SIMBAD references

The distribution of positron-annihilation line emission from the Galactic Center (GC) is arguably the most important gamma-ray line measurement that OSSE can make. After nearly 20 years of observations, the source of the positrons is still unknown. Although OSSE has scanned the GC many times, there are only two scans with the collimator perpendicular to the plane. They show a significant offset from the GC of (-1.36±0.33°; ∼4σ). The centroid is consistent with the black hole source 1E 1740.7-2942. We have developed a new pointing and background strategy for OSSE to maximize sensitivity to this offset. We have analyzed test data in this mode from a blank section of sky, and found it to be well-behaved; a week of actual GC data in this mode are now in hand. These data should be able to confirm the offset from the GC, but more observations are needed to distinguish a point source from an extended source.