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\IBVShead{5261}{17 April 2002}

\IBVStitle{Emission-line flare of ES~560B}


\IBVSauth{SIL'CHENKO, O.K.$^1$; MOISEEV, A.V.$^2$}

\IBVSinst{Sternberg Astronomical Institute, University av.13, 119992 Moscow,
Russia, \\ e-mail: olga@sai.msu.su}
\IBVSinst{Special Astrophysical Observatory, Nizhnij Arkhyz, 369167
Karachaevo-Cherkesia, Russia, \\ e-mail: moisav@sao.ru}

\SIMBADobjAlias{ADS 2594B}{** ES 560B}
\IBVSkey{spectroscopy}
\IBVSabs{Strong Balmer emission lines are found in the spectrum of ES 560B.}
\IBVSabs{It is for the first time during our observations of this star} 
\IBVSabs{from 1996. The star is a fainter component of the visual double} 
\IBVSabs{system, its V ~ 10.5 .}

\begintext

ES~560 (ADS~2594) is a visual double star with the separation of
9\fsec5. We observed it spectroscopically starting from 1996
to calibrate an orientation of the frame of the Multi-Pupil Field
(Fiber) Spectrograph of the 6m telescope (MPFS, Afanasiev et al. 1996,
{\tt http://www.sao.ru/$\sim$gafan/devices/mpfs/}) during our investigation
of stellar populations in the centers of nearby galaxies; later the
brighter component of the double, ES~560A, which has a spectral
classification of K2 (SIMBAD), was used by us as a line-of-sight
velocity standard for cross-correlation with the galactic spectra.
The spectra of ES~560B were not used but were still stored in the
archive of our observational data. The detailed log of the observations
of ES~560 with the MPFS is given in Table~1.

\vskip 5mm

\centerline{Table 1. Log of the spectral observations of ES 560 with the MPFS
of the 6m telescope}
\vskip 3mm
\begin{center}
\begin{tabular}{lcrlc}
\hline
Date & UT & Exposure & Detector & Spectral range \\
&  (start)  & (s) && \\
\hline
1996 Aug 17 & 00:50 & 180 & CCD K585 $520 \times 580$ & 4800--5600~\AA \\
1996 Aug 18 & 01:14 & 180 & CCD K585 $520 \times 580$ & 4800--5600~\AA \\
1996 Oct 10 & 03:19 & 180 & CCD ISD017A $1040 \times 1160$ & 4100--5500~\AA \\
1997 Nov 1 &  02:38 & 270 & CCD K585 $520 \times 580$ & 4700--5500~\AA \\
1998 Jan 19 & 16:12 & 180 & CCD K585 $520 \times 580 $ & 4700--5500~\AA \\
2001 Jan 28 & 19:00 &  60 & CCD TEK $1024 \times 1024$ & 4250--5600~\AA \\
2001 Sep 22 & 01:58 & 240 & CCD TEK $1024 \times 1024$ & 4250--5600~\AA \\
2002 Mar 9 & 17:03 & 240 & CCD TEK $1024 \times 1024$ & 4250--5600~\AA \\
\hline
\end{tabular}
\end{center}

\vskip 5mm

During our last observational run with the MPFS, in March 2002, we have
exposed ES~560 as usually and have immediately seen that the spectrum
of ES~560B has noticeably changed: now it contains strong narrow Balmer
emission lines, H$\beta$ with $EW=13$\AA\ and H$\gamma$ with $EW=11$\AA\
falling into our spectral range. Careful examination of the past spectra
of ES~560B, starting from August~1996, has revealed an early presence of
the very weak emission H$\beta$ comparable to the noise; now its
intensity has risen by more than an order. The full evolution of
the spectrum of ES~560B in the spectral range of 4300--5450~\AA\
is shown in Figure~1; all the continua are normalized to the same level
and after that subtracted, and the shift along the ordinate axis for
every spectrum is selected to provide a clear presentation.

\IBVSfig{10cm}{5261-f1.eps}{The evolution of the spectrum ES~560B from 1996 to
present (two spectra of August~1996 are co-added); the continua are normalized
and subtracted}

\vskip 5mm

\centerline{Table 2. The ratio of brightnesses of two components of
the double star ES 560}
\vskip 3mm
\begin{center}
\begin{tabular}{lc}
\hline
Date  &  $Flux_B/Flux_A$ \\
\hline
1996 Aug 17 &  0.086 \\
1996 Aug 18 &  0.076 \\
1996 Oct 10 &  0.060 \\
1997 Nov 1 &   0.048 \\
1998 Jan 19 &  0.047 \\
2001 Jan 28 &  0.115 \\
2001 Sep 22 &  0.187 \\
2002 Mar 9 &   0.037 \\
\hline
\end{tabular}
\end{center}

\vskip 5mm

By using SIMBAD, we have made cross-identification of ES~560B over some
catalogues. The star is mentioned as V~577 Per by Kazarovets et al. (1999)
in their 74$^{\rm th}$ list devoted to variable stars discovered by HIPPARCOS
(the Hipparcos name of ES~560 is HIC~16563). A more close examination
of their description shows that HIPPARCOS has presented a photometry
of two the stars of the double system ES~560 {\bf together}, and the summed
magnitude varies by some 0.1 mag. What star is variable, ES~560A or ES~560B,
has been still unknown. With our integral-field spectroscopy, we are able
to calculate a ratio of the two star's fluxes; these ratios measured in the
100~\AA\ -band centered at the wavelength of 5000~\AA, from 1996 to March
2002, are presented in Table~2, and their typical accuracy is 0.003--0.005.
One can see that the relative brightness of
ES~560B varies by a factor of 5, the SIMBAD information about
$V_A=8\fmm3$ and $V_B\approx 10\fmm5$ being related rather to the close-to-high
state of ES~560B. Since the summed magnitude, according to HIPPARCOS,
varies by $\sim 0.1$ mag, and not by 1.5 mag as it would be if the brighter
component changes its flux by a factor of 5, we should conclude that
it is ES~560B that is the variable star. Interestingly, the bright flare of emission lines occurred some month later (March 9, 2002) than the maximum continuum brightness (September 22, 2001, according to Table 2). We can suggest that
there is either a few month delay between the emission line and continuum brightening, or the two events are two independent short flares with fast
evolution.

What may be the nature of the variable star ES~560B? Another result
of the cross-identification of ES~560 over various catalogues is
a finding that a bright extreme-ultraviolet source is related to this
double system. Firstly, ROSAT WFC survey in the spectral range of
60--200~\AA\ (Pounds et al. 1993), and later EUVE all-sky survey
at the wavelength of 100~\AA\ (Bowyer et al. 1994) have mapped a
rather bright source near ES~560 -- 2RE J033314+461549 and EUVE 0333+462.
As the spatial resolution of both surveys is about one arcminute,
it is impossible to identify the UV source with ES~560A or ES~560B
unambiguously. Mason et al. (1995) made spectral observations
of the optical counterparts of the ROSAT EUV sources, and have found
weak emission activity in both stars: an emission line of CaII3933
with $EW=0.3$~\AA, but no Balmer emissions, in ES~560A and
an emission line H$\alpha$ with $EW=2$\AA\ in ES~560B; the latter
star is classified by them as dMe. The possibility that ES~560B
is a flaring late-type dwarf may explain the most phenomena described
here; however it is somewhat strange that a rather bright ($V\approx 10.5$)
nearby flaring star has not been discovered yet.

We thank Profs. N.N. Samus and M.M. Katsova for the very helpful
discussions of this work in the field quite new for us. This research
has made use of the SIMBAD database, operated at CDS, Strasbourg, France.

\vskip 1cm


\references

Afanasiev, V.L., Dodonov, S.N., Drabek, S.V., Vlasyuk, V.V., 1996,
      {\it MPFS Manual}., Nizhnij Arkhyz: SAO RAS Publ.

Bowyer, S., Lieu, R., Lampton, M. et al., 1994, {\it ApJ Suppl. Ser.},
        {\bf 93}, 569

Kazarovets, E.V., Samus, N.N., Durlevich, O.V. et al., 1999,
    {\it IBVS}, {\bf 4659}, 1

Mason, K.O., Hassall, B.J.M., Bromage, G.E. et al., 1995,
      {\it MNRAS}, {\bf 274}, 1194

Pounds, K.A., Allan, D.J., Barber, M.A., et al. 1993, {\it MNRAS},
    {\bf 260}, 77

\endreferences

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