M. Orio1,2
Abstract.
X-ray grating spectra have opened a new window on nova physics.
High signal-to-noise spectra have been obtained
for 12 novae after the outburst in the last 13 years, with the Chandra
and XMM-Newton gratings. They offer the only way to probe the
temperature, effective gravity and chemical composition of the hydrogen burning white dwarf
before it turns off. These spectra also
allow an analysis of the ejecta, which can be photoionized by the
hot white dwarf, but more often seem to undergo collisional ionization.
The long observations required for the gratings have revealed
semi-regular and irregular
variability in X-ray flux and spectra. Large short term variability is especially evident
in the first weeks after the ejecta have become
transparent to the central supersoft X-ray source.
Thanks to Chandra and XMM-Newton, we
have discovered violent phenomena in the ejecta,
discrete shell ejection, and clumpy emission regions. As expected, we have
also unveiled the white dwarf characteristics. The peak white dwarf
effective temperature in the targets of our samples
varies between ≥400,000 K and over a million K, with
most cases closer to the upper end, although for two novae only
upper limits around 200,000 K were obtained.
A combination of results from different X-ray satellites and instruments, including
Swiftand ROSAT, shows that
the shorter is the supersoft X-ray phase, the lower
is the white dwarf peak effective temperature, consistent with
theoretical predictions. The peak temperature is
also inversely correlated with t2, the time for a decay by 2 mag
in optical.
I strongly advocate the use of white dwarf atmospheric models
to obtain a coherent physical picture of the hydrogen burning process
and of the surrounding ejecta.
Keywords: stars: novae, cataclysmic variables -- stars: winds, outflows -- X-rays: binaries
1INAF-Padova, vicolo dell' Osservatorio, 5, I35122 Padova, Italy
2Department of Astronomy, University of Wisconsin, 475 N. Charter Str., Madison, WI 53704, USA