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\IBVShead{5028}{7 February 2001}

\IBVStitle{CCD Observation of MWC 560 = V694 M\lowercase{on}}

\IBVSauth{Ishioka, R.; Uemura, M.; Kato, T.}

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

\SIMBADobj{MWC 560}

\begintext

MWC\,560 (= V694\,Mon) is a symbiotic-like star whose character is
still not clear. This star was discovered as an emission-line object
$V=12\fmm5$ by Merrill and Burwell (1943). Sanduleak and Stephenson
(1973) gave a spectral classification of M4ep. One interesting
phenomenon of MWC\,560 is its flickering. Bond et al.\ (1984) first
noted the rapid variation with amplitudes of about 0\fmm2 and time
scales of a few minutes. Flickering is a common property of CVs, but is
rarely seen on symbiotic stars (Dobrzycka et al.\ 1996).

Tomov et al.\ (1990) reported an outburst reaching $V = 9\fmm1$,
associated with remarkable blue-shifted absorption lines with
velocities up to $-$6500~km\,s$^{-1}$. The strong shift implied a
high-velocity and highly-collimated jet. These absorption lines showed
rapid variability (Tomov et al.\ 1990, 1997). The magnitude dropped to
$V\simeq 10.2$ by 1993 and has remained at this mean value up to now.
The interval between the 1990 outburst and the 1995 small maxima agrees
with the orbital period${}\sim 1930$~day (Miko{\l}ajewski et
al.\ 1998). At present the orbital phase may be the same as in 1990 and
1995, but the object is not in outburst now.

In this paper, we report time-resolved CCD observations of MWC\,560 to
detect flickering. The CCD photometric observations were done on 13
nights using ST-7E (unfiltered or $R_c$-filtered) and ST-7
(unfiltered) cameras attached to a 25-cm Schmidt--Cassegrain telescope
at Kyoto University and an $R_c$-filtered PixelVision camera (SITe
SI004AB chip, CryoTiger-cooled) attached to a 60-cm reflector at Ouda
Station (Ohtani et al.\ 1992). The exposure times were 15~s and 5~s,
respectively. The images were dark-subtracted, flat-fielded and
analyzed with the Java$^{\rm TM}$-based aperture photometry package
developed by one of the authors (TK) and with IRAF APPHOT package (IRAF
is distributed by National Optical Astronomy Observatories in U.S.A.),
respectively. We determined the differential magnitude of MWC\,560
using the comparison star GSC 5396.1090 (Tycho $V=10.66$, $B-V=1.73$,
so $R_c\simeq 9.7$, Skiff, 1998) whose constancy was confirmed with the
check star GSC 5396.491. Table~1 is a summary of the observations.

$R_c$-magnitude of MWC\,560 is about 9\fmm8. Although the nightly average
magnitudes seem to show slight variability both of the $R_c$-filtered
and unfiltered data sets, more observation is needed to confirm this.

\begin{table}
\caption{Summary of observations}
\begin{center}
\begin{tabular}{crrccrc}
\hline
Start / end & \lower6pt\hbox{$N^a$} & Exp & \lower6pt\hbox{Filter} & $\sigma^b$ & $\Delta{\rm mag}^c$ & Site$^d$ \\[-4pt]
(JD${}-{}$2450000) & & (sec) & & (mag) & (mag) & \\
\hline
1834.191 / .311 & 637 & 5.0 & $R_c$ & 0.037 & \,0.383 & O \\
1839.240 / .303 & 295 & 15.0 & none & 0.027 & $-$0.024 & K1 \\
1844.242 / .283 & 401 & 5.0 & $R_c$ & 0.020 & \,0.427 & O \\
1849.314 / .332 & 55 & 5.0 & $R_c$ & 0.037 & \,0.450 & O \\
1850.295 / .359 & 310 & 5.0 & $R_c$ & 0.025 & \,0.408 & O \\
1852.280 / .327 & 471 & 5.0 & $R_c$ & 0.026 & \,0.272 & O \\
1855.205 / .245 & 220 & 15.0 & $R_c$ & 0.043 & \,0.193 & K2 \\
1858.279 / .352 & 59 & 15.0 & $R_c$ & 0.064 & \,0.250 & K2 \\
1871.120 / .251 & 469 & 15.0 & none & 0.077 & $-$0.084 & K2 \\
1872.124 / .372 & 975 & 15.0 & none & 0.045 & $-$0.004 & K2 \\
1873.144 / .369 & 961 & 15.0 & none & 0.016 & $-$0.091 & K2 \\
1874.161 / .367 & 810 & 15.0 & none & 0.031 & $-$0.091 & K2 \\
1875.134 / .371 & 832 & 15.0 & none & 0.052 & $-$0.087 & K2 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{cl}
$^a$ & number of frames \\
$^b$ & standard deviation of differential magnitudes of 
the comparison star: \\
& comparison${}-{}$check \\
$^c$ & nightly averaged differential magnitude of MWC\,560 \\
$^d$ & O: at Ouda, K1: at Kyoto with ST-7, K2: at Kyoto with ST-7E \\
\end{tabular}
\end{center}
\end{table}

\IBVSfig{21cm}{5028-f1.ps}{Light curve and power spectrum on individual night data}

\vskip-11pt
We analyzed the data set observed on six nights at Kyoto using DFT
(discrete Fourier transform) algorithm. The light curves used for DFT
analysis and the results are shown in Fig.~1. The abscissa is time in
Julian Date or frequency, and the ordinate is differential magnitude or
relative power. These spectra are shown at the frequencies higher than
that corresponding to the period equal to the half-duration of each
observational run. The arrows in Fig.~1 are put for help and do not
show precise peaks. 1 sigma of the comparison is about 0\fmm02--0\fmm08,
so the variation is real.

As shown in Fig.~1 left, we detect quasi-periodic modulation from
minutes to hours with typical amplitude about 0\fmm2. Fig.~1 right
shows some peaks from 20~min to 2~hours, but there is no peak coherent
over the whole data set. The resultant time scales of variability and the
apparent lack of rigid coherence have confirmed the findings in
previous studies: Michalitsianos et al.\ (1993) found quasi-periodic
variations with periods of 24, 35, and 58~min, superposed on hourly
variability. Tomov et al.\ (1996) detected a periodicity of 70~min
which was coherent over a few days. Dobrzycka et al.\ (1996) obtained
22~min period.

Our observation seems to suggest that the general characteristics of
short-term variability in V694 Mon have been stable nearly a decade.
The lack of coherence suggests that these dominant oscillations are
quasi-periodic, rather than strictly periodic as expected from the
magnetic rotator model (Tomov et al.\ 1992; Shore et al.\ 1994). We
are going to continue this observation in order to clarify the
nature of these periodicities.

\smallskip

We thank to K. Matsumoto and H. Iwamatsu for helping our observations.
And we acknowledge editors' helpful suggestions. 


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