Observations of nuclear reactors on satellites with a balloon-borne gamma-ray telescope.

Gamma rays at energies of 0.3 to 8 megaelectron volts (MeV) were detected on 15 April 1988 from four nuclear-powered satellites including Cosmos 1900 and Cosmos 1932 as they flew over a double Compton gamma-ray telescope. The observations occurred as the telescope, flown from a balloon at an altitude of 35 kilometers from Alice Springs, Australia, searched for celestial gamma-ray sources. The four transient signals were detected in 30 hours of data. Their time profiles show maxima with durations of (21 +/- 1) and (27 +/- 1) seconds (half-width at half maximum) for the lower two satellites and (85 +/- 5) and (113 +/- 7) seconds for the remaining two. Their durations place the origin of the two shorter signals at orbital radii of 260(+40)(-60) and 260 +/- 60 km above the earth and the two longer at 800(+100)(-300) and 800(+250)(-300) kilometers. Their luminosities for energies >0.3 MeV are then (6.1 +/- 1.5) x 10(15), (3.9 +/- 1.0) x 10(15), (1.10 +/- 0.28) x 10(16), and (1.30 +/- 0.32) x 10(16) photons per second. The imaging of the strongest signal indicates a southeastern direction passing nearly overhead. The energy spectrum can be fit to an exponential with index 2.4 +/- 1.4. These transient events add to the already large backgrounds for celestial gamma ray sources.