COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1971 Konkoly Observatory Budapest 1981 June 1 HU ISSN 0374-0676 PHOTOELECTRIC OBSERVATIONS OF THE FLARE STAR EV Lac IN 1980 A program of photoelectric observations of flare stars has been started at the National Astronomical Observatory of the Bulgarian Academy of Sciences using a 60 cm Cassegrain reflector with a photoelectric equipment. In this paper we report the observations of the flare star EV Lac during the summer of 1980. The photoelectric equipment consists of a one channel photometer (described by A. Tomov, Dissertation, 1977, Sofia) and a photoncounting.system. The photomultiplier used is the EMI 9789 QB. The transformation of our instrumental ubv system to the international UBV system is given by the equations: Delta V = Delta v - 0.12.Delta(b-v) Delta (B-V)= 1.14.Delta(b-v) Delta (U-B)= 1.08.Delta(u-b) The monitoring intervals in U.T. as well as the total monitoring time for each night are given in Table I. The observations have been carried out in "u" colour with an integration time of 1 sec. Generally, one registration has been made every 3 sec, but when a flare was noticed, an-every-1sec-registration has been switched over. In this way for every flare a time-resolution of 1 sec was achieved. During the total of 21h32m monitoring time 11 flares were observed, the characteristics of which are given in Table II. For each flare following characteristics are given: a. The date and universal time of flare-maximum. b. The duration before and after maximum (t_b and t_a respectively). c. The total duration of the flare. d.The intensity ratio I_f / I_0, where I_f is the flare maximum intensity less sky background and I_0 is the quiet state intensity less sky background. e. The increase of the star magnitude at flare maximum: Delta m(u)=2.5 log(I_f / I_0) where "u" is the ultraviolet magnitude of the star in our instrumental system. f. The standard deviation of random noise fluctuations in mag. sigma(mag)=2.5 log ((I_0 +sigma) / I_0) The light curves of the observed flares in colour "u" are shown in Figs. 1-11. We want to make brief comments about some interesting features of individual flares. Flare No 3 shows a distinct pre-flare intensity minimum with a duration of about 40 sec and with a superposed pre-flare. The "anti-flare"lies some 0.07 mag below the mean intensity level. About 2 min after the maximum of flare No 3, an increased value (up to 20%) of "sigma" for random-noise is observed. This could probably mean that some of the small "noise-peaks" contain flare activity. This is probably the case for flare No 5, too, again 2-3 min after the flare maximum. Flares No 4 and No 5 form a 'double-flare" with two peaks of similar shape and amplitude at 3.5 min apart from each other. The reason for considering them different flares is that before flare No 5 intensity reached almost quiet state level. Flare No 7 shows that the maximum light with Delta m_f=0.75 mag is reached within 7 sec. For flare No 11, 6-point averages have been plotted, because of the long duration of the flare. With the total duration of 20 min this is the most powerful flare in this report. We want to thank Ing. A. Ustabashiev for his valuable technical assistance. Table I Date Monitoring Total Effective 1980 intervals(U.T.) Monitoring Time July 14/15 23h39m25s-23h46m20s,234635-235018, 235039-235616,235630-000120, 000136-000713,000730-001520, 001533-002447,002507-003323, 003340-004227,004243-004908, 004926-005745,005800-010708, 010726-011643,011658-012426, 011241-013244,013300-013650. 1h53m21s 15/16 232753-233254,233308-234716, 234734-000407,000421-002825, 002843-004449,004505-010150, 010206-011851,011909-013125. 2 01 28 16/17 225149-230107,230252-231212, 231246-231740,231816-232744, 232751-235328,235357-001544, 001658-003843,003857-005959, 010015-011347,011358-013009. 2 32 57 17/18 225405-230043,230107-230842, 230857-231347,231405-232300, 232317-233145,233158-234237, 234254-235309,235321-000850, 000905-002121,002135-002351. 1 27 21 August 2 222312-223448,223548-223651. 0 12 39 4/5 224711-225930,225945-231643, 231652-233102,233122-235140, 235200-235240,235252-002630, 002641-004241,004322-010918, 010933-012017,012131-015048, 015104-020015. 3 09 11 5/6 230111-232857,232913-002200, 002317-005439,005502-012549, 012610-020036. 2 57 28 6/7 222826-224611,224631-224711, 224725-230338,230355-232440, 232509-233746,233759-234418, 234439-234518,234533-000637, 000652-002556,002612-005414, 005523-011750,011807-013410, 013434-014012. 3 07 16 7 223421-224305,224320-231321. 0 48 45 8/9 222616-223104,223121-223541, 223551-224347,224401-225303, 225314-230402,230416-231027, 231038-231834,231849-232721, Table I (continued) 8/9 232832-233636,233648-234401, 234411-000356,000407-001522, 001535-002428,002441-003420, 003439-004033,004042-010131, 010145-010855,010907-011623, 011639-014853,014912-015308. 3 21 42 Total 21h32m,08s Table II Characteristics of the Flares Observed Flare Date U.T. t_b t_a Duration Delta m sigma No. 1980 max min min min I_f / I_0 mag mag 1 14.07 23h43m31s 1.3 2.8 4.1 1.42 0.38 0.06 2 15.07 00 59 20 1.3 1.5 2.8 1.22 0.22 0.05 3 15.07 23 38 03 1.7 3.5 5.2 1.48 0.42 0.06 4 5.08 00 16 27 0.6 1.9 3.5 1.90 0.69 0.07 5 5.08 00 19 43 0.4 4.4 4.8 1.93 0.71 0.075 6 5.08 23 15 20 0.2 1.2 1.4 1.38 0.35 0.06 7 6.08 00 11 42 0.1 4.1 4.2 1.99 0.75 0.06 8 6.08 22 58 16 0.4 1.5 1.9 1.36 0.34 0.07 9 7.08 00 07 27 0.1 0.7 0.8 1.40 0.37 0.07 10 7.09 00 47 38 0.8 5.3 6.1 2.10 0.81 0.07 11 9.08 23 11 50 4 16 20 1.62 0.53 0.09 [FIGURE 1] [FIGURE 2] [FIGURE 3] K.P. PANOV M.K. TSVETKOV Department of Astronomy with National Astronomical Observatory Bulgarian Academy of Sciences Sofia-1000, 7-th November Str. 1 Bulgaria