Optical and Near-Infrared Photometric Observation during the Superoutburst of the WZ Sge-Type Dwarf Nova, V455 Andromedae
ABSTRACT We report on optical and infrared photometric observations of a WZ Sge-type dwarf nova, V455 And during a superoutburst in 2007. These observations were performed with the KANATA (V, J, and K_s bands) and MITSuME (g', Rc, and Ic bands) telescopes. Our 6-band simultaneous observations allowed us to investigate the temporal variation of the temperature and the size of the emitting region associated with the superoutburst and short-term modulations, such as early and ordinary superhumps. A hot (>11000 K) accretion disk suddenly disappeared when the superoutburst finished, while blackbody emission, probably from the disk, still remained dominant in the optical region with a moderately high temperature (~8000 K). This indicates that a substantial amount of gas was stored in the disk even after the outburst. This remnant matter may be a sign of an expected mass-reservoir which can trigger echo outbursts observed in several WZ Sge stars. The color variation associated with superhumps indicates that viscous heating in a superhump source stopped on the way to the superhump maximum, and a subsequent expansion of a low-temperature region made the maximum. The color variation of early superhumps was totally different from that of superhumps: the object was bluest at the early superhump minimum. The temperature of the early superhump light source was lower than that of an underlying component, indicating that the early superhump light source was a vertically expanded low-temperature region at the outermost part of the disk. Comment: 14 pages, 12 figures, PASJ accepted
arXiv:0908.4164v1 [astro-ph.SR] 28 Aug 2009
PASJ: Publ. Astron. Soc. Japan , 1–??,
c ? 2009. Astronomical Society of Japan.
Optical and Near-Infrared Photometric Observation
during the Superoutburst of the WZ Sge-Type Dwarf Nova,
Risako Matsui1, Makoto Uemura2, Akira Arai1, Mahito Sasada1, Takashi Ohsugi2,
Takuya Yamashita3, Koji Kawabata2, Yasushi Fukazawa1, Tsumefumi Mizuno1,
Hideaki Katagiri1, Hiromitsu Takahashi2, Shuji Sato4, Masaru Kino4, Michitoshi Yoshida5, Yasuhiro
Shimizu5, Shogo Nagayama5, Kenshi Yanagisawa5, Hiroyuki Toda5,
Kiichi Okita5, and Nobuyuki Kawai6
1Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1,
2Astrophysical Science Center, Hiroshima University, Kagamiyama 1-3-1,
3National Astronomical Observatory of Japan, Osawa 2-21-1,
Mitaka, Tokyo 181-8588, Japan
4Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602
5Okayama Astrophysical Observatory, National Astronomical Observatory of Japan,
Kamogata Okayama 719-0232
6Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
(Received 2009 March 27; accepted 2009 June 8)
We report on optical and infrared photometric observations of a WZ Sge-type dwarf nova, V455 And
during a superoutburst in 2007. These observations were performed with the KANATA (V , J, and Ks
bands) and MITSuME (g′, Rc, and Ic bands) telescopes. Our 6-band simultaneous observations allowed us
to investigate the temporal variation of the temperature and the size of the emitting region associated with
the superoutburst and short-term modulations, such as early and ordinary superhumps. A hot (>11000 K)
accretion disk suddenly disappeared when the superoutburst finished, while blackbody emission, probably
from the disk, still remained dominant in the optical region with a moderately high temperature (∼8000 K).
This indicates that a substantial amount of gas was stored in the disk even after the outburst. This
remnant matter may be a sign of an expected mass-reservoir which can trigger echo outbursts observed
in several WZ Sge stars. The color variation associated with superhumps indicates that viscous heating
in a superhump source stopped on the way to the superhump maximum, and a subsequent expansion of a
low-temperature region made the maximum. The color variation of early superhumps was totally different
from that of superhumps: the object was bluest at the early superhump minimum. The temperature of
the early superhump light source was lower than that of an underlying component, indicating that the
early superhump light source was a vertically expanded low-temperature region at the outermost part of
Key words: accretion, accretion disk—stars:
novae, cataclysmic variables—stars:individ-
Cataclysmic variables are semi-detached binary systems
consisting of a primary white dwarf and a secondary red
star. A Roche-lobe-filling red star loses mass through
the inner Lagrangian point and the white dwarf accretes.
Dwarf novae are a group of cataclysmic variables, show-
ing repetitive outbursts having amplitudes of 2–8 mag
(Warner 1995). In the quiescent state of dwarf novae, the
optical emission is a superposition of the flux from sev-
eral components, that is, the thermal emission from the
white dwarf and the secondary star, and free-free emission
from an optically thin accretion disk and a hot spot where
the gas stream from the secondary hits the disk (Szkody
1976). In an outburst, the thermal emission from an op-
tically thick disk is dominant (Clarke et al. 1984; Horne
et al. 1990). SU UMa-type dwarf novae are a subgroup of
dwarf novae, exhibiting two types of outburst: a short nor-
mal outburst and a long superoutburst. During superout-
bursts, their light curves show short-term periodic mod-
ulations, called “superhumps”, which have a period that
is a few percent longer than the orbital period (Warner
It is widely accepted that dwarf nova outbursts can be
explained by two types of instabilities in the accretion
disk (Osaki 1996). The first instability is a thermal insta-
2 Author(s) in page-head [Vol. ,
bility (H¯ oshi 1979). According to the thermal instability
model, the accretion disk can take only two thermally sta-
ble states. One is a low-viscosity, cool disk consisting of
neutral hydrogen gas. The other one is a high-viscosity,
hot disk consisting of ionized gas. The disk with partially
ionized gas is predicted to be thermally unstable. Dwarf
nova outbursts can be interpreted as a state transition
from the cool to hot state, which occurs when the disk
density reaches a critical value in the cool state.
The second instability is a tidal instability (Whitehurst
1988; Osaki 1989). According to this model, the disk be-
comes tidally unstable, and deforms to an eccentric disk
when the disk radius reaches the 3:1 resonance radius. A
strong tidal torque works in an eccentric disk, leading to
bright superoutbursts observed in SU UMa stars. An ec-
centric disk is expected to show prograde precession in the
inertial frame of binary systems. The superhump period,
slightly longer than the orbital period, can naturally be
explained by this precession.
SU UMa stars, which only experiences superoutbursts
(O’Donoghue et al. 1991; Osaki 1995). They are charac-
terized by quite long (∼10 yr) intervals of superoutbursts.
WZ Sge stars have received attention because they show
peculiar variations which are not seen in ordinary SU UMa
stars, and whose mechanisms are poorly understood.
WZ Sge stars tend to show echo outbursts after the
main superoutburst (Kato et al. 2004).
the thermal–tidal instability model, the amount of gas in
the disk should be minimum just after an superoutburst
(Osaki 1989). The long duration of the echo outburst
phase and its sudden cessation are, hence, problematic
for the disk instability model. Hameury et al. (2000) pro-
posed that an echo outburst is caused by an enhanced
mass-transfer rate from the secondary. Patterson et al.
(2002) reported that the observation of WZ Sge supported
that scenario. On the other hand, Osaki et al. (2001)
proposed that an echo outburst can be triggered if the
disk viscosity remains high just after the outburst, and
the gas is supplied from the outer disk (Kato et al. 1998;
Kato et al. 2004). Hellier (2001) suggested that a sub-
stantial amount of gas may be stored between the 3:1 res-
onance and the tidal-limit radius in binary systems having
an extreme mass ratio (M2/M1<
type dwarf novae. Observational evidence for such a mass
reservoir for echo outbursts has, however, not been estab-
lished (Uemura et al. 2008; Kato et al. 2008).
WZ Sge-type dwarf novae exhibit unique short-term pe-
riodic modulations only appearing in a very early phase
of superoutbursts. They are called “early superhumps”,
whose period is in agreement with the orbital period
(Patterson et al. 1981; Kato et al. 1996). Since the ampli-
tude of the early superhump depends on the inclination
angle of binary systems, it is probably attributed not to
the variation due to viscous heating, but to a geometric
effect of the accretion disk (Kato 2002). It is proposed
that a part of the disk is vertically expanded, and has
a non-axisymmetric structure (Kato 2002; Osaki, Meyer
2002; Kunze, Speith 2005).
∼0.1), such as WZ Sge-
Fig. 1. V -band CCD image of the field of V455 And observed
on 15 September 2007 with the KANATA telescope. The field
of view is 7’×7’. V455 And and comparison stars are indicated
by the black bars.
V455 And was discovered as a dwarf nova candidate
from the Hamburg Quasar Survey. Follow-up observations
showed an orbital period of 81.09 min and a spectrum
similar to that of WZ Sge (Araujo-Betancor et al. 2004;
Araujo-Betancor et al. 2005). The first-ever recorded out-
burst of V455 And was discovered on 2007 September
41. As reported in the following section of this paper,
V455 And was actually confirmed to be one of the WZ Sge
stars from observations during the outburst. The object
became a very bright source, reaching the 8th magnitude
in the V band at maximum. It, hence, provided a great
chance to study WZ Sge phenomena in detail. We per-
formed optical–near-infrared multi-band photometric ob-
servations in order to provide the color variation associ-
ated with a superoutburst as well as early and ordinary
superhumps. In this paper, we report on the results of our
observation. We, furthermore, investigate the temporal
variation of the temperature and the size of the emitting
region of the disk using the multi-band data. In section 2,
the observation and image reduction are described. Our
observational results are shown in section 3. The impli-
cation of the results is discussed in section 4. Finally, we
summarize our findings in section 5.
We performed photometric observations at two sites.
First, observations at Higashi-Hiroshima Observatory
were carried out with the 1.5-m KANATA telescope. We
used the instrument “TRISPEC (Triple Range Imager
and SPECtrometer with Polarimetry)” attached to the
Cassegrain focus of the telescope (Watanabe et al. 2005).
Photometric observations were performed simultaneously
No. ]Running Head3
Table 3. Magnitude of the comparison stars
13.17 ± 0.01
12.70 ± 0.01
12.12 ± 0.01
11.63 ± 0.01
10.89 ± 0.02
10.26 ± 0.02
12.45 ± 0.01
12.20 ± 0.01
11.85 ± 0.01
11.50 ± 0.02
11.05 ± 0.02
10.71 ± 0.02
in the V , J, and Ksbands. The exposure times of V , J,
and Ks-band observations were 10 or 30, 2 or 5, and 1 s,
respectively, depending on the sky condition. An example
of the V -band images is shown in figure 1. Second, the
observations at Okayama Astrophysical Observatory were
carried out with the 50-cm MITSuME telescope. The ob-
servations were performed simultaneously in the g′, Rc,
and Ic bands. The exposure times of the three bands
were between 5 and 60s. The journal of the observations
is given in tables 1 and 2 for the Higashi-Hiroshima and
Okayama observatories, respectively.
After dark subtraction and flat fielding, we performed
aperture photometry, and obtained differential magni-
tudes of the object relative to the comparison stars.
The comparison stars that we used are indicated in
figure 1.Comparison stars 1 and 2 are located at
RA = 23h34m04.s19, DEC = +39◦21′24′′.1, and RA =
23h34m15.s09, DEC = +39◦22′47′′.4, respectively. The
optical and near-infrared magnitudes of the comparison
stars were quoted from Henden (2006)2and the 2MASS
catalog (Skrutskie et al. 2006), respectively. The magni-
tude of the comparison star in the g′band was converted
from the B and V magnitudes with the following formula
(Smith et al. 2002):
g′= V +0.54(B−V )−0.07.
The magnitudes of the comparison stars are listed in ta-
The constancy of the magnitude of comparison star 1
was checked with comparison star 2. No significant vari-
ation was seen over ∼ 0.04 mag in the relative magni-
tude between comparison stars 1 and 2 throughout our
observation. In the following sections, we show the results
using comparison star 1. We confirmed that the magni-
tudes calculated with comparison star 2 are in agreement
with those with comparison star 1 within the errors in all
bands. The magnitude errors of the comparison star were
included in those of the object in the following analysis
for the spectral energy distribution (SED).
The interstellar extinction in the direction of V455 And
is estimated to be small, AV = 0.34, according to the
database of Schlegel et al. (1998). This can be considered
as being an upper-limit of the extinction in V455 And.
The actual extinction is probably significantly smaller
than the upper-limit, since V455 And is a nearby source;
the distance is estimated to be 90±15pc (Araujo-Betancor
Time ( days from the outburst maximum )
20 30 40 50
Fig. 2. Light curve and color variation during the outburst.
The upper panel shows the light curves in the V and J bands
represented by the filled and open squares, respectively. The
lower panel shows the color variation of V − J. The figure
includes observations for ∼2 months, from 4 September to 30
October 2007. The abscissa is the elapsed days from the out-
burst maximum (5.5 September 2007(UT) [JD 2454349.0]).
Errors of the photometric points are also shown, but they are
smaller than the symbol size.
et al. 2005). It is difficult to perform an accurate correc-
tion of the extinction with our available data.
paper, we neglect the interstellar extinction.
3.1.The 2007 Outburst of V455 And
Figure 2 illustrates the overall light curve and color vari-
ation of V455 And during the 2007 outburst. The out-
burst of V455 And was discovered on 4 September 2007.
We started to observe the object just after its discovery.
The object was rapidly rising with −5.8magday−1in the
V band during our first night observation on 4 September.
On the next day, 5 September, the outburst reached the
maximum when the magnitude was 8.69±0.01 in the V
band. Hereafter, we denote the time as T, the elapsed
days from the outburst maximum, defining T = 0.0 as 5.5
September 2007(UT) (JD 2454349.0).
After the object reached the maximum, it had declined
from V = 8.7 to 11.9 for 17 d. The fading rate was calcu-
lated to be 0.31±0.02magday−1in the V band at T =0–5.
It, then, significantly decreased to 0.13±0.01 magday−1
at T = 9–15. A change of the fading rate occurred at
T ∼ 6. Such a feature is commonly observed in WZ Sge-
type dwarf novae (Kato et al. 2001). The outburst con-
tinued at least until T ∼17, and then a rapid fading from
the outburst was observed at T = 20. The rapid fading
4Author(s) in page-head [Vol. ,
Table 1. Observation log for the KANATA telescope
T (days) Time [+JD2454000]
∗Magnitudes are averaged ones in each run.
No. ] Running Head5
Table 2. Observation log for the MITSuME telescope
T (days)Time [+JD2454000]
∗Magnitudes are averaged ones in each run.
stopped at 2.7 mag brighter than the quiescence in the
V band. Then, the object again started gradual fading.
Those temporal behaviors are common to the light curves
of the other wave-bands, that is, the g′, Rc, Ic, and Ks
bands. V455 And exhibited no echo outburst, which has
been observed in several WZ Sge stars (Kato et al. 2004).
The color index took a minimum value of V − J =
−0.13±0.01 when the brightness was at the maximum.
The color index gradually increased with time during the
outburst. As can be seen in the lower panel of figure 2, the
color curve also shows a break at T ∼ 6. The color sud-
denly changed to be red, V −J ∼ 0.8, at the same time
as when the outburst finished. After the outburst, the
color remained almost constant at ∼0.8, while the bright-
ness continued a gradual decline. Thus, the brightness—
color relation in outburst was different from that after the
outburst. This result suggests that a dominant radiation
mechanism or component changed when the outburst fin-
3.2. SED Variation during the Outburst
Using 6-band photometric observations, we investigated
the radiation mechanism, the size, and the temperature
of the emitting region during and after the outburst.
Figure 3 shows the SEDs of the optical–infrared region.
The top, middle, and bottom panels show the SED at
T = 5, 13, and 22, respectively.
We need to develop an SED model to obtain physical
parameters of the emitting region for both the outburst
and post-outburst states. It is considered that the ther-
mal emission from the optically thick disk is dominant
in the optical range during dwarf nova outbursts (Clarke
et al. 1984; Horne et al. 1990). It has been proposed,
however, that WZ Sge stars have more complex emission-
components and structure of the accretion disk (Smak
1993; Nogami et al. 2009).
quiescent states, furthermore, it has been believed that
several components can contribute to the optical–near-
infrared emission, for example, the white dwarf, the opti-
cally thick/thin disk, the hot spot, and the secondary star.
In the post-outburst and