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\IBVShead{5098}{30 May 2001}

\IBVStitle{Outburst Photometry of DX A\lowercase{nd}}

\IBVSauth{Kato, Taichi; Nogami, Daisaku$^*$}

\IBVSinsto{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}

\IBVSx{Hida Observatory, Kyoto University, Gifu 506-1314, Japan,
e-mail: nogami@kwasan.kyoto-u.ac.jp}

\SIMBADobj{DX And}
\IBVStyp{ UGSS }
\IBVSkey{dwarf nova, photometry}

\begintext

DX And is a well-known dwarf nova with a long orbital period ($P=0\fday4405$,
Bruch et al.\ 1997).  DX And is also famous for its long outburst recurrence
time (270--330 d, Simon 2000).  Only few known cataclysmic variables (CVs)
have similar characteristics.  The best known among these CVs is the
famous old nova GK Per, which at the same time shows dwarf nova-type
outbursts.  GK Per is also known as an intermediate polar (IP)\@.
The dwarf nova outbursts of GK Per is unique in that they show standstills
during their rises (cf.\ Kim et al.\ 1992).  Another noteworthy feature of
GK Per is the presence of quasi-periodic oscillations (QPOs) with periods
2--30 min in outburst (Kato et al., in preparation).  Since DX And has
similar system and outburst parameters, we attempted to search for the
possible existence of similar modulations during an outburst of DX And.
The outburst we studied was the 1996 December one, whose initial rise was
detected on December 10 by R. J. Modic at visual magnitude of 13.9.
We undertook CCD observations starting on the next night.

The observations were done on eight nights between 1996 December 11
and 1997 January 3, using a CCD camera (Thomson TH~7882, $576 \times 384$
pixels, on-chip $2\times 2$ binning adopted) attached to the Cassegrain
focus of the 60-cm reflector (focal length${}=4.8$~m) at Ouda Station, Kyoto
University (Ohtani et al.\ 1992).  An interference filter was used which
had been designed to reproduce the Johnson $V$ band.  The exposure
time was 30--50 s depending on the brightness of the object.  The frames
were first corrected for standard de-biasing and flat fielding, and were
then processed by a microcomputer-based aperture photometry package
developed by one of the authors (TK)\@.  The magnitudes were determined
relative to GSC 3242.510 ($V=12.72$, $B-V=+0.47$), whose constancy was
confirmed using GSC 3242.216 ($V=13.33$, $B-V=+0.38$).  The magnitudes are
taken from Misselt (1996).  A total of 355 useful frames were obtained.
Barycentric corrections to observed times were applied before the following
analysis.  Table 1 lists the log of observations, together with nightly
averaged magnitudes.

\begin{table}
\caption{Nightly averaged magnitudes of DX And}
\begin{center}
\begin{tabular}{ccccr}
\hline
start$^a$ & end$^a$ & mean mag$^b$ & error$^c$ & $N^d$ \\
\hline
50428.889 & 50428.892 & 0.637 & 0.008 &   5 \\
50438.856 & 50439.006 & 0.296 & 0.002 & 330 \\
50439.861 & 50439.862 & 0.597 & 0.013 &   3 \\
50440.853 & 50440.854 & 1.037 & 0.058 &   3 \\
50441.855 & 50441.856 & 1.419 & 0.049 &   3 \\
50443.009 & 50443.010 & 1.796 & 0.059 &   3 \\
50444.877 & 50444.879 & 2.356 & 0.024 &   5 \\
50451.870 & 50451.872 & 2.289 & 0.020 &   3 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{l@{ }l}
$^a$ & BJD${}-{}$2400000 \\
$^b$ & Magnitude relative to GSC 3242.510 \\
$^c$ & Standard error of nightly average \\
$^d$ & Number of frames \\
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10cm}{5098-f1.ps}{Light curve of the 1996 December outburst
of DX And}

\vskip-11pt
Figure 1 shows the light curve drawn from the data in Table 1.  The
decline from the maximum was quite linear, at a mean rate of 0.35 mag
d$^{-1}$.  This value would be useful in further calibration of the
Bailey's relation (cf.\ Szkody and Mattei 1984).  On JD
2450438--2450439, we undertook a long time-series to search for
possible QPOs.  The exposure times were 30~s, yielding a net time
resolution of 37 s.  Figure~2 shows the enlarged light curve.  The
upper panel shows the raw observations.  The lower panel shows averaged
observations in 0.008-d bins, after subtracting the linear decline.
Except for slow, small-amplitude variations with a characteristic
time-scale of 0\fday05 (Figure 2, lower panel), no significant
periodicity was found between 0\fday001 and 0\fday1.  This observation
excludes the presence of large-amplitude QPOs as seen in GK Per.

Both GK Per and DX And show slowly developing outbursts, likely
explained by inside-out propagation of disk instability (Kim et al.\ 1992;
Simon 2000).  There may be a chance that unique features (e.g.\ QPOs)
during outbursts of GK Per is reproduced in DX And, if the origin
of such features is related to the outburst mechanism.  The present
observation, however, does not support this possibility, and suggests
that the manner of development of disk instability may not be the major
cause of QPOs during outbursts of GK Per.  Alternately, the IP-nature
may play a more important role in producing the QPOs in GK Per.
The consequence of the IP-nature on the disk oscillations still needs
to be investigated, both observationally and theoretically.

\IBVSfig{10cm}{5098-f2.ps}{Enlarged light curve of DX And.  The upper panel
shows raw individual observations.  The lower panel shows the averages
and standard errors in 0.008-d bins, after subtracting the linear decline}

The authors are grateful to VSNET members for promptly and publicly
notifying the outburst.

\references

Bruch, A., Vrielmann, S., Hessman, F. V., Kochsiek, A., Schimpke, T.,
1997, {\it A\&A}, {\bf 327}, 1107

Kim, S.-W., Wheeler, J. C., Mineshige, S., 1992, {\it ApJ}, {\bf 384},
269

Misselt, K. A., 1996, {\it PASP}, {\bf 108}, 146

Ohtani, H., Uesugi, A., Tomita, Y., Yoshida, M., Kosugi, G., Noumaru,
J., Araya, S., Ohta, K., 1992, {\it Memoirs of the Faculty of Science,
Kyoto University, Series A of Physics, Astrophysics, Geophysics and
Chemistry}, {\bf 38}, 167

Simon, V., 2000, {\it A\&A}, {\bf 364}, 694

Szkody, P., Mattei, J. A., 1984, {\it PASP}, {\bf 96}, 988

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