Transient Gamma‐Ray Spectrometer Observations of Gamma‐Ray Lines from Novae. III. The 478 keV Line from⁷Be Decay

Abstract

The W ind spacecraft, which carries the Transient Gamma Ray Spectrometer (TGRS), moves in an extremely elliptical orbit that largely avoids EarthÏs trapped radiation belts and albedo c-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 1995È1997, including Ðve known individual events and possible undiscovered individual events. The TGRS design also uses W indÏs 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 isotopeÏs 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.

Explosive lithium production in the classical nova V339 Del (Nova Delphini 2013)

“…For nearby bright novae, there have been several attempts to detect the 478 keV gamma-ray line produced by the decay of 7 Be. However, no definite detection has been reported because of the insufficient sensitivity 23,24 . The 7 Be absorption lines in the near UV spectrum of V339 Del are found in highly blue-shifted (∼ 1, 000 km s −1 ) flows, which have been blown off by the outburst.…”

Binary Systems and Their Nuclear Explosions

“…More recent analyses of novae during the period 1995-1997, have been possible thanks to the Transient Gamma-Ray Spectrometer (TGRS) on board the Wind satellite. The flux limits from TGRS were a factor of 10 smaller than those from SMM observations, but the upper limits on 7 Be ejected masses did not improve by the same factor, mainly because novae observed with TGRS were at larger distances than those observed with SMM (Harris et al 2001).…”

Novae in gamma-rays

“…Regarding the 7 Be line at 478 keV, the first search from the galactic center and from some particular novae was performed with SMM/GRS by Harris, Leising & Share (1991). Three individual novae, Nova Aql 1982, Nova Vul 1984 #2 and Nova Cen 1986, were observed, and the upper limits derived ranged between 8.1 × 10 −4 and 2.0 × 10 −3 phot cm −2 s −1 (see as well Harris et al (2001)). The corresponding upper limits of 7 Be ejected masses ranged between 5 × 10 −8 and 6.3 × 10 −7 M ⊙ (Harris, Leising & Share 1991).…”