Transient Gamma‐Ray Spectrometer Observations of Gamma‐Ray Lines from Novae. II. Constraining the Galactic Nova Rate from a Survey of the Southern Sky during 1995–1997

Abstract

The good energy resolution (3-4 keV FWHM) of the Transient Gamma-Ray Spectrometer (TGRS) on board the Wind spacecraft makes it sensitive to Doppler-shifted outbursts of 511 keV electron-positron annihilation radiation, the reason being that the Doppler shift causes the cosmic line to be slightly offset from a strong instrumental background 511 keV line at rest, which is ubiquitous in space environments. Such a cosmic line (blueshifted) is predicted to arise in classical novae because of the annihilation of positrons from β-decay on a timescale of a few hours in an expanding envelope. A further advantage of TGRS—its broad field of view, containing the entire southern ecliptic hemisphere—has enabled us to make a virtually complete and unbiased three-year search for classical novae at distances up to 1 kpc. We present negative results of this search and estimate its implications for the highly uncertain Galactic classical nova rate and for future space missions.