A Transient Mev Range Gamma-Ray Source Observed By Heao-1

The UCSD/MIT hard X‐ray and gamma‐ray instrument on the HEAO–1 surveyed the region near the galactic center three times during its lifetime in 1977–1979. During the Sept.–Oct. 1977 scan, a gamma‐ray source in the 300–650 keV range was detected south of the galactic center. The source was below threshold sensitivity in the Spring and Fall of 1978. The source was detected with the medium energy phoswich scintillation counters which operated over the 80 keV–2 MeV range, had an area of 42 cm2 each, and a 17° FWHM aperture. The source is centered on lII=−2.3°, bII =−12.1°, with a 905 confidence error circle of ∼3.5° radius. The flux in the 333–583 keV range was (1.98±0.35)×10−5 ph/cm2‐sec‐keV and was constant within statistics during the one month period the source was in the field of view. The spectrum can be characterized as a Gaussian in the range 300≤E≤650 keV with a FWHM of 290±60 keV centered on 490±30 keV. The source is tentatively identified with the 5.57 hr period low mass X‐ray emitting binary system...