Session 76 - Exploding Stars.
Display session, Thursday, June 13
Great Hall,

## [76.07] On the X-ray Spectral Evolution of Nova Cygni 1992 in the First Two Years After Outburst

S. Balman, H. Ogelman (U. of Wisconsin-Madison), J. Krautter (Landessternwarte, Germany)

Nova V1974 Cygni 1992 has been accepted as a prototype of classical nova owing to its unique data set in almost all wavelengths. We present a further analysis of the archival X-ray data obtained by ROSAT PSPC on several pointings from 1992 April 22 to 1993 December 3. The soft X-ray emission arising from the H-burning white dwarf envelope (between 0.1-0.7 keV energy range) is fit with a new nova blackbody emission model that accounts for the absorption of X-ray photons by enhanced abundances in the white dwarf atmosphere (MacDonald 1994, private communication). A complete evolution of the white dwarf on a Hertzsprung-Russell diagram is presented using two different atmosphere models of NeO and CO rich compositions. The soft X-ray flux derived using the new NeO blackbody model is in the range (1.7-2.3)\times 10^-7 erg\ s^-1\ cm^-2 during the constant bolometric luminosity phase. The peak photospheric temperature \sim 51 eV (5.9 \times 10^5 K) is reached at about day 500 with a photospheric radius of (0.96-1.4)\times 10^9 cm for a 2-3 kpc source distance. In addition, we present the spectral development of the hard X-ray emission above 0.7 keV thought to originate from shocks within the nova wind. The hard X-ray emission evolves independently from the soft component reaching a maximum \sim 150 days after the outburst with a peak unabsorbed flux \sim 1.8 \times 10^-11\ erg\ s^-1\ cm^-2. The total hard X-ray energy dissipated in the ejecta is about 0.01 % of the initial explosion energy.