Detection of a gamma-ray spectral line from the galactic-center region

A balloon-borne experiment timed to observe a lunar occultation of the X-ray source GX 3+ 1 was conducted on 1971 November 20, from Parana', Argentina. Good data were obtained in the 30 keV-2.4 MeV energy band with two 24 FWHM instruments for over four hours, including the hour-long occultation. The observed spectra confirm our 1970 measurements of a spectral line superposed on a power-law continuum radiated from the galactic-center region. The combination of the 1970 and 1971 data places the line 5.3 standard deviations above the continuum. The line energy is 476 i 24keV; its intensity is (1.8 l 0.5) x l0- photons cm-2 1 A result of the occultation observation is that, while it is ruled out as the origin of the bulk of the radiation from the galactic center region, a relatively faint, hard-exponential continuum is radiated by OX 3+1, a result in agreement with an extrapolation of that measured at lower energies by the LRL group. The identification of the source of the 476-keV radiation is unclear, although there was a 2.3 standard deviation decrease in the spectral line during the occultation. Subject headings: galactic nuclei - gamma rays - lunar occultation