Transient Gamma‐Ray Spectrometer Observations of Gamma‐Ray Lines from Novae. III. The 478 keV Line from7Be Decay

The "Wind" spacecraft carrying the Transient Gamma Spectrometer (TGRS) movesin an extremely elliptical orbit that largely avoids Earth's trapped radiationbelts and albedo gamma-radiation. The TGRS therefore enjoys a relatively lowlevel of background that is also extremely stable. We show how this stabilityenables modeling of the time variability of background lines, which in turnenables a novel technique of background subtraction to be used in the detectionof transient astrophysical lines. We apply a simple version of this method tothe line at 478 keV that is expected to arise from nucleosynthesis of 7Be innearby novae. This search covers the entire southern ecliptic hemisphere during1995-1997, including five known individual events, and possible undiscoveredindividual events. The TGRS design also uses "Wind"'s 3 s rotation period tomodulate signals from the Galactic Center (GC). We use this feature of theinstrument to search for a quasi-constant level of 478 keV emission from theaccumulation of 7Be from several novae that are expected to occur in thedirection of the GC during that isotope's 53 d half-life. We derive upperlimits on the transient (single-nova) emission which improve on previous limitsby about an order of magnitude, and limits on the steady (many-nova) emissionwhich represent a factor 2 improvement. Only weak limits can be placed on thekey parameters in the nucleosynthesis and ejection of 7Be, however.