COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 2972 Konkoly Observatory Budapest 29 December 1986 HU ISSN 0374 - 0676 PHOTOELECTRIC OBSERVATIONS OF FLARE STAR EV Lac IN 1984 Within the framework of the program of investigation of flare stars in the solar neighbourhood and stellar aggregates patrol observations of the flare star EV Lac have been carried out in the Rozhen National Astronomical Observatory in the autumn of 1984 (Tsvetkov et al., 1986). These observations were made with the 60-cm Cassegrain telescope, using an EF_1 one-channel electrophotometer (Panov et al., 1981). The accuracy in the U-band was a sigma <=0.09, integration time 1s. For the patrol observations of EV Lac together with its optical companion (an A-type star of constant brightness; V=12.00, B=12,74 and U=12.95 according to Andrews and Chugainov, 1969) a diaphragm of diameter 1.5 mm (41.25 arcsec) was used. This has to be taken into consideration when using the data listed in the table for recorded flares where no corrections have been made for stellar companions. For 15h42m43s effective observation time 17 flare events were detected some of which forming separate groups of flares. Table I lists the total effective observation time in each individual night, and the respective number of detected flare events during the patrol observations of EV Lac. Table II lists data on these flare events, viz. date and the moment of maximum brightness in universal time, duration before and after this maximum (t_b and t_a, respectively) and total duration of the flare; value of the relation (I_f+I_0)/I_0 for the maximum of the flare, where I is the intensity deflection less sky background of the quiet star and IF is the, total intensity deflection less sky background of the star plus flare, the integrated intensity of the flare over its total duration, including pre-flares, if present, P= (I_f-I_0)/I_0 dt, the increase of the apparent magnitude of the star at flare maximum delta m_u=2.5log (I_f/I_0), where u is the uv magnitude of the star in the instrumental system, the standard deviation of random noise fluctuation sigma(mag)= 2.5log (I_0+sigma)/I_0 during the quiet-state phase immediately preceding the beginning of the flare, and the air mass at flare maximum. Fig. 1 shows the light curves of spike flares observed in the u band. During the patrol observations a number of estimations of the brightness of EV Lac were made using standards C_1 and C_2 according to Pettersen (1980) and its brightness was found to be comparatively constant. Table I Total monitoring time Date T_eff Observed flares 18/19 Oct. 1984 3h 23m 12s 8 19/20 Oct. 1984 5 35 07 4 20/21 Oct. 1984 1 40 01 2 21/22 Oct. 1984 1 45 26 1 23/24 Oct. 1984 1 04 58 1 10/11 Nov. 1984 2 13 59 1 Total: on six nights 15h 42m 43s 17 Table II Characteristics of the flares observed t_b t_a D_min Delta m_u X Date U T_max min min mag (I_f-I_0)/ sigma m_u I_0 P mag 1984 1 18 Oct. 20h33m43s 0.120 3.280 3.400 0.729 0.310 0.59 0.05 1.04 2 -"- 21 02 57 0.533 22.383 22.916 3.503 6.236 1.63 0.05 1.06 3 -"- 22 17 49 6.183 11.517 17.700 0.860 1.573 0.67 0.04 1.19 A 22 11 46 0.133 >2.867 >3.000 0.223 >0.163 0.22 0.04 B 22 14 51 0.217 >0.800 >1.017 0.532 0.240 0.46 0.04 1.18 C 22 15 40 0.017 >0.650 >0.667 0.361 0.108 0.33 0.04 1.18 D 22 16 23 0.067 >0.933 >1.000 0.288 0.134 0.27 0.04 1.18 E 22 17 49 0.500 11.517 12.017 0.860 0.928 0.67 0.05 1.19 4 -"- 22 31 19 0.008 0.008 0.017 1.465 0.024 0.71 0.05 1.22 5 -"- 22 31 26 0.008 0.008 0.017 1.013 0.017 0.41 0.05 1.22 6 -"- 22 31 33 0.008 0.008 0.017 4.654 0.078 1.77 0.05 1.22 7 -"- 22 31 36 0.008 0.008 0.017 1.006 0.017 0.41 0.05 1.22 8 19 Oct. 00 44 36 -- 9.067 >9.067 >0.458 >0.170 0.41 0.07 1.85 9 -"- 19 13 43 0.380 1.720 2.100 0.192 0.105 0.19 0.05 1.00 10 -"- 20 15 58 0.008 0.025 0.033 1.951 0.065 1.17 0.06 1.02 11 -"- 21 01 23 0.100 3.950 4.050 0.283 0.178 0.27 0.05 1.06 12 -"- 21 37 28 2.75 1.20 3.95 0.230 0.288 0.23 0.05 1.12 13 20 Oct. 20 19 54 0.833 2.117 2.95 0.241 0.197 0.23 0.04 1.03 14 -"- 21 00 32 0.917 1.55 2.467 0.368 0.449 0.34 0.05 1.07 15 21 Oct. 18 59 10 0.633 2.333 2.966 0.250 0.188 0.24 0.05 1.00 16 23 Oct. 20 56 32 0.366 1.716 2.083 0.420 0.380 0.38 0.05 1.08 17 10 Nov. 18 34 37 0.150 1.817 1.967 1.022 2.589 0.77 0.09 1.01 [FIGURE 1] The following mean values of the brightness of EV Lac in the instrumental ubv system were established: v=10.04+-0.04; b=11.42+-0.05; u=12.24+-0.05 Using the following equations our instrumental system was converted into the international UBV system as follows: DeltaV=(0.105Delta(B-V); Delta(B-V)=1.118Delta(b-v); Delta(U-B)=0.800Delta(u-b) Figure 1 shows the light curves of spike flares observed in the u band. Before discussing the physical conditions causing flare events of such a short duration and relatively high amplitude up to Delta m_u=2m.49 **, (No.6) it is necessary to make sure that there are neither instrumental errors nor other influences. Current printed records of spike flares of EV Lac and other flare stars are of very great importance for the elucidation of their nature. Over the last decade a number of astronomers engaged in patrol observations of flare stars, were faced with similar phenomena still insufficiently corroborated. There are several factors accounting for the difficulties; duration of the flares is close to the integration time of the device used for the patrol observations; the phenomenon occurs mainly in the U-band, and with simultaneous observations the amplitudes in the B-band are generally by approximately 1 stellar magnitude smaller; possible interference that may cause spikes in the device. **, If EV Lac's companion is taken into consideration in estimating the amplitude of flare No.6, its influence would increase the value Delta m_u=0m.723. A most reliable evidence of the existence of such flares would be provided by simultaneous observations of high time resolution, varying from 0.1s to 1s, in the ultraviolet band simultaneously on two telescopes. In this sense the publication of the present results may be regarded as launching a call to put forward such a program. Another reason is that as early as in 1972, Moffett gave valid grounds for the project of making high time resolution patrol observations. Parallelly an increasing number of authors obtain results suggestive of similar spike flares - in the optical spectral region of flare stars UV Cet, EV Lac and AD Leo (Cristaldi and Rodono, 1970, 1973; Gershberg and Petrov, 1985; Zalinian and Tovmassian, 1986; Ichimura and Shimizu, 1986). Beskin et al. (1985) could not detect such flare events on the 6m telescope of the Special Astrophysical Observatory, when a MANIA device was used to detect small-scale time changes in flare events in some UV Ceti type stars. On the other hand, Lang et al. (1983) detected similar spike flares of 0.1s duration in the radiorange during a flare event in AD Leo. In view of the contentions made regarding the physical conditions in the atmosphere of flare stars at the time of their activity (Ambartsumian, 1954; Ambartsumian, Mirzoyan, 1982) it is of primary importance to prove the existence of rapid flare activity of UV Ceti stars, of small-scale time structure of the flare, and the existence of spike flares, as well. M.K. Tsvetkov, A.P. Antov, A.G. Tsvetkova Department of Astronomy and National Astronomical Observatory Bulgarian Academy of Sciences Sofia 1784, Lenin Blvd No.72 Bulgaria References: Ambartsumian V.A., 1954, Commun. Byurakan Obs, No.13, Ambartsumian V.A., Mirzoyan L.V., 1982. Astroph. Sp. Sc. 84, 317 [BIBCODE 1982Ap&SS..84..317A ] Andrews A.D., Chugainov, P.F. 1969, IBVS No. 370. Beskin, G.M., Neizvestnii, S.I., Plahotnichenko. V.L., Pustilnik, L.A., Cheh, S.A., Shvartsman, V.F., and Gershberg, R.E., 1985, Proc. Symposium "Flare Stars and Related Objects", Byurakan, Ed. Mirzoyan, L.V., 60 p. [BIBCODE 1986fsro.conf...60B ] Cristaldi, S., Rodono M., 1970 Astr. Aph. Suppl, 2, 223. [BIBCODE 1970A&AS....2..223C ] Cristaldi, S., Rodono M., 1973 Astr. Aph. Suppl, 10, 47. [BIBCODE 1973A&AS...10...47C ] Gershberg, R.E., Petrov P.P., 1985, Proc. Symp. "Flare Stars and Related Objects" Byurakan, Ed, L.V. Mirzoyan, 38 p. [BIBCODE 1986fsro.conf...38G ] Ichimura, K. Shimizu, Y., 1986, Tokyo Astron. Bull., Sec. Ser., No. 276, 3187 [BIBCODE 1986TokAB.276.3187I ] Lang, K., Bookbinder, J., Golub, L., Davis, M.M., 1983, Ap. J., 272, L15 [BIBCODE 1983ApJ...272L..15L ] Moffett T.J., 1972, Nature Physical Science, 240, No. 98, 22 [BIBCODE 1972NPhS..240...41M ] Panov, K.P., Pamukchiev I.Ch., Christov, P.P., Petrov, D.I., Notev,P.I., and Kotsev, N.G., 1982,Comptes rendus of the Bulgarian Acad, Sci, 35, No. 6, 717 Pettersen B.R., 1980, A.J. 85, No 7, 871 [BIBCODE 1980AJ.....85..871P ] Tsvetkov, M.K., Antov, A.P. and Tsvetkova, A.G., 1986, Comm. Konkoly Obs. No. 86, 423 [BIBCODE 1986CoKon..86..423T ] [CoKon No. 86] Zalinian, V.P., Tovmassian, H.M., 1986, Comm. Konkoly Obs, No. 86, 435 [BIBCODE 1986CoKon..86..435Z ] [CoKon No. 86]