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\IBVShead{5453}{2 October 2003}


\IBVStitle{The R Band Light Curves of Eclipses of U~G\lowercase{em} in Outburst}


\IBVSauth{\v{S}IMON, V.}

\IBVSinsto{Astronomical Institute, Academy of Sciences of the Czech Republic, 251~65 Ond\v{r}ejov, Czech Republic; e-mail: simon@asu.cas.cz}

\SIMBADobj{U Gem}

\begintext


     U~Gem is one  of just a few known eclipsing dwarf novae (e.g. Krzeminski, 
1965). Large variations of the eclipse  profiles  in  quiescence and outburst 
offer a  unique  opportunity  to  study  the  changes  in  the  disk  and the 
interaction  of  the  disk  with the  mass  stream  (bright  spot).  Here  we 
report  an  analysis of  the  $R$  band  CCD  observations  of eclipses which 
occurred  during  the peak of the outburst. As far as we know, no photometric 
observations of eclipses  of U~Gem in the red spectral region during outburst 
were published previously.

%\vskip 0.4cm

     The $R$-filter  CCD images  were  obtained by Maksutov 180/1000~mm, SBIG 
ST-6 in the  Astronomical  Institute in Ond\v{r}ejov during the February~2002 
outburst  and  covered three eclipses. The exposure times were 90 and 70 sec. 
The star USNO-A2.0~1050-05472483  was  used as  the  comparison  star  while 
USNO-A2.0~1050-05473287  served  as  the  check   star.  The  variable,  the 
comparison  star  and the check star were placed on the same image. These CCD 
data are available at the IBVS website as {\tt 5453-t2.txt}. 

\IBVSedata{5453-t2.txt}
%\IBVSdataKey{5453-t2.txt}{U Gem}{photometry}

     The position  of our  CCD observations  is shown in Fig.\,1. The one-day 
means of the visual observations  from the AFOEV  database  (CDS, Strasbourg, 
France) are superimposed and  show that the CCD data were obtained during the 
peak of the outburst. 


\IBVSfig{6cm}{5453-f1.eps}{The profile of the February 2002 
outburst  of U~Gem. The empty circles represent the one-day 
means  of  the  AFOEV  visual  observations.  The  vertical 
lines mark  the  centers  of the series of the $R$ band CCD 
observations. }
%%\IBVSfigKey{5453-f1.eps}{U Gem}{light curve}

\IBVSfig{10cm}{5453-f2.eps}{The  profiles  of  the  eclipses 
observed in the $R$ band during the top of the February 2002 
outburst  of  U~Gem. The  relative  magnitudes  for both the 
variable and the check star are plotted. }
%%\IBVSfigKey{5453-f2.eps}{U Gem}{light curve}

\IBVSfig{9cm}{5453-f3.eps}{The details of the profiles of the 
eclipses observed during the February 2002 outburst of U~Gem. 
The  number  at  each  curve  refers to the label in Fig.\,2. 
The smooth  curves  represent  the  fits  by  the code {\tt HEC13}. 
The  ephemeris  of Smak  (1993), valid  for the moment of the 
mid-eclipse of the bright spot in quiescence, was used. }
%%\IBVSfigKey{5453-f3.eps}{U Gem}{light curve}


     The profiles of  the  eclipses  observed  in the  $R$ band  are shown in 
Fig.\,2. The  first  eclipse  might occur  during  the  very late rise to the 
outburst maximum  because  a trend of an increasing brightness is superposed. 
The depth of  the eclipses  is very  small, of  the  order  of 0.05~mag($R$). 
The light  curves  were  therefore  smoothed  by  the  code {\tt HEC13}, written by 
Dr.~P.~Harmanec and based on the  method of Vondr\'{a}k (1969 and 1977). This 
method  can  fit  a smooth  curve  to  a complicated  course of the data. The 
descending and ascending branches of eclipses 1 and 3 were fitted separately.

     The fits to  the profiles of the individual eclipses are shown in detail 
in Fig.\,3. The orbital  phases were calculated according to the ephemeris of 
Smak  (1993): \\$T_{\rm  min}~=  2\,437\,638.82627~+~0.1769061898~E$,  which is 
valid for the moment of the mid-eclipse of the bright spot in quiescence.

     We determined the  timings  of  the  three  eclipses  by  the  method of 
bisectors to  take  the  asymmetry  of the  profiles into account. The minima 
determined  from  the  observed  and  smooth  curves, representing mainly the 
middle  part  of the  eclipse  depth, were in good agreement. A difference of 
approx. $-0.005$  days  was  found  only for the first eclipse. The values of 
$O-C$ in Table~1 were calculated using the ephemeris of Smak (1993).


\begin{table}[!hb]
\centerline{\normalsize Table 1. Minima timings of U~Gem determined from the smooth curves}
\vskip 3mm
\begin{center}
\begin{tabular}{ccc}
\hline
      JD       &  Epoch  &  $O-C$ (days)  \\
\hline
2452321.3327   &  82996  &    0.0003      \\
2452321.5097   &  82997  &    0.0004      \\
2452322.3960   &  83002  &    0.0022      \\
\hline
\end{tabular}
\end{center}
\end{table}


     In quiescence, the deep, narrow eclipse  of U~Gem  with  a steep ingress 
and egress (Krzeminski, 1965) is caused  by an occultation of the bright  spot 
by the secondary star while the white dwarf and the inner disk region are not 
eclipsed  (Warner  and  Nather,  1971). On  the  contrary, the profiles of the 
eclipses obtained at the peak of the 2002 outburst in the $R$ band  appear to 
be very broad, often with  extended wings. In addition, there is no sign of a 
bump characteristic  for the interval of the  best  visibility of the  bright 
spot, which  is  very  prominent  in  quiescence. This  behaviour  bears some 
resemblance to  that  observed  in  the $V$ band by Naylor and la~Dous (1997) 
during the 1995 April outburst.

     Fig.\,3 shows  that  the full  width  of eclipse can  be as large as 0.2 
phases. The  very  broad wings of  the  eclipses  are  characteristic  for an 
eclipse of an expanded, highly brightened  accretion  disk. Nevertheless, the 
phases of mid-eclipse (Table~1)  are close to the phase of the eclipse of the 
spot, not  of  a disk. The  moment  of the  superior conjunction of the white 
dwarf in U~Gem (i.e. when the  mid-eclipse of the disk should occur) advances 
the moment of the mid-eclipse of the bright spot by  $0.025\pm0.005$ phase in 
quiescence (Smak,  2001). The moment of minimum occurs earlier during outburst 
than in  quiescence  (Krzeminski,  1965). However, our  observations show that 
the minima during outburst occur systematically  later  than suggested by the 
ephemeris of Smak  (1993), and  later  than the course of the $O-C$ values in 
his Fig.\,1. This can be explained  by a recent increase of the period length 
$P_{\rm orb}$. We note that the minima observed by Naylor and la~Dous (1997), 
when recalculated  according  to the  ephemeris  of Smak (1993), have negative 
$O-C$'s during the 1995 April outburst and $O-C\approx0.0012$~day in the 1996 
February quiescence.

     Significant  cycle-to-cycle  changes of the modulation, mainly the slope 
of the ascending branches of the consecutive eclipses  1 and 2  and a sharper 
bottom part of eclipse 2, were detected. A similar asymmetry is apparent also 
in one  night  of the  1995  April  outburst (Naylor and la~Dous, 1997). Small 
changes of  the  disk profile  can give  rise to  the  asymmetry of a grazing 
eclipse. A comparison  with  the  in-outburst  Doppler  tomography  of  He~II 
$\lambda$4686  (Groot, 2001) reveals  that  the  part  of  the disk on which a 
spiral  shock arm is developing lies between the white dwarf and the observer 
between phases \mbox{$\sim0.0-0.15$.} Large  changes  of  the  profile  of the disk, 
connected with  a  non-uniform   brightness  distribution, may  occur  there. 
Another possibility could be a deformation of  the  disk around the impact of 
the stream (emerging from eclipse), caused by an increase of the mass outflow 
from  the  donor  star  due  to  an irradiation  during outburst (Smak, 1995). 
Nevertheless, the  spiral  arms are  a more  likely  explanation because they 
appear  sooner  than the  heating  of  the  donor  (Fig.\,2  in  Groot, 2001). 
Variable, vertically  extended  structures of unclear origin are really known 
to be present in U~Gem and to give rise to the X-ray dips at  phases 0.15 and 
0.7 (Mason  et al., 1988) and  UV dips at phase 0.1 (Naylor and la~Dous, 1997). 
The spiral structure of the disk may be the common  cause  of the  variations 
observed  in  various  spectral  regions  during  outburst  but  simultaneous 
multiwavelength  observations are highly desired in this respect.



\vskip 0.8cm

\noindent
Acknowledgements:  
This  research  has made use of  NASA's  Astrophysics  Data System Abstract 
Service, and the  AFOEV database, operated at CDS, France. I am indebted to 
Dr.~Harmanec  for providing me with the program  {\tt HEC13}. The  support by the 
project ESA PRODEX INTEGRAL 14527 is acknowledged. 




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\endreferences

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