TGRS Observations of Positron Annihilation in Classical Novae
M. J. Harris, D. M. Palmer, J.E. Naya
B. J. Teegarden, T. L. Cline, N. Gehrels, R. Ramaty
H. Seifert
Abstract
The TGRS experiment on board the Wind spacecraft has many advantages
as a sky monitor - broad field of view ( ~ 2 p
centered on the south ecliptic pole), long life (1994-present), and
stable low background and continuous coverage due to Wind 's
high altitude high eccentricity orbit. The Ge detector has sufficient
energy resolution (3-4 keV at 511 keV) to resolve a cosmic positron
annihilation line from the strong background annihilation line from
b-decays induced by cosmic ray impacts on the instrument, if the
cosmic line is Doppler-shifted by this amount. Such lines (blueshifted)
are predicted from nucleosynthesis in classical novae. We have searched
the entire TGRS database for 1995-1997 for this line, with negative
results. We thereby obtain an unbiased limit on the highly-uncertain
Galactic nova rate. We carefully examined the times around the known
nova events during this period, also with negative results. The
upper limit on the nova line flux in a 6-hr interval is
typically < 3.8 ×10-3
photon cm-2 s-1 (4.6 s). We performed the same
analysis for times around the outburst of Nova Vel 1999, obtaining a
worse limit due to recent degradation of the detector response
caused by cosmic ray induced damage. This result, < 1.4 ×10-2 photon cm-2 s-1 (3 s),
should be regarded as preliminary.
File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:18.