HARD X-RAY SPECTRAL FEATURE DETECTED IN THE REGION OF MCG-8-11-11

During a balloon flight of the High Energy Astronomical Telescope on 1 995 July 29, a region of the sky of 2.5 degrees width around the Seyfe rt galaxy MCG 8-11-11 was studied over the photon energy range 20-300 keV. An excess at the 9 sigma level-well described by a power-law phot on spectrum with a slope of alpha = 2.1 +/- 1.2, plus a Gaussian line centered at 112 +/- 6 keV having a flux of (9.0 +/- 3.4) x 10(-4) phot ons cm and a FWHM of 32 +/- 6 keV-was detected in the energy range 20- 160 keV. A contemporary detection of a strong gamma-ray flux in the ve to system of the telescope suggests that the observed spectral feature may be the result of double-Compton backscattering of a 511 keV line source incident on an external cloud. If associated with the Seyfert g alaxy MCG 8-11-11, the present measurement would provide the first evi dence for electron-positron pair annihilation flux emission from an ac tive galactic nucleus. An observation of the Crab Nebula as an in-flig ht calibration source of the telescope is also decribed.