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\IBVShead{5562}{10 September 2004}

\IBVStitle{Rapid Variations in RS~O\lowercase{ph} Observed by OMC/INTEGRAL}


\IBVSauth{\v{S}IMON, V.; HUDEC, R.; HROCH, F.}

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

\SIMBADobj{RS Oph}

\begintext
     
     RS~Oph is  a relatively  bright  symbiotic  star with the orbital period 
$P_{\rm  orb}=460$~days, the inclination angle of $30\deg-40\deg$ and 
the giant component underfilling its lobe (Dobrzycka and Kenyon 1994). RS~Oph 
is a long-period cousin of cataclysmic variables (CVs) because  in contains a 
white dwarf (WD), as suggested also from the fact that it is a recurrent nova 
with five observed explosions (e.g. Warner 1995). Beside these outbursts, the 
quiescent  brightness  fluctuates  on time  scales of months and years mostly 
between  11 and  12 mag$_{\rm vis}$, sometimes  reaching  10~mag$_{\rm  vis}$ 
(e.g. Dobrzycka and Kenyon 1994, Oppenheimer and Mattei 1996). Also the rapid 
optical  variations  on the  time  scale of tens of minutes, similar to those 
often seen  in short-period CVs, were  reported  several  times  (e.g. Walker 
1977, Dobrzycka et al. 1996).

     RS~Oph was repeatedly observed  by the  $INTEGRAL$  satellite during its 
Galactic Plane Scans (GPS). Here we report on the observations of this object 
by the  Optical  Monitoring Camera (OMC) onboard $INTEGRAL$. This instrument, 
equipped  with  Johnson  $V$ filter  and a CCD detector, is able to carry out 
rapid photometry (Mas-Hesse et al. 2003). The observing strategy used for GPS 
consists  of so-called {\it science  windows} during  which a given region of 
the sky is observed by the co-aligned instruments  for  about  1.5~hours. OMC 
obtains  a series  of images  during  this period. Most images used  for this 
study were secured with a 100 sec. exposure time. A smaller part was obtained 
with  a 30 sec. exp. time; these  measurements  often possess slightly larger 
observational errors.

     The observed light curves are displayed in Figure~1. It can be seen that 
both night  to night  and  rapid variations of brightness occurred during 
our observations. In  order to allow for a comparison of the amplitude of the 
variations  in the  individual science windows (hereafter abbreviated as sets 
ABCDE),  the  magnitudes  were  transformed  into  intensities,  setting  the 
intensity equal to unity at 11.4~mag($V$).

     A search for the cycle-lengths in the OMC data set was carried out using 
weighted wavelet Z-transform  (WWZ) (Foster 1996). This  method, based on the 
Morlet  wavelet, enables  one to  determine  period and amplitude of unevenly 
sampled  time  series. WWZ  indicates  whether  or not  there  is  a periodic 
fluctuation  at  a given  time  at a given  frequency and is a measure of the 
confidence of a frequency at a given moment. This method is thus suitable for 
a search  for the periods which undergo variations during the observation. It 
can be used for discovering possible temporarily existing periods in  RS~Oph. 
The value of the parameter $c$  which  determines how rapidly  the  analyzing 
wavelet  decays  was set to 0.0125, which is commonly used  for variable star 
light  curves. WWZ  transform could be made for sets BCDE and the results are 
shown in Figure~2. 

\newpage

\IBVSfig{14cm}{5562-f1.eps}{
The $V$ band light curve of RS~Oph from OMC in the magnitude 
{\bf (a)}  and intensity  scale {\bf (b)}. The horizontal dashed line 
denotes  the  level  in  which  the intensity  was  set to unity. The 
individual  sets  (science  windows)  are indicated  by  the  capital 
letters ABCDE. These sets are plotted on expanded axes in {\bf cdef}. 
The distances between the ticks on both the abscissa and ordinate are 
identical for all four panels. The points are connected by the dashed 
line for convenience to show the profile of the variations. The error 
bars  of the  individual  measurements are also marked; in some cases 
the errors are smaller than the size of the symbols. }
\IBVSfigKey{5562-f1.eps}{RS Oph}{light curve}

    Our observations were obtained in the orbital phases $\phi=0.9848-0.9935$ 
according to  the ephemeris $T_{\rm icg}=2450000+460~E$ (Dobrzycka and Kenyon 
1994) where  $T_{\rm  icg}$ refers  to the  inferior  conjunction  of the red 
giant. This implies that our observations were obtained at primary eclipse if 
RS~Oph were an eclipsing  system. In spite of the low inclination angle, this 
is still  an important phase because for example the wind flow from the giant 
toward the WD can still influence our view into the vicinity of the WD. 

\IBVSfig{21cm}{5562-f2.eps}{
Weighted wavelet Z-transform of the individual sets of RS~Oph. 
WWZ indicates whether or not there is a periodic fluctuation at a given 
time  at a given  frequency. The  method of Foster (1996) was used. The 
dotted  curves  indicate  the maximum of WWZ. The range of the ordinate 
is identical for all four panels  while  the length of the abscissa for 
each  set  corresponds  to the  length of  the given  science window of 
$INTEGRAL$. }
\IBVSfigKey{5562-f2.eps}{RS Oph}{spectrum}

\vspace*{-0.4cm}

     We observed RS~Oph at various levels of brightness, with the lower value 
close to  11.45~mag($V$), the  lowest  one at  which  the  flickering of this 
object was  analysed  previously, as summarized by Anupama  and  Mikolajewska 
(1999). We detect rapid  variations  of  the  brightness  in  all  sets  with 
sufficiently  long  coverage  (BCDE). The  largest peak-to-peak amplitudes of 
about  0.3~mag($V$)  were  observed  for  sets~C  and D. The amplitude of the 
flickering  in RS~Oph tends to increase  with  the increasing  mean  level of 
intensity. This  speaks in  favour  of the  origin of both the flickering and 
``constant" optical luminosity from the same source. The short  time  scale of 
the flickering places its most probable location to the close vicinity of the 
WD. The  interpretation  that the  inner  disk  region  is the  source of the 
flickering  is supported  also  by  the  rapid variations  of the  He~II 4686 
emission (Sokoloski 2002). We note that the relation between the amplitude of 
the rapid and long-term variations is in the same sense  as the  behaviour of 
CH~Cyg  (Sokoloski  2002, Sokoloski and Kenyon  2003) and  T~CrB (Anupama and 
Mikolajewska 1999). Our observations are also in agreement with Bruch (1992), 
i.e. that  the  luminosity of the flickering and ``constant" source in CVs are 
correlated.

     WWZ enabled us to detect  a typical frequency of the flickering for each 
of sets~BCDE. Although  the duration  of each set is not long enough to prove 
the coherence of the frequency  over long time scales, we  can state that the 
typical  frequency was 30--50  cycles/day  (i.e. period of 48--29 min) during 
our observations. The frequency tends to vary with the varying mean intensity 
of the  sets~-- set~B  which possessed  a lower  mean intensity than sets~CDE 
displayed the flickering with  a lower frequency. The frequency displayed the 
variations  also  in the course of the set, as can be clearly seen for set~D. 
All this contradicts the origin  of the  flickering from  the rotation of the 
magnetized  WD. We also  stress that  the typical  periods  of the flickering 
found here are  quite  discordant with the period of $81\pm2$~min reported by 
Dobrzycka et al. (1996). 

     We find a complicated relation  between  the amplitude of a flare in the 
flickering and its duration. The amplitude decreases with the decrease of the 
cycle-length (and hence the duration of the flare) in set~D (compare Figures~1 
and~2). On the contrary, set~B  which possesses a lower  mean intensity level 
displays  a longer  cycle-length  with a smaller amplitude in comparison with 
set~D. The level of the ``constant" intensity thus plays a role in this regard.

\noindent
Acknowledgements: 
Based on  observations  with  $INTEGRAL$, an ESA project with instruments 
and science  data  centre  funded by ESA member states (especially the PI 
countries: Denmark,  France,  Germany,  Italy, Switzerland, Spain), Czech 
Republic  and  Poland, and  with the participation of Russia and the USA. 
The  analyzes of cataclysmic variables represent a part of the $INTEGRAL$ 
Core  Programme  (CP, topic  5.5, responsible  scientist  R.~Hudec).  Our 
study  was supported  by  the project  ESA PRODEX INTEGRAL 14527. We made 
use of the  code  developed  by G.~Foster  and  available at 
{\tt http://www.aavso.org/data/software/wwz.shtml}.



\references

Anupama, G.C., Mikolajewska, J., 1999, {\it A\&A}, {\bf 344}, 177

Bruch, A., 1992, {\it A\&A}, {\bf 266}, 237

Dobrzycka, D., Kenyon, S.J., 1994, {\it AJ}, {\bf 108}, 2259

Dobrzycka, D., Kenyon, S.J., Milone, A.A.E., 1996, {\it  AJ}, {\bf 111}, 414

Foster, G., 1996, {\it AJ}, {\bf 112}, 1709 

Mas-Hesse, J.M., Gimenez, A., Culhane, J.L., et al., 2003, {\it A\&A}, {\bf 411}, L261

Oppenheimer, B.D., Mattei, J.A., 1996, {\it IAU Symp. No.165}, p.457

Sokoloski, J.L., 2002, {\it Symbiotic Stars Probing Stellar Evolution}, astro-ph/0209101

Sokoloski, J.L., Kenyon, S.J., 2003, {\it ApJ}, {\bf 584}, 1027

Walker, A.R., 1977, {\it MNRAS}, {\bf 179}, 587

Warner, B., 1995, {\it Cataclysmic Variable Stars}, Cambridge Univ.
 Press, Cambridge

\endreferences


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