SMM OBSERVATIONS OF GAMMA-RAY TRANSIENTS .1. A SEARCH FOR VARIABLE EMISSION AT MEV ENERGIES FROM 5 GALACTIC AND EXTRAGALACTIC SOURCES

Transient emission at energies near 1 MeV has been reported by previou s experiments on time scales of weeks to months from the Galactic cent er. the Crab Nebula, and Cyg X-1, and on shorter time-scales from NGC 4151 and Cen A. The spectra of these events fall into two broad classe s: a broad line-like feature centered near 1 MeV, and continuum emissi on (or a very broad feature) extending from approximately 600 keV up t o several MeV. These features have been interpreted theoretically in t erms of emission from hot pair-dominated plasmas, which may be the nec essary positron source implied by reports of narrow e-e+ annihilation lines from Cyg X-1 and the Galactic center. In this paper, data accumu lated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) betwee n 1980 and 1989 have been searched for evidence of these two types of features. We find no compelling evidence for transient 1 MeV broad-lin e emission on time scales of order 12 d or longer when any of the sour ces are in the GRS field of view, upper limits on the transient line f lux during any 12 d period are typically approximately 4.5 x 10(-3) ga mma (cm2 s)-1. The same is true of variability of the continuum betwee n 0.6-7 MeV, for which the upper limits are characteristically approxi mately 2 times the nominal flux from the Crab for any 12 d period. Our analysis was not sensitive enough either to confirm or to reject seve ral reports from other experiments of transient emission in the 0.6-7 MeV continuum during 1980-1989. We withdraw the statement in Share et al. (1993) that our upper limit for one such event, a transient from t he Crab Nebula in 1980 Spring, is inconsistent with the measurement of Ling & Dermer (1991). We did not detect any transient emission in the 1 MeV feature preceding or coinciding with reported emission of the 0 .511 MeV annihilation line from the Galactic center in 1988 1989. We d iscuss briefly the implications of this result for models of the annih ilation of positrons produced in the Galactic center source 1E 1740.7 - 2942.