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artist concept of Gamma

* Mission Overview

The Gamma observatory was a Soviet-based spacecraft launched on 11 July 1990. The satellite was in an orbit of ~ 375 km altitude, 51.6 degrees inclination. Gamma offered the possibility of locating COS-B sources with a precision of a few arcminutes. The mission lasted ~ 2 years.

* Instrumentation

The science instrumentation included the Gamma-1 telescope (with a Telezvezda star tracker), the Disk-M telescope, and the Pulsar X-2 telescope. All 3 instruments were mounted coaxially on the observatory, allowing for simultaneous observation of any given region of the celestial sphere.

The Gamma-1 telescope was the principle instrument aboard the Gamma observatory. It was designed to perform detailed studies in the energy range 50 MeV - 6 GeV. The system consisted of 2 scintillation counters and a gas Cerenkov counter. It had an effective area of ~2000 cm2. Its angular resolution at 100 MeV was 1.5 degrees. The imaging field of view was +/- 2.5 degrees. The energy resolution was 12% at 100 MeV. Rather late in the game, it was decided to include a tungsten coded aperture mask which could be moved in and out of the field of view. The mask itself was made of 2 1-D masks each having a unit cell size of 1 mm. The basic resolution achieved by the mask was ~20 arcminutes for the weakest sources (4-sigma). Shortly after launch, the power to the spark chamber failed. This resulted in the angular resolution for most of the mission being only ~ 10 degrees. The telescope was a joint Soviet-French endeavor.

The Telezvezda star tracker worked together with the Gamma-1 telescope. It had a 6 deg x 6 deg field of view, and a 5th stellar magnitude sensitivity. It had an angular resolution of 2 arcmin, making it possible to determine the Gamma-1 pointing direction to the same accuracy.

The Disk-M telescope was designed to measure fluxes in the range 20 keV - 5 MeV. The detector consisted of NaI scintillation crystals. The angular resolution of the telescope was ~ 25 arcmin. The Disk-M telescope stopped working not long after launch.

The Pulsar X-2 telescope covered 2-25 keV with ~ 30 arcmin resolution and a 10 deg x 10 deg field of view. The telescope was a joint Soviet-French endeavor.

* Science

Studies were made of the Vela pulsar, the galactic center region, the Cygnus binaries, the Heming gamma source in Taurus, and Her X-1. The observatory also gathered information about the high-energy emissions of the Sun during peak solar activity.

*Other information

  • Ramsden 1983, Adv Sp Res, 3,#4, pp.31-38.
  • Akimov et al. 1991, Astronomicheskiy Zhurnal, 17, #6, pp.501-504.
  • Polezhayev and Poluektov 1991, Zemlya I Vselennaya, #3, pp. 2-9.

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