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\IBVShead{5256}{11 April 2002}


\IBVStitle{OBSERVATIONS OF THE FLARE STAR EV L\lowercase{ac} IN 2000-2001}

\IBVSauth{Zalinian, V.P.$^1$; Tovmassian, H.M.$^2$; Cardona, O.$^2$}

\IBVSinst{Byurakan Astrophysical Observatory of the Armenian National Academy of Sciences}

\IBVSinst{Instituto Nacional de Astrof\'{\i}sica \'Optica y Electr\'onica, AP 51 y 216, 72000, Puebla, Pue, M\'exico}

\SIMBADobj{EV Lac}


\begintext


In 2000-2001 we continued observations of the flare star EV~Lac with the
two-channel (U and B) fast photometer (Zalinian \& Tovmassian 1989) installed
at the 40 cm telescope of the Byurakan Astrophysical Observatory (BAO),
Armenia. Observations with this photometer allowed us to study the light curve
of a flare with high time resolution (0.5 s). In previous observations we
have detected very short spiky flares of a duration less than a second with
rising times of about 0.1 s (Zalinian \& Tovmassian 1987, Tovmassian \&
Zalinian 1988, Zalinian \& Tovmassian 1997, Tovmassian et al. 1997a). Fast
flares of a duration of a few seconds have been registered also by Tsvetkov et al.
(1986) and Shvartsman et al. (1988). Tovmassian et al. (1997b) showed that the
spiky flares occur mostly after normal, longer lasting flares, and are
systematically bluer than the preceding main flares (Tovmassian et al. 1997b).

The log of observations is presented in Table~1. The night sky background was
measured several times during one observing run. Therefore, the total duration
of observation of the star itself is less than the time between the start and
the end of observations. Observations were made with integration time 0.5~s. 

\begin{table}[htb]
\caption[]{The log of observations of EV Lac in 2000-2001.}
\begin{center}
\begin{tabular}{cccc}
\hline
Date & Start & End & Total duration \\
     & UT    & UT  & h m            \\
\hline
21 Aug 2000 & 20 00 & 20 56 & 0 45 \\
23 Aug 2000 & 19 13 & 22 46 & 2 25 \\
25 Aug 2000 & 18 45 & 22 53 & 2 35 \\
26 Aug 2000 & 19 10 & 19 20 & 0 10 \\
26 Aug 2000 & 19 30 & 22 29 & 2 30 \\
27 Aug 2000 & 16 47 & 22 49 & 3 10 \\
28 Aug 2000 & 18 55 & 00 32 & 4 15 \\
04 Sep 2000 & 18 08 & 22 30 & 2 45 \\ 
08 Oct 2001 & 17 20 & 18 52 & 1 10 \\
24 Oct 2001 & 17 00 & 19 25 & 2 05 \\
\hline
\end{tabular}
\end{center}
\end{table}

Since the aim of our observations was detection of fast flares, we measured
the brightness of EV Lac in U and B in relation to its quiescent state. It is
known that EV Lac is a variable star with an amplitude $\Delta V=0\fmm08$
(Pettersen, Kern \& Evans 1983). In observations with the 40 cm telescope made
with integration time 0.5 s such variations are certainly below the accuracy
of our measurements. The comparison star C2 from Pettersen et al. (1983) was
observed once in a night for controlling the registration system of the
photometer.

During 22 hours of monitoring of EV Lac we detected three flares. Light
curves of the detected flares, and also the variations of $(U-B)$ magnitudes
of the star during flares, are presented in Figures~1-4. Each point on the 
$U-B$ graph is the average of 5 preceding and 5 subsequent measurements.

The flare on 23~August has relatively sharp rise and decline in the initial
phase, followed by a slow decline (Figure~1). Unfortunately, the declining part
of the flare is interrupted for the sky background measurement. However, the
end of decline is seen on the record made after the sky background
measurement. The total duration of the flare exceeds 14 min. One may notice a
small decrease of the emission of the star during about 70 s which took
place before the flare.

The slow rise of the flare on 26~August starts at about 19\hr13\mm (Figure~2).
After $\sim2$ m the brightness rose sharply during about 20 s. The slow
decline is observed during about 3\mm40\ss. Then observation was interrupted
for sky background measurement. Observations resumed after about 10 m, when
the star was at quiescent state.

The flare on 4~September has a composite nature (Figure~3). There is a short
spiky preflare at 18\hr 47\mm 30\sec. The main flare started $\sim10$ s later and
lasted $\sim 20$ s. It consists, probably, of a few spiky flares. The
central part of the light curve of the preflare and main flare is presented in
Figure~4. About 2 m later two small and short flares are seen in U. However,
their presence in B is not certain. Generally some of the spikes in U are not
certainly seen in B due, probably, to their relative faintness, they are
smaller of the $3\sigma$ level.

In Table~2 the following parameters of the detected flares: the date
of the flare, the flare magnitudes $\Delta U$ and $\Delta B$, the $(U-B)$
color of the star at the peak of the flare, and the total duration of the
flare are given.

\begin{table}[htb]
\caption[]{The parameters of the EV Lac flares.}
\begin{center}
\begin{tabular}{ccccl}
\hline
Date & $\Delta U$ & $\Delta B$ & $(U-B)$ & Total \\
     &            &            &                & duration \\
\hline
23 Aug 2000 & $3\fmm1$ & $1\fmm1$  & $-1\fmm20$ & $14^m 10^s$ \\
26 Aug 2000 & $1\fmm6$ & $0\fmm8$  & $-0\fmm05$ & $ >4^m$ \\
04 Sep 2000$^{\dag}$ & $1\fmm9$ & $0\fmm6$  & $-0\fmm55$ & $\sim1^s$\\
04 Sep 2000 & $2\fmm5$ & $0\fmm7$  & $-1\fmm05$ & $ 0^m 20^s$ \\ 
\hline
\end{tabular}
\end{center}
\end{table}

VPZ is grateful to the CONACYT Project 34564-E and to the INAOE for financial
support and hospitality during the reduction of observational data and
preparation of paper at INAOE.\\


\IBVSfig{6cm}{5256-f1.ps}{The flare of EV Lac on 23 August 2000. In the gap of the light curve the sky background was measured.}

\IBVSfig{6cm}{5256-f2.ps}{The flare of EV Lac on 26 August 2000.}

\IBVSfig{6cm}{5256-f3.ps}{The flare of EV Lac on 4 September 2000. It consists, possibly, of a few spiky flares.}

\newpage

\IBVSfig{6cm}{5256-f4.ps}{The light curve of the main flare of EV Lac on 4 September 2000 with higher resolution.}


\references

Pettersen, B.R.,  Kern, G.A. \& Evans, D.S., 1983, {\it A\&A}, {\bf 123}, 184

Shvartsman, V.F., Beskin, G.M., Gershberg, R.E., Neizvestni, S.I., Plakhotenkova, V.L. \& Pustilnik, L.A., 1988, {\it Izv. Crimean AO}, {\bf 79}, 71

Tovmassian, H.M. \& Zalinian, V.P., 1988, {\it Astrofizika}, {\bf 28}, 131

Tovmassian H.M., Recillas, E., Cardona, O. \& Zalinian, V.P., 1997a, {\it IBVS}, No. 4465

Tovmassian H.M., Recillas, E., Cardona, O. \& Zalinian, V.P., 1997b, {\it Rev. Mex. AA}, {\bf 33}, 107

Tsvetkov, M.K., Antov, A.P. \& Tsvetkova, A.G., 1986, {\it Comm. Konkoly Obs. Hung. Acad. Sci.}, No. 86, 435  \BIBCODE{1986CoKon..86..423T}

Zalinian, V.P., \& Tovmassian, H.M., 1987, {\it IBVS}, No. 2992

Zalinian, V.P., \& Tovmassian, H.M., 1989, {\it Contr. Byurakan Obs.}, {\bf
61}, 142

Zalinian, V.P. \& Tovmassian, H.M, 1997, {\it IBVS}, No. 4464

\endreferences


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