SMM OBSERVATIONS OF GAMMA-RAY TRANSIENTS .3. A SEARCH FOR A BROADENED, REDSHIFTED POSITRON-ANNIHILATION LINE FROM THE DIRECTION OF THE GALACTIC-CENTER

We have searched the 1980-1988 Solar Maximum Mission gamma-ray spectrometer data for transient emission on timescales from hours to approximately 12 days of broad gamma-ray lines at energies congruent-to 400 keV, which were reported by the HEAO 1 and SIGMA experiments from two sources lying toward the Galactic center. The lines have been interpreted as the product of the annihilation of positrons in pair plasmas surrounding the black hole candidate 1E 1740.7-2942 and the X-ray binary IH 1822-371. Our results from a combined exposure of approximately 1.5 x 10(7) s provide no convincing evidence for transient emission of this line on any timescale between approximately 9 hr and approximately 1 yr. Our 3 sigma upper limits on the line flux during approximately 12 day intervals are characteristically 4.8 x 10(-3) photon cm-2 s-1, while for approximately 1 day intervals our 3 sigma upper limits are characteristically 4.9 x 10(-3) photon cm-2 s-1. These results imply a duty cycle of less than 1.3% for the transient line measured from IH 1822-371 during a approximately 3 week interval in 1977 by HEAO 1, and a duty cycle of less-than-or-equal-to 0.8% for the transient line detected in 1990 and 1992 from 1E 1740.7 - 2942 on approximately 1 day timescales by SIGMA.