\documentstyle[twoside, epsf]{article}
 
\input ibvs2.sty 
 
\begin{document} 
\IBVShead{5365}{21 January 2003} 
 
\IBVStitle{On the Variability of the Dwarf Nova EM Cygni}  

\IBVSauth{Spogli, Corrado$^1$; Fiorucci, Massimo$^1$; Raimondo, Gabriella$^2$} 
 
\IBVSinst{Department of Physics and Astronomical Observatory, Perugia University, Perugia, Italy} 
\IBVSinst{INAF-Teramo Astronomical Observatory, Collurania, Teramo, Italy} 
 
\SIMBADobj{EM Cyg}

\IBVStyp{ UGZ } 
\IBVSkey{photometry} 

\IBVSabs{In this paper we report BVRI observations of EM Cyg obtained in}
\IBVSabs{the years 1997, 1998 and 2000.} 
\IBVSabs{The optical flux distribution during all the outburst cycle} 
\IBVSabs{can be described as the emission of the secondary 4000 K star}
\IBVSabs{plus a steady-state accretion disk.}


\begintext
 
Dwarf novae (DNe) are a type of cataclysmic binary stars that undergo quasi
periodic  eruptions of a few magnitudes. In EM Cyg the increase in brightness
is of about 2~mag, from $V\simeq14\fmm4$ to $V\simeq12\fmm5$ (Downes \& Shara 
1993), and generally occurs every 20-30 days.
The variable is both an eclipsing binary and a double-lined spectroscopic binary. 
Initially it was classified as a nova-like, but successive observations allowed the 
classification to be refined and EM Cyg was included in the class of DNe. 
The usually regular eruption cycle is occasionally interrupted by 
irregular low-amplitude fluctuations in brightness which suggested that it 
belongs to the Z Cam type subclass (see Downes \& Shara 1993).
As usual for this class of objects, the infrared emission of EM Cyg is dominated
by the late-type (K2V) secondary star (Jameson et al. 1981), whereas optical 
data suggest that the primary's light is dominated by the contributions of 
accretion disk and bright spot.

From measurements of the radial velocity variations, Robinson (1974) was able
to derive the orbital elements and the masses of the components of the binary
system. However, the spectrum of the secondary star was heavily veiled by the
strong continuum arising from the accretion disk, and the measured radial
velocities were of relatively low accuracy.
More accurate results were obtained by Stover et al. (1981), who estimated
the orbital period (P=6.98 h), the masses of the accreting white dwarf and the
mass-losing secondary star: $M_1 = (0.56\pm0.05) M_\odot$ and
$M_2 = (0.76\pm0.10) M_\odot$.
Recently, North et al. (2000) discovered that the spectrum is contaminated by
light from a K2-5 V star, in addition to the K-type mass donor star. They
revised the value of the mass ratio that combined with the orbital inclination
$i=67\deg\pm2\deg$ leads to masses of $M_1 = (1.12\pm0.08) M_\odot$ and
$M_2 = (0.99\pm0.12) M_\odot$.


\begin{table}
\centerline{{\bf Table~1:} $BV(RI)_C$ magnitudes of EM Cyg}
\begin{center}
\begin{tabular}{cccccc}
\hline
\hline
Date & JD (2450000+) &$B$        & $V$       &      $R_C$     &       $I_C$        \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
28/06/97 & 628.585 & 13.41 $\pm$ 0.11 & 12.95 $\pm$ 0.05 & 12.59 $\pm$ 0.05 &       	      \\
02/07/97 & 632.550 & 13.88 $\pm$ 0.09 & 13.20 $\pm$ 0.07 & 12.72 $\pm$ 0.05 & 12.32 $\pm$ 0.05\\
06/07/97 & 636.537 & 14.05 $\pm$ 0.09 & 13.45 $\pm$ 0.03 & 12.93 $\pm$ 0.05 & 12.34 $\pm$ 0.04\\
07/07/97 & 637.547 & 14.01 $\pm$ 0.08 & 13.40 $\pm$ 0.02 & 12.91 $\pm$ 0.04 & 12.33 $\pm$ 0.04\\
08/07/97 & 638.565 & 14.26 $\pm$ 0.08 & 13.58 $\pm$ 0.03 & 13.04 $\pm$ 0.04 & 12.42 $\pm$ 0.04\\
09/07/97 & 639.551 & 14.06 $\pm$ 0.08 & 13.48 $\pm$ 0.03 & 12.92 $\pm$ 0.04 & 12.35 $\pm$ 0.04\\
10/07/97 & 640.562 & 14.21 $\pm$ 0.08 & 13.45 $\pm$ 0.04 & 13.03 $\pm$ 0.04 & 12.43 $\pm$ 0.04\\
11/07/97 & 641.529 & 14.14 $\pm$ 0.10 & 13.50 $\pm$ 0.03 & 12.95 $\pm$ 0.03 & 12.40 $\pm$ 0.04\\
12/07/97 & 642.541 & 14.11 $\pm$ 0.09 & 13.57 $\pm$ 0.04 & 12.98 $\pm$ 0.03 & 12.38 $\pm$ 0.04\\
25/07/97 & 655.441 & 12.52 $\pm$ 0.08 & 12.35 $\pm$ 0.04 & 12.12 $\pm$ 0.03 & 11.72 $\pm$ 0.04\\
26/07/97 & 656.443 & 12.64 $\pm$ 0.08 & 12.45 $\pm$ 0.03 &       	    &                 \\
28/07/97 & 658.446 & 12.99 $\pm$ 0.06 & 12.72 $\pm$ 0.02 & 12.42 $\pm$ 0.04 & 11.96 $\pm$ 0.04\\
30/07/97 & 660.455 & 13.50 $\pm$ 0.08 & 13.17 $\pm$ 0.02 & 12.79 $\pm$ 0.04 & 12.28 $\pm$ 0.05\\
01/08/97 & 662.421 & 13.95 $\pm$ 0.08 & 13.23 $\pm$ 0.03 & 12.80 $\pm$ 0.04 & 12.30 $\pm$ 0.04\\
04/08/97 & 665.412 & 14.40 $\pm$ 0.08 & 13.81 $\pm$ 0.03 & 13.24 $\pm$ 0.05 & 12.65 $\pm$ 0.05\\
07/08/97 & 668.419 & 14.20 $\pm$ 0.11 & 13.45 $\pm$ 0.05 & 13.10 $\pm$ 0.05 & 12.50 $\pm$ 0.05\\
14/08/97 & 675.405 & 12.97 $\pm$ 0.05 & 12.65 $\pm$ 0.04 & 12.35 $\pm$ 0.03 & 11.95 $\pm$ 0.05\\
18/08/97 & 679.399 & 14.27 $\pm$ 0.04 & 13.54 $\pm$ 0.03 & 13.05 $\pm$ 0.03 & 12.59 $\pm$ 0.04\\
20/08/97 & 681.395 & 14.15 $\pm$ 0.11 & 13.57 $\pm$ 0.03 & 13.15 $\pm$ 0.03 & 12.53 $\pm$ 0.05\\
22/08/97 & 683.391 & 14.18 $\pm$ 0.10 & 13.46 $\pm$ 0.04 & 12.98 $\pm$ 0.03 & 12.55 $\pm$ 0.04\\
29/08/97 & 690.384 & 13.55 $\pm$ 0.08 & 13.14 $\pm$ 0.02 &       	    &      	      \\
17/09/97 & 709.349 & 14.34 $\pm$ 0.11 & 13.59 $\pm$ 0.03 & 13.03 $\pm$ 0.03 & 12.45 $\pm$ 0.05\\
18/09/97 & 710.350 & 14.14 $\pm$ 0.09 & 13.50 $\pm$ 0.02 & 13.00 $\pm$ 0.03 & 12.39 $\pm$ 0.05\\
24/09/97 & 716.325 & 12.99 $\pm$ 0.05 & 12.67 $\pm$ 0.02 & 12.38 $\pm$ 0.03 & 11.95 $\pm$ 0.05\\
04/10/97 & 726.308 & 14.04 $\pm$ 0.08 & 13.41 $\pm$ 0.02 & 12.95 $\pm$ 0.03 & 12.37 $\pm$ 0.04\\
05/10/97 & 727.321 & 14.12 $\pm$ 0.13 & 13.48 $\pm$ 0.04 &       	    &      	      \\
16/10/97 & 738.253 & 13.55 $\pm$ 0.06 & 13.07 $\pm$ 0.03 &       	    &      	      \\
17/10/97 & 739.329 & 12.51 $\pm$ 0.05 & 12.31 $\pm$ 0.03 & 12.08 $\pm$ 0.03 & 11.70 $\pm$ 0.04\\
18/10/97 & 740.285 & 12.35 $\pm$ 0.06 & 12.20 $\pm$ 0.03 & 11.98 $\pm$ 0.04 & 11.62 $\pm$ 0.04\\
19/10/97 & 741.286 & 12.61 $\pm$ 0.06 & 12.39 $\pm$ 0.03 & 12.19 $\pm$ 0.04 & 11.80 $\pm$ 0.05\\
22/10/97 & 744.282 & 13.05 $\pm$ 0.06 & 12.77 $\pm$ 0.04 & 12.45 $\pm$ 0.03 & 12.02 $\pm$ 0.04\\
\hline
31/08/98 & 1057.450 & 13.52 $\pm$ 0.04 & 13.03 $\pm$ 0.04 & 12.61 $\pm$ 0.04 & 12.27 $\pm$ 0.05\\
01/09/98 & 1058.416 & 13.54 $\pm$ 0.04 & 13.08 $\pm$ 0.05 & 12.58 $\pm$ 0.04 & 12.18 $\pm$ 0.05\\
02/09/98 & 1059.333 & 13.07 $\pm$ 0.04 & 12.74 $\pm$ 0.04 & 12.46 $\pm$ 0.04 &       	       \\
\hline
19/08/00 & 1776.357 & 13.45 $\pm$ 0.04 & 13.04 $\pm$ 0.04 & 12.72 $\pm$ 0.04 & 12.18 $\pm$ 0.04\\
20/08/00 & 1777.385 & 13.37 $\pm$ 0.04 & 12.98 $\pm$ 0.04 & 12.69 $\pm$ 0.04 & 12.21 $\pm$ 0.04\\
21/08/00 & 1778.333 & 13.60 $\pm$ 0.07 & 13.01 $\pm$ 0.05 & 12.66 $\pm$ 0.04 & 12.07 $\pm$ 0.04\\
25/08/00 & 1782.328 & 13.21 $\pm$ 0.05 & 12.83 $\pm$ 0.05 & 12.61 $\pm$ 0.05 & 12.09 $\pm$ 0.05\\
29/08/00 & 1786.331 & 13.45 $\pm$ 0.05 & 13.06 $\pm$ 0.06 & 12.68 $\pm$ 0.05 & 12.16 $\pm$ 0.05\\
11/09/00 & 1799.313 & 13.04 $\pm$ 0.09 & 12.75 $\pm$ 0.06 & 12.42 $\pm$ 0.04 & 11.98 $\pm$ 0.05\\
12/09/00 & 1800.312 & 12.77 $\pm$ 0.08 & 12.59 $\pm$ 0.04 & 12.39 $\pm$ 0.03 & 11.96 $\pm$ 0.04\\
15/09/00 & 1803.308 &                  & 13.01 $\pm$ 0.04 & 12.67 $\pm$ 0.04 & 12.14 $\pm$ 0.04\\
22/09/00 & 1810.299 & 14.22 $\pm$ 0.04 & 13.54 $\pm$ 0.04 & 13.04 $\pm$ 0.03 & 12.43 $\pm$ 0.05\\
12/10/00 & 1830.293 &                  &  	          &   	             & 11.74 $\pm$ 0.04\\
28/10/00 & 1846.246 &                  &                  &                  & 12.38 $\pm$ 0.04\\
\hline
\end{tabular}
\end{center}
\end{table}


\IBVSfig{9cm}{5365-f1.eps}{$V$ light curve of EM Cyg during the summer-autumn 
1997. Circles represent the data here reported, while the small crosses are 
visual estimates available from VSNET}
%\IBVSfigKey{5365-f1.eps}{EM Cyg}{light curve}

\vspace*{-0.4cm}

We observed this dwarf nova at the Perugia Astronomical Observatory
from June to October 1997, and from August to October 2000.
We have also observed EM Cygni at the Teramo Astronomical Observatory
in August-September 1998, for a total of 45 different observational nights. 
Table 1 shows all the $BVR_CI_C$ magnitudes with the Julian Date and the date 
of the night (not the UT date).
The results presented here are part of a project devoted to gain multi-band
light curves of a sample of DNe, with the goal of increasing the historical
database and information on this class of variable sources. In particular
we are interested in identifying the accretion disk emission during all the
outburst cycle, because DNe offer the best conditions in which to
study the uncertain physics of the disk.


All the data are obtained in the $BVR_CI_C$ Johnson-Cousins broad bands.
The instruments used and the photometric techniques have been already
described in Spogli et~al.~(1998). In this work, we used the calibration
stars reported in Misselt (1996) with the number 2, 4, 5, 8, and 11. Moreover
we calibrated these comparison stars with the $I_C$ filter by observing,
on photometric nights, several standard stars (Landolt 1992) having $(B-V)$
from $-$0.2 to 1.4, over a wide range of airmass.
The weighted means and standard deviations of the obtained values are:
$I_C(2) = 12.28 \pm 0.06$,
$I_C(4) = 12.51 \pm 0.06$,
$I_C(5) = 13.00 \pm 0.05$,
$I_C(8) = 10.99 \pm 0.05$,
and $I_C(11) = 12.12 \pm 0.08$.

Figure 1 shows the V data during the summer-autumn 1997, and the visual
estimates available from VSNET
({\tt http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/}). The comparison shows
that we have a good coverage of all the outburst phases. Table 2 remarks
the principal photometric characteristics of all the dataset: the extreme
magnitudes and the average color indices at maximum and at minimum.

\vskip 2mm
\centerline{{\bf Table 2:} Photometric characteristics of EM Cyg from our observations}
\begin{center}
\begin{tabular}{lccccccc}
\hline
\hline
          &~~~$B$~~~&~~~$V$~~~&~~~$R_C$~~~&~~~$I_C$~~~& $(B-V)_{ave}$ & $(V-R_C)_{ave}$ & $(V-I_C)_{ave}$\\
Maximum   &12.3 & 12.2 & 12.0 & 11.6 & 0.2 & 0.2 & 0.6\\
Minimum   &14.4 & 13.8 & 13.2 & 12.7 & 0.7 & 0.5 & 1.1\\
\hline
\end{tabular}
\end {center}


\IBVSfig{9cm}{5365-f2.eps}{$BVR_CI_C$ data points converted in density flux at minimum (left panel) and at maximum (right panel). Dotted lines represent the 
secondary star, while the dashed lines represent the theoretical emission of a 
steady-state accretion disk. 
The integrated light coming from the emission disk and the secondary (bold 
lines) is a rough but good approximation of the EM Cyg emission in the optical 
region}
%\IBVSfigKey{5365-f2.eps}{EM Cyg}{spectrum}

We computed the flux density of EM Cyg using the same procedure described
in Spogli et al. (1998), adopting the interstellar reddening $E(B-V)$=0.03 as 
reported by Bruch (1984). The spectral distribution is dominated by the 
emission of the secondary star during the minimum, and we tried to isolate its 
contribution with the simple assumption that the overall emission is mainly due
to the secondary and the accretion disk. In this phase we have neglected the 
presence of another red star (North et al. 2000) because the spectral types are
similar (K2-5V).
Figure 2 shows an example of decomposition of the spectral emission in two 
components.
We have considered the canonical emission of a steady-state accretion disk 
$F(\lambda) \propto \lambda^{-7/3}$, while the emission of the secondary is 
simulated via Kurucz's spectra of dwarf stars with solar metalicity 
(Kurucz 1993). The best fit in all the phases of the outburst cycle
is obtained considering the emission of a 4250 K star, 
and an accretion disk that varies of a factor ten from the passive 
cooling of the quiescence to the brightness of the outburst maximum. 
This model is obviously too simplistic and doesn't take into account all the
components of the system, but reproduces quite well the variability dynamics 
during the outburst. Another component (probably the hot spot) or a physical 
model of the accretion disk may be necessary to compensate the strong blue 
emission during the outburst.

\references
Bruch, A., 1984, {\it A\&AS}, {\bf 56}, 441
%%\BIBCODE{1984A&AS...56..441B}

Downes, R. A., Shara, M. M., 1993, {\it PASP}, {\bf 105}, 127
%%\BIBCODE{1993PASP..105..127D}

Jameson, R. F., King, A. R., Sherrington, M. R., 1981, {\it MNRAS}, {\bf 195}, 235
%%\BIBCODE{1981MNRAS.195..235J}

Kurucz, R., L., 1993, {\it ATLAS9 Stellar Atmosphere Programs and Grid (CD-ROM 13)}
%%\BIBCODE{1993KurCD..13.....K}

Landolt, A. U., 1992, {\it AJ}, {\bf 104}, 340
%%\BIBCODE{1992AJ....104..340L}

Misselt, K. A., 1996, {\it PASP}, {\bf 108}, 146
%%\BIBCODE{1996PASP..108..146M}

North, R. C., Marsh, T. R., Moran, C. K. J., et al., 2000, {\it MNRAS}, {\bf 313}, 383
%%\BIBCODE{2000MNRAS.313..383N}

Robinson, E. L., 1974, {\it ApJ}, {\bf 193}, 191
%%\BIBCODE{1974ApJ...193..191R}

Spogli, C., Fiorucci, M., Tosti, G., 1998, {\it A\&AS}, {\bf 130}, 485
%%\BIBCODE{1998A&AS..130..485S}

Stover, R. J., Robinson, E. L., Nather, R. E., 1981, {\it ApJ}, {\bf 248}, 696
%%\BIBCODE{1981ApJ...248..696S}

\endreferences


\end{document}
