The Super Soft Source (SSS) phase of novae is the last major
evolutionary phase during which the hottest layers closest to the
surface of the white dwarf can be observed until the hydrogen content
of the accreted material is consumed or ejected. The X-ray and UV
evolution of novae during their outbursts has been determined using
Swift monitoring observations. Deeper, continuous XMM-Newton and
Chandra observations have been taken, guided by the long-term
evolution determined by Swift. The time when bright SSS emission
becomes visible, the turn-on time of the SSS phase, depends on the
evolution of the nova ejecta. Accurate predictions are currently not
possible from evolutionary models, but ...view middle of the document...
Furthermore, most recently, a 54-s transient period was found
in Swift data of the nova V339Del (Beardmore et al. 2013) and
in an XMM-Newton observation (Ness et al. 2013b), both taken
during the SSS phase. Meanwhile, no 54-s periodic signal was
seen in a Chandra observation that was also taken during the
SSS phase (Nelson et al. 2013). A second Chandra observation
was taken on day 114, but data are not yet publicly available.
In addition to novae, a similar short period of ∼ 67 s has
been seen by Odendaal et al. (2014) in the prominent persistent
SSS Cal 83 which they interpret as the spin period of the white
dwarf. While this is close to the break-up period, they argue that
the white dwarf may be spun up by accretion disc torques.
Evidently, this type of short-period oscillations originates
from the surface of the white dwarf hosting nuclear burning as
it was only seen in white dwarfs emitting SSS spectra. It will be
of interest whether the 35-54 s periods in the novae are related
to the 67-s period in the persistent SSS Cal 83. If the origin is
not the spin period of the white dwarf, it might be related to the
interiors or the nuclear burning regions, and understanding these
processes might give a new diagnostic method to determine
fundamental parameters such as the mass of the underlying
white dwarf. It is therefore of interest to have as many systems
as possible with short-period oscillations in order to identify
those properties that drive them.
We have searched for short-period variations in all systems
containing bright SSS emission, focusing here on XMM-Newton
and Chandra observations while a similar project based on all
Swift XRT data will be presented by Beardmore et al. For those
systems with short-period oscillations, we study the evolution
of signal strength and peak period.
We first describe in brevity the general observation techniques
and the observations used in this article in Sect. 2. We
then describe the timing analysis in Sect. 3 and the results in
Sect. 4, where we dedicate an extra subsection to each system in
which short-term oscillations are present, closing with Sect. 4.6
in which we briefly describe those observations in which no periodic
signal was found. We discuss our results in Sect. 5 and
summarise our findings and conclusions in Sect. 6.
The large X-ray observatories XMM-Newton and Chandra
observe from highly elliptical orbits allowing long uninterrupted
observations up to 2 days. The instrumentation provides for
high resolution in time and energy using various combinations
of CCD detectors and dispersive gratings. For more details, we
refer to the corresponding instrument papers, e.g., Jansen et al.