Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. III. The 478 keV Line from 7Be Decay

Abstract

The "Wind" spacecraft carrying the Transient Gamma 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 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 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 (GC). 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 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 7Be, however.