic gamma-ray emission and some point sources

The correlation between diffuse galactic gamma rays and gas tracers is studied using the final COS-B database and H i and CO surveys covering the entire galactic plane. A good quantitative fit to the gamma rays is obtained, with a small galacto-centric gradient in the gamma-ray emissivity per hydrogen atom. The average ratio of H₂ column density to integrated CO temperature is determined, the best estimate being (2.3 ± 0.3) × 10² molecules cm^–2 (K km s^–1)^–1. Strictly taken, this value is an upper limit. The corresponding mass of molecular hydrogen in the inner galaxy, derived using both 1st and 4th quadrants, is 1.0 × 10⁹ M .The softer gamma-ray spectrum towards the inner galaxy found in previous work can be attributed to a steeper emissivity gradient at low energies and/or to a softer gamma-ray spectrum of the emission distributed like molecular gas. A steeper emissivity gradient at low energies could be related to cosmic-ray spectral variations in the Galaxy, to different distributions of cosmic-ray electrons and nuclei, or to a contribution from discrete sources. A softer spectrum for the emission associated with molecular clouds may be physically related to the clouds themselves (i.e., cosmic-ray spectral variations) or to an associated discrete source distribution.New results on the temporal and spectral characteristics of the high-energy (50 MeV to 5 GeV) gammaray emission from the Vela pulsar are presented. The whole pulsed flux is found to exhibit long-term variability. Five discrete emission regions within the pulsar lightcurve have been identified, with the spectral characteristics and long-term behaviour being different. These characteristics differ significantly from those reported earlier for the Crab pulsar. However, geometrical pulsar models have been proposed (e.g., Morini, 1983; Smith, 1986) which could explain many of these features.     

Initial results from COMPTEL onboard GRO

COMPTEL is the first imaging telescope to explore the MeV gamma-ray range (0.7 to 30 MeV). At present, it is performing a complete sky survey. In later phases of the mission selected celestial objects will be studied in more detail. The data from the first year of the mission have demonstrated that COMPTEL performs very well. First sky maps of the inner part of the Galaxy clearly identify the plane as a bright MeV-source (probably due to discrete sources as well as diffuse radiation). The Crab and Vela pulsar lightcurves have been measured with unprecedented accuracy. The quasars 3C273 and 3C279 have been seen for the first time at MeV energies. Both quasars show a break in their energy spectra in the COMPTEL energy range. The 1.8 MeV line from radioactive ²⁶A1 has been detected from the central region of the Galaxy and a first sky map of the inner part of the Galaxy has been obtained in the light of this line. Upper limits to gamma-ray line emission at 847 keV and 1.238 MeV from SN 1991T have been derived. Upper limits to the interstellar gamma-ray emissivity have been determined at MeV-energies. Several cosmic gamma-ray bursts within the field-of-view have been located with an accuracy of about 1°. On 1991 June 9, 11 and 15, COMPTEL observed gamma-ray emission (continuum and line) from three solar flares. Also neutrons were detected from the June 9 and June 15 flares.     

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.     

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.     

Gamma-ray sources observed by COS-B

Space Science Reviews - The nature of the fine-scale structure in the gamma-ray distribution is not yet disclosed. Considerable debate is going on whether these structures which appear point-like...     

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.     

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.     

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.