[vsnet-preprint 98] LX And paper

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

LX And paper

Dear all,

   The following paper is accepted for publication in IBVS No. 5346.

   The PostScript version and figures are available at:
   http://vsnet.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/LX_And-2/

Regards,
Taichi Kato

===

\documentstyle[twoside,epsf]{article}

\input{ibvs2.sty}

\begin{document}

\IBVShead{xxxx}{xx xxx 2002}

\IBVStitle{Dramatic Change of Outburst Properties in LX And}

\IBVSauth{Kato,~Taichi$^1$}
\vskip 5mm

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

\IBVSobj{LX And}
\IBVStyp{UGSS:}
\IBVSkey{dwarf nova, outbursts}

\begintext

   LX And was originally discovered as an RV Tau star during a survey
for RR Lyr-type stars (Kinman et al. 1982).  Uemura et al. (2000)
revealed thet the object actually shows dwarf nova-type outbursts.
Following this new classification, Morales-Rueda and Marsh (2002)
spectroscopically confirmed the dwarf nova-type nature.
Although the orbital period has not been established, the outburst
characteristics (Uemura et al. 2000) and infrared colors
(Hoard et al. 2002) make a classification of an SS Cyg-type dwarf nova
likely.

   The mean periods of LX And have been reported to be 36.469 d
(Kinman et al. 1982) or 21--30 d, most likely $\sim$26 d
(Uemura et al. 2000).  The object has been closely monitored by visual
observers around the world since the discovery of the dwarf nova-type
nature.  From the recent outburst detections, we noticed a dramatic
change of the outburst properties of this object.

\IBVSfig{10cm}{lxand-fig1.eps}{Light curve of LX And based on VSNET
observations and Uemura et al. (2000).  The large and small dots, and open
squares represent positive and negative (upper limit) visual observations,
and CCD observations, respectively.
}

   Figure 1 shows the long-term light curve based on the observations
reported to VSNET (http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/) and
Uemura et al. (2000).  The accuracy
of the visual observations are usually 0.2--0.3 mag, which would not
affect the following discussion.  Table 1 shows the list of outbursts.
The initial date of the outburst detections and the observed maxima
are given.  When there are sufficient data around the peak brightness,
we took an average of the observations.

\begin{table}
\begin{center}
Table 1. Outbursts of LX And. \\
\vspace{10pt}
\begin{tabular}{cccccc}
\hline\hline
JD$^a$ & Max & JD$^a$ & Max & JD$^a$ & Max \\
\hline
51484 & 12.2 & 51950 & 12.6 & 52337 & 12.9 \\
51511 & 13.0 & 51990 & 14.2$^b$ & 52466 & 14.2$^b$ \\
51535 & 12.8 & 52123 & 13.4 & 52496 & 12.9 \\
51567 & 12.7 & 52142 & 13.0 & 52512 & 12.9 \\
51603 & 12.3 & 52155 & 12.5 & 52528 & 13.1 \\
51769 & 12.4 & 52189 & 12.3 & 52547 & 12.8 \\
51814 & 12.5 & 52229 & 13.0 & 52565 & 13.1 \\
51850 & 12.4 & 52263 & 14.7$^b$ & 52578 & 13.6 \\
51881 & 13.5 & 52290 & 12.5 & 52594 & 13.4 \\
51912 & 12.2 & 52319 & 13.2 &       &      \\
\hline
 \multicolumn{6}{l}{$^a$ JD-2400000.} \\
 \multicolumn{6}{l}{$^b$ True maximum probably missed.} \\
\end{tabular}
\end{center}
\end{table}

   Table 2 shows the mean outburst intervals of the continuous segments
of the light curve (there are unavoidable gaps in observation around the
solar conjunction).  The mean intervals and the errors were determined
by using a linear fit to the observed times of the outbursts.

   The data clearly demonstrate that this object shows a large variation
(16--36 d) of the outburst mean intervals (Figure 2).  We have ruled out,
by a close inspection of the entire data, a possibility that a period
doubling (30--36 d and 16 d) is not a result of missed outbursts.
A dense CCD light curve by Uemura et al. (2000) also rejects this
possibility.

\begin{table}
\begin{center}
Table 2. Mean outburst intervals. \\
\vspace{10pt}
\begin{tabular}{cccc}
\hline\hline
Start &  End  & Period & Error \\
\multicolumn{2}{c}{JD-2400000} & (d) & (d) \\
\hline
51484 & 51603 & 29.4 & 1.4 \\
51769 & 51990 & 35.6 & 0.8 \\
52123 & 52155 & 16.0 & 1.7 \\
52155 & 52337 & 31.0 & 1.4 \\
52496 & 52594 & 16.5 & 0.3 \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{7cm}{lxand-fig2.eps}{Variation of the mean outburst intervals
of LX And.
}

   The ratio between the maximum mean interval $\Delta T_{\rm max}$ and
the minimum mean interval $\Delta T_{\rm min}$ is 2.2, which far exceeds
the typical values (1.2--1.5) reported in other SS Cyg-type dwarf novae
(Bianchini 1990).  LX And is thus shown to be a rare system with a huge
variation of long-term outburst intervals.  Although it was not clearly
demonstrated, a report of a long-term variation of mean magnitudes
(Kinman et al. 1982), which originally classifed the object to be an RVb
type star, may have been a result of a similar long-term trend.

   As suggested by Bianchini (1990) and Ak et al. (2001), such a long-term
variation can be attributed to solar-type activity cycle in a cataclysmic
binary.  LX And would be a promising target for a future more comprehensive
work in search of a further signature of a solar-type activity cycle.

   An alternative explanation is the dramatic change of the state of
the accretion disk, as demonstrated in the SU UMa-type dwarf nova
V503 Cyg by Kato et al. (2002).  In this case, we do not necessarily
require a variable mass-transfer rate, but would require a still
unidentified mechanism causing the state changes in the accretion disk.

\vskip 3mm

We are grateful the members of VSNET for reporting crucial observations.
This work is partly supported by a grant-in-aid (13640239)
from the Japanese Ministry of Education, Culture, Sports, Science and
Technology.

\references

Ak, T. and Ozkan, M. T. and Mattei, J. A., 2001, A\&A, {\bf 369}, 882

Bianchini, A., 1990, AJ, {\bf 99}, 1941

Hoard, D. W., Wachter, S., Clark, L. L., Bowers, T. P., 2002, ApJ,
   {\bf 565}, 511

Kato, T., Ishioka, R., Uemura, M., 2002, PASJ, in press
   (astro-ph/0209233)

Kinman, T. D., Mahaffey, C. T., Wirtanen, C. A., 1982, AJ, {\bf 87}, 314

Morales-Rueda, L., Marsh, T. R., MNRAS, 2002, {\bf 332}, 814

Uemura, M., Kato, T., Watanabe, M., 2000, IBVS No. 4831

\endreferences

\end{document}

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