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\input ibvs2.sty

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\IBVShead{5139}{17 July 2001}

\IBVStitle{A Deep Dip during an Outburst in the Old Nova, Q Cygni}

\IBVSauth{Nogami, D.$^{1,2}$; Kunjaya, C.$^{3,2}$; Kato, T.$^2$; Masuda, S.$^2$}

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

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

\IBVSinst{Dept.\ of Astronomy, Institut Teknologi Bandung, Bandung
40132, Indonesia,\\ \indent e-mail: kunjaya@sirius.as.itb.ac.id}

\SIMBADobj{Nova Cyg 1876}
\IBVStyp{ NA }
\IBVSkey{old nova, photometry}
\IBVSabs{We report time-series photometry around a 1994 brightening}
\IBVSabs{in an old nova, Q Cyg. A deep tough was caught around}
\IBVSabs{the maximum, but the mechanism is still unsolved.}

\begintext

Q Cyg is an old fast nova ($t_3\sim11$ d) which reached $V=3\mm$ at the
maximum of the 1876 outburst. In quiescence after the outburst
($V\sim15\mm$) this object has been revealed to show large-amplitude
modulations, which are called ``stunted'' outbursts by Honeycutt et al.\
(1998, and see references therein for earlier studies on this
behavior). Honeycutt (2001) listed that the mean amplitude, the
typical spacing, and the mean full-width-at-half-maximum (FWHM) of
outbursts are 1\fmm0, about 200 d, and 24 d, respectively. Although
$\sim10$ old novae other than Q Cyg is now known to exhibit such
``stunted'' outbursts (Honeycutt 2001), the mechanism is still a
mystery. To investigate the accretion disk in outburst, we carried out
time-resolved photometry during an outburst in 1994, detected by T.
Vanmunster (private communication).

We made the observations at Ouda Station, Kyoto University. A 60-cm
reflector (focal length${} = 4.8$ m) and a CCD camera (Thomson TH~7882,
$576 \times 384$ pixels with on-chip $2 \times 2$ binning) attached to
the Cassegrain focus were used (for more information of the
instruments, see Ohtani et al.\ 1992). We adopted a Johnson $V$-band
interference filter. Table 1 gives the journal of the observations.
After standard de-biasing and flat fielding, the frames were processed
by a microcomputer-based aperture photometry package developed by one
of the authors (TK).

The magnitudes of the object were measured relative to a local
standard star, Q Cyg--31 ($V=13.375$, $B-V=+0.754$) in Henden and
Honeycutt (1997). Heliocentric corrections to observed times were
applied before the following analysis. Relative magnitudes between
the comparison star and a local field star were measured to confirm
the constancy of the comparison star within 0\fmm04 during our runs
and to calculate the errors listed in Table 1.

The long-term light curve derived from our observations of this
outburst is drawn in Figure 1. This is of a typical decline shape of
outburst in Q Cyg, while the decline rate of 0.07 mag\,d$^{-1}$ is a
little large for this star (cf.\ Honeycutt et al.\ 1998). In our
observations, we could not detect any short-term periodic modulations,
such as superhumps and quasi-period oscillations (QPOs), in a period
range of 0.002--0.2 d.

\IBVSfig{10cm}{5139-f1.ps}{Light curve of Q Cyg between 1994 July 10 and
22}

\IBVSfig{10cm}{5139-f2.ps}{The deep dip observed on 1994 July 10}

\clearpage

\begin{table}
\caption{The observation summary}
\begin{center}
\begin{tabular}{rcccccc}
\hline
\lower6pt\hbox{Date} & HJD & HJD & Exposure & \lower6pt\hbox{Error$^2$} & Mean & \lower6pt\hbox{$N^4$} \\[-3pt]
 & start$^1$ & end$^1$ & time (s) & & $V$ mag$^3$ & \\

\hline
1994 July 10 & 49544.098 & 49544.200 & 300 & 0.03 & 1.038 & 17 \\
 15 & 49548.987 & 49549.106 & 100 & 0.03 & 1.398 & 87 \\
 16 & 49550.096 & 49550.132 & 180 & 0.03 & 1.461 & 16 \\
 17 & 49551.029 & 49551.131 &  90 & 0.05 & 1.553 & 82 \\
 18 & 49551.966 & 49552.130 &  90 & 0.05 & 1.580 & 134 \\
 20 & 49554.003 & 49554.255 &  90 & 0.05 & 1.609 & 149 \\
 21 & 49554.961 & 49555.269 &  90 & 0.05 & 1.628 & 230 \\
 22 & 49555.975 & 49556.059 & 100 & 0.14 & 1.605 & 47 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{l@{ }l}
$^1$ & HJD${}-{}$2400000 \\
$^2$ & Nominal error for 1 point \\
$^3$ & Magnitude relative to a local standard star ($V=13.375$) \\
$^4$ & Number of frames \\
\end{tabular}
\end{center}
\end{table}

The most remarkable feature in our data is a deep dip ($\triangle
m\simeq0.65$) with a duration of about 1 hour observed on 1997 July 10
(Figure 2). We examined the original images to reject the
possibilities of effects of clouds and other natural/artificial
factors and ensured the existence of the dip. Although two similar
dips were detected in Q Cyg by Honeycutt et al.\ (1998), the dip in
the present data seems to have a different nature than those dips
because of two reasons: 1) the duration was quite different (1 hour
versus several-tens of days), and 2) the dip in our data occurred in a
bright phase, but those dips were observed in ``quiescence'',
following two separate outbursts.

This may be an eclipse of the accretion disk by the secondary star.
This interpretation is, however, not plausible, since the low
inclination angle is implied by the fact that any orbital feature has
never been detected in spite of the long observational history of this
old nova. There is another possibility of an eclipse by the third body,
although its orbit may be needed to be more inclined from the orbital
surface of the primary and the secondary stars and the duration may be
too short. The third possibility is a transient increase of absorption
by mass sporadically ejected (cf.\ BZ Cam, Ringwald \& Naylor
1998; AT Cnc, Nogami et al.\ 1999). To solve this
difficult problem and to understand the nature of the oscillations in
old novae, we would encourage extensive time-series photometry and
spectroscopy to seek for orbital feature and an evidence of mass
ejection.

\bigskip

We thank Mr.\ Maehara and Mr.\ Baba for kind helping our observations at
Ouda Station.

\references

Henden, A. A., Honeycutt, R. K., 1997, {\it PASP}, {\bf 109}, 441

Honeycutt, R. K., 2001, {\it PASP}, {\bf 113}, 473

Honeycutt, R. K., Robertson, J. W., Turner, G. W., 1998, {\it AJ},
{\bf 115}, 2527

Nogami, D., Masuda, S., Kato, T., Hirata, R., 1999, {\it PASJ}, {\bf
51}, 115

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

Ringwald, F. A., Naylor, T., 1998, {\it AJ}, {\bf 115}, 286

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