3. Methods and results
To recognize features in flares one usually requires an amplitude of the order of 0.1 mag (Gershberg 1986; Ventura et al. 1995; Konstantinova-Antova and Antov, 1995). The digital filtering technique (FT) enables an appreciable increase in the photometric quality of the light curve by reduction of atmospheric and photon noises. The FT can be made by convolving the series of counts n(i) with the filter pulse response characteristic coefficients h(i)
Here the index refers to the counts modified by filtering. For typical noise spectra that have approximately an uniform spectral density, the initial and the postfiltered variances and satisfy the condition (Jenkins et al. 1969)
where is the correlation interval of the filtered series. The quantity can also be considered as a time-resolving power in the post-filtered data or as the effective filter band width. We consider filter the data by a low-pass filter with a Gaussian window
where m is expressed in units of the sampling time t. According to (3), the correlation interval of the filtered series is then . This filtering technique allows to reach an accuracy comparable or even below that due to counting statistics at the cost of decrease in time resolution. Using the FT, our observations revealed small-scale features in flare events of EV Lac with limiting amplitude
lower by a factor at least of the order of 3 compared to usual observations.
Fig. 1 shows the light curves of EV Lac detected on 1990 August 27, 00:45 UT in the U and V bands simultaneously. The observations presented here were obtained at Peak Terskol with the two-channel photometer and a time resolution of 0.5 s. The prefiltering technique developed by Zhilyaev et al. (1994) was applied for detection of small-scale activity. The standard deviations of for quiet state are shown. It is particularly remarkable that the classical flare event in the U band with an amplitude of -1.2 mag was accompanied by a dip in the V band at the same time. The decay and the rise times in V are of 2 to 3 s. The fading duration is of about 5 s and has an amplitude of 0.07 mag. The U flux increase and decrease are roughly exponential. The time span of the flare event in U equals about 10 s.
Fig. 2 exhibits a more extreme case of flare on EV Lac observed on 1995 August 4, 23:23:20 UT. This remarkable event was registered at two distant telescopes operated simultaneously. The U-band light curve was obtained at Peak Terskol. The B-band monitoring was carried out at the Belogradchik Observatory in Bulgaria. Raw U data indicate a flare with an effective width of 0.6 s and an amplitude of mag. We could not identify any significant variations in the raw B data prior to filtering. After filtering a drop by mag for 1.5 s was found at the time of the flare. The light curves shown in Fig. 2, have been smoothed by a Gaussian filter with an effective bandwidth of 0.6 s. The bars shown are the standard deviations for the filtered data in the quiet state of the star.
The EV Lac observations, shown in Fig. 3 were made with the 1.25 m telescope AZT-11 in Crimea and the UBVRI photometer-polarimeter described by Piirola (1984) and Kalmin & Shakhovskoy (1995) on 1996 October 5, 23:47:23 UT. The monitoring was carried out in UBVRI simultaneously with a time resolution 0.1 s. The detected flare event is notable for many reasons, including its unique behavior in the different colors. The emission flare itself is of short duration and appears with amplitudes of -0.55, -0.05, -0.01, -0.007, -0.005 in UBVRI bands respectively. The time delay between the flare in RI and in UBV is about 10 s. There is a clear post-flare emision in U lasting for more than 150 s. Before and after the flare there is an intensity drop visible in V, R and I and partly in B. The I band drop has clearly defined boundaries with e-folding decay and rise time of about 20 s and 90 s long. The situation with the other bands is more uncertain due to lower signal to noise ratios. It is obvious that such fadings are phenomenon related to the flare beginnings before and with a minimum at the flare peak. Note that the fading amplitudes in the BVRI bands are quite small and ranged from 0.02 to 0.01 mag. Such low amplitude variations lie usually below the detection limit for raw photometric data. Their discovery became possible by use the filtering technique.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998