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A broad $\gamma$-ray line (FWHM $>200$ keV) has been reported on two separate occasions from distinct sources in the Galactic Center region, at energies below 0.511 MeV: at $\sim 0.46$ MeV from V1223 Sgr in 1977 by HEAO-1\/ (Briggs 1990, Briggs et al. 1991) and at $\sim 0.48$ MeV from 1E 1740.7--2942 on 1990 13 October by SIGMA (Bouchet et al. 1991). These features have been interpreted as a 0.511 MeV line arising from annihilation of positrons in a hot accretion disk surrounding a black hole, thermally broadened and gravitationally redshifted (reviewed by Ramaty et al. 1992). We have searched in SMM\/ Gamma Ray Spectrometer (GRS) data for transient emission of this feature from the broad region of the Galactic Center containing the two sources (which, at $\sim 12 \deg$ apart, are not resolved by the GRS). The data were obtained between 1981 and 1989, and were searched for transient emission of the feature on time-scales between $\sim 1$ d and $\sim 1$ yr by the method of Harris et al. (1991, 1993). We find no evidence for significant emission in such a feature, down to 3 $\sigma$ upper limits which are characteristically \(\simeq 3 \times 10^{-3} \) $\gamma$ cm$^{-2}$ s$^{-1}$ over time-scales 12 d or greater, and \(\simeq 1.2 \times 10^{-2} \) $\gamma$ cm$^{-2}$ s$^{-1}$ over time-scales $\sim 1$ d. For comparison, Briggs (1990) measured a transient flux of \(6 \times 10^{-3} \) $\gamma$ cm$^{-2}$ s$^{-1}$ from V1223 Sgr on a time-scale of a few months, and Bouchet et al. (1991) detected \(1.3 \times 10^{-2} \) $\gamma$ cm$^{-2}$ s$^{-1}$ from 1E 1740.7--2942 on a time-scale $\sim 1$ d. We deduce a duty cycle for this transient at the reported intensities, from either source, of $<$ 1.3\% on $\sim 12$ d time-scales, or of $\leq$ 0.3\% on $\sim 1$ d time-scales. This work was supported by NASA grant DPR--W17, 808.
L. Bouchet et al., 1991, ApJ 383, L45 \\ M.S. Briggs, PhD Thesis, Univ. of California, San Diego \\ M.S. Briggs et al., IAUC 5229 \\ M.J. Harris et al., 1991, Bull AAS 23, 1440 \\ M.J. Harris et al., 1993, ApJ in press \\ R. Ramaty et al., 1992, ApJ 392, L63 \\
