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[vsnet-preprint 74] EY Cyg paper
EY Cyg paper
Dear Colleagues,
The following paper is accepted for publication as IBVS No. 5268.
The figures are available at:
http://vsnet.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/EY_Cyg/
Regards,
Taichi Kato
===
\documentstyle[twoside,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx April 2002}
\IBVStitle{Outburst Photometry of EY Cyg}
\IBVSauth{Kato,~Taichi$^1$, Uemura,~Makoto$^1$, Ishioka,~Ryoko$^1$}
\vskip 2mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: (tkato,uemura,ishioka)@kusastro.kyoto-u.ac.jp}
\IBVSobj{EY Cyg}
\IBVStyp{UGSS}
\IBVSkey{dwarf nova, photometry}
\begintext
EY Cyg is a well-known dwarf nova with a very long cycle length.
In spite of its relatively bright magnitude ($V\sim$11.4 at maximum,
$V\sim$15.5 at minimum), no time-resolved CCD or photoelectric photometry
has been yet published. We observed EY Cyg on two outburst occasions in
1992 and 2001. Between these outbursts, only one additional outburst
(1997 January) is known, which was poorly observed because of the
unfavorable seasonal condition.
The 1992 observations were done on three nights between April 5 and 11,
using a CCD camera (Thomson TH~7882, 576 $\times$ 384 pixels, on-chip
3 $\times$ 3 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 {\it V} band. The exposure time was
20--40 s depending on the transparency. The 2001 observations were done
on 12 nights between November 15 and December 4, using an unfiltered ST-7E
CCD camera attached to a Meade 25-cm Schmidt-Cassegrain telescope,
located in Kyoto University. The exposure time was 30 s.
The frames were first corrected for standard
de-biasing (Ouda data) or dark subtraction (Kyoto data) and flat-fielding,
and were then processed by a microcomputer-based aperture photometry
package (Ouda data) or Java$^{\rm TM}$-based aperture photometry package
developed by one of the authors (TK).
We used two comparison stars GSC 2673.525 (Tycho-2 magnitude $V$=10.89
$\pm$0.06, $B-V$=+0.46$\pm$0.08) and GSC 2673.2950 (Tycho-2 magnitude
$V$=11.47$\pm$0.10, $B-V$=+0.31$\pm$0.14), whose constancy during the
run was confirmed by inter-comparison. The magnitudes of EY Cyg were
determined relative to the sum of these two stars (Ouda data) or
relative to GSC 2673.525 (Kyoto data). Barycentric corrections to observed
times were applied before the following analysis. Table 1 lists the
log of observations, together with nightly averaged magnitudes.
\begin{table}
\begin{center}
Table 1. Nightly averaged magnitudes of EY Cyg \\
\vspace{10pt}
\begin{tabular}{cccccc}
\hline
Start$^a$ & End$^a$ & Mean mag$^b$ & Error$^c$ & $N$$^d$ & Site \\
\hline
48718.257 & 48718.337 & 1.481 & 0.002 & 192 & Ouda \\
48720.209 & 48720.329 & 1.764 & 0.002 & 332 & Ouda \\
48724.207 & 48724.327 & 2.967 & 0.002 & 258 & Ouda \\
52229.061 & 52229.073 & 0.496 & 0.012 & 24 & Kyoto \\
52230.049 & 52230.066 & 0.673 & 0.006 & 42 & Kyoto \\
52233.020 & 52233.028 & 1.485 & 0.005 & 22 & Kyoto \\
52234.062 & 52234.082 & 1.856 & 0.009 & 49 & Kyoto \\
52235.032 & 52235.053 & 2.170 & 0.006 & 50 & Kyoto \\
52236.019 & 52236.044 & 2.439 & 0.013 & 61 & Kyoto \\
52237.023 & 52237.044 & 2.506 & 0.031 & 43 & Kyoto \\
52239.018 & 52239.041 & 2.674 & 0.055 & 46 & Kyoto \\
52240.010 & 52240.020 & 3.064 & 0.054 & 21 & Kyoto \\
52241.007 & 52241.021 & 2.934 & 0.024 & 35 & Kyoto \\
52245.007 & 52245.020 & 2.908 & 0.058 & 27 & Kyoto \\
52247.989 & 52248.007 & 2.969 & 0.028 & 42 & Kyoto \\
\hline
\multicolumn{5}{l}{$^a$ BJD$-$2400000.} \\
\multicolumn{5}{l}{$^b$ Relative magnitude (see text).} \\
\multicolumn{5}{l}{$^c$ Standard error of nightly average.} \\
\multicolumn{5}{l}{$^d$ Number of frames.} \\
\end{tabular}
\end{center}
\end{table}
Figure 1 shows the light curves of the 1992 and 2001 outbursts drawn
from nightly averaged magnitudes by this study. Both sets of observations
covered the decline from outbursts. The object showed on both occasions
a linear fading at a rate of 0.30 mag d$^{-1}$ (1992) or 0.28 mag d$^{-1}$
(2001) for the first seven nights. This rate of decline can be thus
considered to be a typical value for EY Cyg.
\IBVSfig{10cm}{fig1.eps}{Light curve of EY Cyg on two outbursts in 1992
and 2001. The zero point for the 1992 observation corresponds
to $V$=10.39$\pm$0.08.
}
\IBVSfig{18cm}{fig2.eps}{Light curve of the 1992 April outburst of EY Cyg.
The zero points are the same as in Fig. 1.
}
Figure 2 shows the nightly light curves (please note the vertical axis
is shifted reflecting the mean brightness of the object) for the 1992
outburst. These light curves show slow modulation with a time-scale of
$\sim$0.1--0.2 d. On the first night (April 5), a slow decline and
a shallow minimum near the end of the run was observed. On April 7,
a fading and brightening clearly defined a rather flat-bottomed minimum.
On April 11, the variation looks more sinusoidal. This observation makes
the unique time-resolved photometry during outburst. To our best knowledge,
these modulations look to more reflect slow quasi-periodic variations rather
than the stable orbital period. A period search between 0.1 and 0.3 d,
using the Phase Dispersion Minimization (PDM) method (Stellingwerf 1978),
has yielded the strongest signals near 0.192 and 0.212 d, but the values
should be treated with caution because each observing runs were not long
enough to adequately assess the possibility of a longer periodicity.
Because of the shortness of each runs, we were not able to test the
presence of this periodicity in the 2001 observation.
The orbital period of EY Cyg has not been yet unambiguously determined.
Hacke and Andronov (1988) gave a photometric period of 0.18228 d from
their photographic observations. Sarna et al. (1995) further obtained
CCD photometry and gave a period of 0.2165 d. Smith et al. (1997)
obtained optical spectra and identified the secondary as an K5--M0 star.
The lack of radial velocity variations observed by Smith et al.
(1997) suggests a low inclination system. Since a K5--M0 companion usually
suggests a longer orbital period (cf. Ritter and Kolb 1998), these
photometric periodicities need to be further examined.
The decline rate observed in this study is 0.28--0.30 mag d$^{-1}$, which
is close to that of DX And (Kato and Nogami 2001), a dwarf nova with an
orbital period of 0.4405 d. From the similarity of outburst cycle lengths
and outburst durations between EY Cyg and DX And, and from the application
of Bailey's relation (Bailey 1975; Szkody and Mattei 1984; Warner 1995) to
the decline rates, we propose a longer orbital period close to that of
DX And.
Regarding short-period oscillations, we detected low-amplitude ($<$0.05
mag), fluctuations with time scales of 10--60 min (small wiggles in
Figure 2), but we could not find no firm periodicity.
\vskip 3mm
This work is partly supported by a grant-in aid (13640239) from the
Japanese Ministry of Education, Culture, Sports, Science and Technology.
Part of this work is supported by a Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists (MU).
\references
Bailey, J., 1975, JBAA, 86, 30
Smith, R. C., Sarna, M. J., Catal\'{a}n, M. S., Jones, D. H. P.,
1997, MNRAS, 287, 271
Hacke, G., Andronov, I. L., 1988, MVS, 11, 74
Kato, T., Nogami, D., 2001, IBVS No. 5098
Ohtani, H., Uesugi, A., Tomita, Y., Yoshida,
M., Kosugi, G., Noumaru, J., Araya, S., Ohta, K.
1992, Memoirs of the Faculty of Science, Kyoto University,
Series A of Physics, Astrophysics, Geophysics and Chemistry,
38, 167
Ritter, H., Kolb, U., 1998, A\&AS, 129, 83
Sarna, M. J., Pych, W., Smith, R. C., 1995, IBVS No. 4165
Stellingwerf, R. F., 1978, ApJ, 224, 953
Szkody, P., Mattei, J. A., 1984, PASP, 96, 988
Warner, B., 1995, Cataclysmic Variable Stars (Cambridge Univ. Press.,
Cambridge)
\end{document}
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