UPPER LIMITS TO THE SOFT GAMMA REPEATER POPULATION FROM THE LILAS EXPERIMENT ABOARD THE PHOBOS MISSION

We report the results of a search for Soft Gamma Repeaters (SGRs) in the data of the Li as and 2 experiments aboard the Soviet Phobos spacecraft, which viewed 4pi sr for 6.4 months. Due to extended low-energy coverage (6 keV and above), their sensitivity to SGRs (greater than or similar to 1.3 x 10(-7) ergs CM-2 s-1) was comparable to that of BATSE for the longer duration SGRs, and slightly better than those of instruments such as Konus, HXRBS, and SIGNE 2MP (Prognoz 9), which detected numerous bursts from SGRs. Because the Lilas detectors consisted of small NaI(Tl) scintillators operating in interplanetary space, they were immune to particle- and magnetospheric-induced events which can masquerade as SGRs. Soft bursts from any of the three known SGRs-or indeed from any Galactic SGR similar to the three known-could have been detected at their previous intensity levels, but no confirmed events were observed. We discuss the implications of this result for the number, activity cycles, and energetics of SGRs in the Galaxy.