Monitoring of the WZ Sge-type system V455 And before, during and after superoutburst.

1

• Monitoring of the WZ Sge-type system V455 And
  before, during and after superoutburst.
• N.A. Katysheva2, S.Y. Shugarov1,2
• 1) Astronomical Institute of the Slovak Academy
  of Sciences, 059 60 Tatranská Lomnica, The Slovak
  Republic
• 2) Sternberg State Astronomical Institute,
  Universitetskij Prosp. 13, 119992, Moscow, Russia

We summed the light curves of V455 And with
the orbital period 0.05630921 d. This ephemeris
Min 24551812.67765 0.05630921E was
determined in paper by Araujo-Betancor et al.
(2005) and works till now, so during the 60000
cycles (till the end of 2009) the orbital period
does not change. On the Fig.4 the phase
light curve of October 2008 is plotted. Two-hump
LC is similar the light curves of WZ Sge-type
stars in quiescence. This LC was constructed from
reduced ?B,V R after removal of the declining
trends and averages .
WZ Sge-type dwarf novae The WZ Sge-type
objects are the most extreme subgroup of SU
UMa-type DNe with a long (several years)
superoutbursts recurrence time. Their orbital
periods are the shortest ones observed for SU
UMa-type stars. Some of them exhibit a complex
post-superoutburst rise of brightness called
rebrightening(s), rarely seen in other SU
UMa-type DNe. One of the most remarkable
signatures of WZ Sge-type objects is a presence
of early superhumps during the earliest
stages of superoutbursts. This feature is
also referred to as orbital superhumps or
outburst orbital humps. Early superhumps have
a period extremely close to the binary
period and commonly show a doublehumped
profile, in contrast to the ordinary superhumps
of SU UMa-type dwarf novae. Early
superhumps are the most discriminative
feature of the WZ Sge-type objects, and
have not been detected in other DNe. V455
And ( HS23313905) was discovered by Gaensicke
et al. from the Hamburg Quasar SUrvey.
Araujo-Betancor et al. (2005) analyzed their data
described the main characteristics of this. They
defined HS2331 as a cataclysmic variable with
very wide spectra of the periods orbital (81.08
min) and superhump ones (83.38 min), non-radial
pulsations of white dwarf (5 6 min) and a
coherent signal at 1.12 min. The spectroscopic
period was about of 3.5 hour. Araujo-Betancor et
al. (2005) proposed a possible superoutburst of
HS23313905 as WZ Sge-type binary. And
superoutburst happened in September, 2007.
The paper by Matsui et al. (2009), Maegara et al.
(2009) described the photometrical observations
of superoutburst. Nogami et al. (2009) carried
out the spectroscopic observations V455 And
during the superoutburst.
Fig. 4. Two-hump phase light curve of the 16-17
October 2008 (below, left). (delta BVR and mean,
folded with the orbital period).

Fig. 1. The del V light curves summed with the
orbital period on the zero-day of the outburst
(05 September 2007). In the left-top corner
gaps on the LC are because of clouds, in the
right-bottom - a part of V-light curve from ?
0.4 till 0.8 (P0.05630921 d).
Our CCD observations of V455 And Most bulk
of our BVR-CCD observations were taken with the
camera Apogee-47a mounted in the 0.6m
Zeiss-reflector at the Crimean laboratory of SAI,
further CCD observations were taken with the
Pictor-416 cameras (0.5m Maksutov (f/4.0)
telescope at the Crimean Laboratory of SAI. We
carried out the monitoring of V455 And from 2004
to 2009 yr. Katysheva and Shugarov (2009)
observed this system before superoutburst (in
2004 ? 2006) and studied the photoplates from
Moscow archive. The limit of these Moscow plates
is about of 12 13 magnitudes. On the 45th
plates from 1907 till 1957 there were not any
sign of outbursts on them. The nine month before
superoutburst of 2007 yr there were no
significant signs of any activities except
mentioned by Araujo-Betancor et al. (2005)
(Katysheva Shugarov, 2009). Zero day of
superoutburst ? the 5th of September 2007.
We carried out the observations on the 5, 8, 9,
10 and 11 and then 17, 18, 19 and 20 September
2007 yr. So it was the beginning of
superoutburst. The further set of observations
was in November of 2007. In Fig.1 we
present the nightly LCs in V-band during the
zero-day of superoutburst. The numerous humps are
seen clearly between phase 04 - 0.8. In Fig.2 we
plotted the overall V-LC and nightly LCs summed
with the orbital period. The complex structure of
LC (with many humps) especially at the beginning
of superoutburst seems to connect with the
turbulent processes in accrecion disc. The
interesting detail is double-peak maxima at 5th
(10 Sept.) and 12th days (17 Sept.) of
superoutburst and (for example) in quiescence
(the 13th and 15th of November 2007). On the LC
of Nov. 18, 2007 there is very clear orbital
minimum. Three days in November 13, 15 and 18
and there are so different LCs in spite of the
transition to quiescence. It says about unsteady
disc processes.


Fig. 4. Two-hump phase light curve of the 16-17
October 2008 (below, left). (delta BVR and mean,
folded with the orbital period).

Fig. 5. Phased light curve of V455 And and the
mean light curve (November 2009) left. At the
right average light curve folded with the
superhump period 0.05735 d.
Early, ordinary and late superhumps The
early superhumps are a remarkable feature of WZ
Sge-type stars. They appear near the maximum
magnitude of superoutburst and have periods
almost identical to the orbital one. Osaki and
Meyer (2002) suggested that a double peaked
profile of early superhumps is manifestation of
the tidal 21 resonance in accretion disks of
binary systems with extremely low mass ratios.
Early superhumps can be explained by a
two-armed spiral pattern of tidal dissipation
generated by the 21 resonance. According to
Kato (2002), the expansion beyound the 31
resonance radius can be responsible for the
appearance of early superhumps. The
common, or ordinary superhumps, with a single
peak profile, appear during the plateau of
superoutburst of WZ Sge-type stars. Their periods
are a few percent longer than the orbital period.
The ordinary superhumps can be explained by the
thermal tidal instability model of an accretion
disk (Osaki, 1989 Whitehurst, 1988). The
presence of the tidal 31 resonance in the disk
(with the radius smaller than the 21 resonance
radius) results in the formation of an eccentric
outer ring undergoing apsidal precession with a
period appreciably longer than the orbital one.
The beating of the orbital and precessional
periods cause periodic variations, identified
as superhumps. This model is supported by
numerical simulations (Bisikalo et al., 2005).
The late superhumps appear several days after
the rapid decline from the plateau of a
superoutburst and may continue for several
hundred cycles after the end of the
superoutburst. According to Kato et al. (2008),
the late superhumps originate in the precessing
eccentric disk near the tidal truncation. The
eccentric disk slowly expands during the
superoutburst decline and finally reaches the
tidal truncation, where the period is stabilized.
Fig. 2. The overall light curve of the beginning
of superoutburst (top) and the nightly light
curves of V455 And (days 4-6 and 12-15 after
outburst (AO)) from top to bottom (HJD
2400000).

• References
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Fig. 3. The nightly and mean orbital phase light
curves (R-band). The ephemeris JDmin
2451812.67765 0.05630921E. The 13th, 15th and
18th of November 2007. (Top) Nov. 13, (bottom,
right -Nov. 15. Bottom Nov. 18)
Physics of Accreting Compact Binaries, July
26(Mon) - 30(Fri), 2010, Kyoto University, Kyoto,
Japan