OBSERVATIONS OF GAMMA - RAY LINES FROM NOVAE . III . THE 478 keV LINE FROM 7 Be DECAY

The Wind spacecraft carrying the Transient Gamma Ray Spectrometer (TGRS) moves in an extremely elliptical orbit that largely avoids Earth's trapped radiation belts and albedo γ-radiation. The TGRS therefore enjoys a relatively low level of background that 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 to be used 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 7 Be 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 (GC). We use this feature of the instrument to search for a quasi-constant level of 478 keV emission from the accumulation of 7 Be from several novae that are expected to occur in the direction of the GC during that isotope's 53 d half-life. We derive upper limits on the transient (single-nova) emission which improve on previous limits by about an order of magnitude, and limits on the steady (many-nova) emission which represent a factor 2 improvement. Only weak limits can be placed on the key parameters in the nucleosynthesis and ejection of 7 Be, however.