2001ApJ...563..950H

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 γ-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 ⁷Be 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 ⁷Be, however.