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\IBVShead{4944}{25 August 2000}

\IBVStitle{\boldmath{$BVRI$} Observations of V503 Cygni in Superoutburst}


\IBVSauth{Spogli, Corrado$^1$; Fiorucci, Massimo$^1$; Raimondo, Gabriella$^2$}

\IBVSinst{Osservatorio Astronomico, Universit\`a di Perugia, Via A. Pascoli
I-06100 Perugia, Italy}
\IBVSinst{Teramo Astronomical Observatory, Collurania, Teramo, Italy}

\SIMBADobj{V503 Cyg}
\IBVStyp{ CWA }
\IBVSkey{photometry}
\IBVSabs{In this paper we report BVRI observations of the dwarf nova}
\IBVSabs{V503 Cyg during a superoutburst}

\begintext

Dwarf novae (DNe) are a subclass of cataclysmic variables
characterized by the presence of sudden increases of brightness
(outbursts) in the optical light curve.  Normal outbursts have
amplitudes in the range 2--5 mag  and they  occur at irregular intervals
of time, typically ranging from about ten days to some months. Some
dwarf novae show ``superoutbursts'' in the light curve, which are
characterized by a larger amplitude and a longer duration than normal
outbursts.  Superoutbursts occur at regular, but not strictly periodic
intervals of time (see, e.g., Osaki 1996).

Since the behaviour of most DNe is unpredictable, it is very difficult
for astronomers to monitor these variables systematically.  During DN
outburst, the rise is very rapid (typically less than a day), the
maximum stands for 2-3 days, and the decline has a typical duration of
4-5 days (Szkody and Mattei, 1984) but it can be as long as a month or
more (Warner 1995). Therefore, for the observation of DNe during all
the outburst cycle, we need a total availability of the telescope for a
considerable amount of time. For this reason most of the optical
observations of DNe were carried out by amateur astronomers through
visual estimations or, recently, with small telescopes equipped with
CCD cameras.  However, the important data so collected are generally
unfiltered or obtained with only one filter.

Multi-band monitoring is of special interest in order to extend the 
work done by amateurs, to study the spectral behavior of the optical 
continuum, and to explore the physics of accretion disks.  
For this reason in a previous paper (Spogli et al.\ 1998) 
we reported $BVR_cI_c$ observations of a small sample of dwarf novae 
during the descending phase after an outburst. The most relevant 
conclusion was that the optical spectral distribution of DNe sometimes 
cannot be well described by steady-state, optically thick, 
accretion disk models. 

V503 Cygni is a dwarf nova of the SU Ursae Majoris class, characterized
by short outbursts lasting $\simeq3$ days and recurring with a period
$\simeq 30$ days, and superoutbursts lasting $\simeq 10$ days and
spaced by $\simeq 88$ days (see, e.g., Harvey et al.\ 1995).  Rises to
maximum light take only $\simeq 1$ day for both types of eruption.
During quiescence a periodic signal of very large amplitude is evident,
with a period of 109 minutes (Szkody et al.\ 1989, Szkody \& Howell
1993).  Superhumping is also evident during superoutbursts, with a
peak-to-trough amplitude of 0.1 mag and a period of 116.7 minutes
(Harvey et al.\ 1995).

We observed this DN at the Astronomical Observatory of
Collurania-Teramo during August-September 1998.  The observations were
taken with the 0.72-m Ritchey--Chr\'etien reflector, equipped with a
Tektronix 512 CCD camera and $B$, V (Johnson), $R_c$, $I_c$ (Cousins)
filters.  Each CCD frame was corrected using bias and dark frames
obtained before and after each series of $BVR_cI_c$ exposures. The
usual flat-field correction was obtained with twilight sky flat
fields.

The CCD frames were processed with MIDAS using typical photometry
packages that calculate the instrumental magnitudes through synthetic
aperture photometry.  Although the telescope scale was relatively
small (0.5 arcsec/pixel), all the magnitudes reported in this paper
have been obtained using an aperture radius of 4 arcsecs for sake 
of homogeneity since this is the same we used in Spogli et al.\ (1998).
This aperture size is equal to about four times the typical image FWHM and, 
since we used the same value for the DN and comparison standard stars, no 
aperture corrections were necessary.

\begin{table}
\caption{$BVR_cI_c$ magnitudes of the selected comparison stars}
\begin{center}
\begin{tabular}{ccccc}
\hline
No. & $B$  & $V$ &  $R_c$ & $I_c$  \\
\hline
 1  &  15.15${}\pm{}$0.05 & 14.22${}\pm{}$0.04 & 13.72${}\pm{}$0.04 & 13.25${}\pm{}$0.04\\
 3  &  15.82${}\pm{}$0.06 & 14.95${}\pm{}$0.04 & 14.41${}\pm{}$0.04 & 13.90${}\pm{}$0.04\\
 4  &  11.75${}\pm{}$0.05 & 11.12${}\pm{}$0.04 & 10.78${}\pm{}$0.04 & 10.50${}\pm{}$0.04\\
 5  &  13.98${}\pm{}$0.05 & 13.33${}\pm{}$0.04 & 12.89${}\pm{}$0.04 & 12.45${}\pm{}$0.04\\
\hline
\end{tabular}
\end{center}

\smallskip

\caption{$BVR_cI_c$ magnitudes of V503 Cyg}
\begin{center}
\begin{tabular}{ccccc}
\hline
JD & \lower6pt\hbox{$B$}  & \lower6pt\hbox{$V$} & \lower6pt\hbox{$R_c$} & \lower6pt\hbox{$I_c$} \\[-3pt]
(2451000\,+) &  & & &  \\
\hline
  52.4199&13.89${}\pm{}$0.03&13.82${}\pm{}$0.02&13.66${}\pm{}$0.05&13.67${}\pm{}$0.02\\
  53.3317&13.80${}\pm{}$0.02&13.75${}\pm{}$0.01&13.61${}\pm{}$0.05&13.64${}\pm{}$0.02\\
  53.4180&13.76${}\pm{}$0.01&13.67${}\pm{}$0.03&13.58${}\pm{}$0.05&13.56${}\pm{}$0.06\\
  56.2881&14.02${}\pm{}$0.04&13.94${}\pm{}$0.02&13.76${}\pm{}$0.01&13.78${}\pm{}$0.02\\
  56.4324&14.12${}\pm{}$0.04&14.01${}\pm{}$0.01&13.94${}\pm{}$0.02&13.92${}\pm{}$0.01\\
  57.3275&14.40${}\pm{}$0.01&14.19${}\pm{}$0.02&14.03${}\pm{}$0.01&14.16${}\pm{}$0.01\\
  57.4824&14.39${}\pm{}$0.04&14.33${}\pm{}$0.02&14.20${}\pm{}$0.01&14.24${}\pm{}$0.03\\
  58.2997&14.28${}\pm{}$0.03&14.28${}\pm{}$0.02&14.16${}\pm{}$0.02&14.22${}\pm{}$0.01\\
  58.4364&14.45${}\pm{}$0.01&14.37${}\pm{}$0.02&14.23${}\pm{}$0.02&14.19${}\pm{}$0.02\\
  59.2819&14.33${}\pm{}$0.03&14.29${}\pm{}$0.02&14.18${}\pm{}$0.02&14.19${}\pm{}$0.03\\
  59.4048&14.49${}\pm{}$0.04&14.40${}\pm{}$0.01&14.25${}\pm{}$0.01&14.26${}\pm{}$0.01\\
  60.3307&14.57${}\pm{}$0.05&14.61${}\pm{}$0.01&14.40${}\pm{}$0.01&14.37${}\pm{}$0.05\\
  63.2908&14.90${}\pm{}$0.02&14.83${}\pm{}$0.01&14.64${}\pm{}$0.02&14.65${}\pm{}$0.04\\
  67.2803&18.10${}\pm{}$0.03&17.71${}\pm{}$0.02&17.44${}\pm{}$0.01&17.64${}\pm{}$0.02\\
\hline
\end{tabular}
\end{center}
\end{table}

We performed differential photometry using some comparison stars
present in the field and reported by Misselt (1996).  In order to check
the $B,V,R_c$ calibrations and to obtain $I_c$ secondary standard
sequences, the comparison stars were calibrated by observing, on
photometric nights, several standard stars (Landolt 1992) having $B-V$
from $-0.2$ to 1.4, over a wide range of airmasses.  The standard
magnitudes of the comparison stars reported in Table 1 are the weighted
means of the values obtained during at least three photometric nights.
The serial numbers reported are as in Misselt (1996).  Considering the standard
deviation, our data are in agreement with the measurements carried out
by Misselt (1996), Henden \& Honeycutt (1997) and Harvey et
al.\ (1995): the differences are always within two standard
deviations.  Moreover we have included the calibration for the $I_c$
filter.

We observed V503 Cyg during the maximum and the phase of decline from a
superoutburst. Probably the superoutburst started less than a day
before our first observation, and the observed maximum was $V \simeq
13.7$ (JD 2451052), a value which can be considered quite typical.
During the night are well evident variations probably due to
superhumping.  In Table 2 we report the $BVR_cI_c$ magnitudes, while the
$V$ light curve can be found in Fig.~1.

\IBVSfig{10cm}{4944-f1.ps}{$V$ light curve of V503 Cyg.}

During the first week after the maximum (from JD 2451053 to 2451060) the 
mean colour indices were $B-V=0.1$, $V-R_c=0.1$, $V-I_c=0.1$, and the 
light curve showed on average a linear decay with the following rates: 
$$\begin{array}{r@{}l}
{\rm d}m(B)/{\rm d}t & {}= 0.11 \pm 0.01~{\rm mag/day} \\
{\rm d}m(V)/{\rm d}t & {}= 0.11 \pm 0.01~{\rm mag/day} \\
{\rm d}m(R_c)/{\rm d}t & {}= 0.10 \pm 0.01~{\rm mag/day} \\
{\rm d}m(I_c)/{\rm d}t & {}= 0.10 \pm 0.01~{\rm mag/day} \\
\end{array}$$

These values are quite similar to those recorded in the superoutbursts
for AL Com, WZ Sge (Patterson et al.\ 1996), 
and V660 Her (Spogli et al.\ 1998). 
After this phase the decline was very rapid and in September 10th we 
observed the minimum ($V \simeq 17.7$). 

Our current knowledge and understanding of the physical processes, that
form the basis of the DN outburst mechanism, are still subject to a lot
of controversy. Although disk-instability models are the most favoured
models, some aspects are still unclear and more observations are
required.  $BVR_cI_c$ observations of dwarf novae allow to evaluate the
optical spectral behaviour and, therefore, they can be used as a test
to compare theoretical models of accretion disk emission. In particular
they can be used to verify the often quoted theoretical flux
distribution of a stationary (infinitely) large accretion disk whose
surface elements radiate as a black body spectra
($F(\nu)\propto\nu^{1/3}$, see, e.g., Warner 1995).  In disk
instability models the accretion disk can accumulate a certain amount
of gas before it gets unstable. When instability is reached, the
accretion of matter to the white dwarf increases dramatically and this
explains the outburst.  When the accretion disk has lost enough mass,
it becomes stable again and the dwarf nova returns to minimum magnitude
(see, for example, Cannizzo et al.\ 1986).  One important feature of
the disk instability picture is that the disk is never in a steady
state: the quiescent disk is rarely close to steady-state, with
temperature distribution usually flatter than $T(R) \propto R^{-3/4}$,
where $R$ is the radial distance from the center of the disk. Systems in
eruption more closely resemble, but never achieve, the steady-state
temperature distribution (Robinson et al.\ 1999; Kenyon 1999).  If the
temperature distribution is flatter, then the disk spectrum is
naturally flatter and the predicted spectral slope is $\alpha \leq
0.33$.

To study the behaviour of the optical continuum of the observed DN
during the outburst, we converted the $BVR_cI_c$ magnitudes in fluxes
using the conversion factors reported by Bessell (1979).  For V503 Cyg
we can neglect the extinction coefficient (Szkody 1985).  Using the
flux values so obtained we note that at the minimum the spectral
distribution is dominated by the emission of the secondary star, while
at the maximum the spectral distribution follows a power law
($F(\nu)\propto\nu^{\alpha}$).  For V503 Cyg we obtain a spectral slope
$\alpha$ in the range between 0.5 and 0.8 during the outburst, a value
that is greater than expected.

Our data confirm that for some sources the steady-state accretion disk
models do not provide a good representation of the optical continuum
during the outburst (see, for example, Spogli et al.\ 1993, 1998).
This is a clear evidence about our poor knowledge of the true radiative
transfer solution in accretion disk, and more efforts must be made to
obtain multi-wavelength observations of DNe.

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\endreferences

\end{document}

