Transient Gamma-Ray Spectrometer observations of gamma-ray lines from novae.: III.: The 478 keV line from 7Be decay

The Wind spacecraft, which carries the Transient Gamma Ray Spectrometer (TGRS), moves in an extremely elliptical orbit that largely avoids Earth's trapped radiation belts and albedo gamma -radiation. The TGRS therefore enjoys a relatively low level of background, which is also extremely stable. We show how this stability enables modeling of the time variability of background lines, which in turn enables a novel technique of background subtraction for use in the detection of transient astrophysical lines. We apply a simple version of this method to the line at 478 keV that is expected to arise from nucleosynthesis of Be-7 in nearby novae. This search covers the entire southern ecliptic hemisphere during 1995-1997, including five known individual events and possible undiscovered individual events. The TGRS design also uses Wind's 3 s rotation period to modulate signals from the Galactic center. We use this feature of the instrument to search for a quasi-constant level of 478 keV emission from the accumulation of 7Be from several novae that are expected to occur in the direction of the Galactic center during that isotope's 53 day half-life. We derive upper limits on the transient (single nova) emission that improve on previous limits by about an order of magnitude and limits on the steady (many nova) emission that represent a factor of 2 improvement. Only weak limits can be placed on the key parameters in the nucleosynthesis and ejection of Be-7, however.