Large amplitude super-soft X-ray intensity variations and a 54 sec QPO in Nova Del 2013 (V339 Del)
ATel #5573; A. P. Beardmore, J. P. Osborne and K. L. Page (U. Leicester)
on 12 Nov 2013; 15:53 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
In ATEL #5505 we reported the rise of the super-soft X-ray source in
Nova Del 2013 (V339 Del) seen by the Swift XRT. Since the last observation
reported there, at 1.5 c/s on day 70.8 after outburst, the soft X-ray
count rate has risen to a peak count rate of ~100 c/s on day 87.9 (10
Nov). All count rates are from grade 0 events only. The rising count
rate was interspersed by a few large dips, one of which reached down
to <1% of the count rate one day earlier. Since day 84.6 (7 Nov) the
interval of large amplitude variations appears to have ended, with no
variations larger than a factor of two up to the most recent
observation on day 89.7 (12 Nov).
We have searched XRT WT light curves for variability on shorter
timescales. An FFT analysis of the data from day 77.5 to 88.6, in
which power spectra of 40 intervals of data of duration 409.6s at 0.1s
binning were averaged, has revealed the presence of a quasi-periodic
oscillation (QPO) and low frequency noise (LFN). Fitting the averaged
power spectrum with a power-law for the LFN, a Gaussian for the QPO
and a constant for the statistical (i.e. Poisson) noise gives a best
fit QPO period of 53.2 +/- 1.2 sec, a QPO FWHM of (3.5 +/- 1.7)e-3 Hz and
a LFN power law index of -1.44 +/- 0.16 (where the errors are
1-sigma). The strongest detection was on day 88.0; the QPO is seen
in the remaining data at a level 4 sigma above the LFN. The coherence
as measured from the average power spectrum is ~12 cycles. The average
power in the Gaussian corresponds to a fractional r.m.s. of 2.5%. Although
not detected in all observations, individual sections of data in which the
QPO is clearly visible (e.g. day 84.4 and 88.0) show a sinusoidal-like
modulation with fractional amplitudes up to 6%.
The large amplitude variation in the super-soft flux is very similar
to that seen from the recurrent/classical novae RS Oph, KT Eri, and V458 Vul (Osborne et al. 2011
ApJ 727, 124, Schwarz et al. 2011 ApJS 197, 31). Its origin is not
certain, but may be due to clumps in the ejecta causing variable and
possibly ionised absorption (Ness et al. 2007 ApJ 665 1334) or white
dwarf photospheric temperature variations (S11). The QPO detected here
is also similar to one seen in RS Oph at 35 sec by the XRT (O11) and
XMM (N07), and in KT Eri also by the XRT at 35 sec (ATEL #2423). The
origin of this short-period QPO is also not certain, possibly being
related to either the spin of the white dwarf or an oscillation in the
nuclear burning rate.
We thank the Swift PI and operations team for their support of these
observations.
