COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1006 Konkoly Observatory Budapest 1975 June 10 FLARE ACTIVITY ON UV CETI, 1975.01 We report observations in the U-band of flare activity on UV Ceti between January 3 and January 8, 1975. The observations were originally planned to provide concurrent optical coverage for an experiment to observe flares of UV Ceti with X-ray equipment aboard the ANS satellite (Grindlay, 1974). Although cloudy weather permitted only 3.7 hours of monitoring, 17 events were recorded with a 60 cm reflector above a detection limit of Udet=15.0 Data for individual flares are listed in Table 1, following a format used on earlier occasions (Kunkel 1968, 1973). The level of activity is determined from the incidence statistic U_0 in the event rate equation R(U) = exp[a(U-U_0)] hr^-1 the lower value of U_0 indicating greater activity. Although the value of the coefficient a may be observed to vary from sample to sample, experience with observations reported so far (Kunkel 1975) is that a unique value of a applies to flare activity of all stars for the flares typically observed. This value is close to unity, and for a given sample of flares the estimate of a approaches this limit as the sample size increases. A simultaneous solution for both a and U_0 yields a = 0.8 + 0.2 and U_0 = 13.0 + 0.2, implying that stronger events dominate the sample. Solving for U_0 alone, with a = 1.06, the value that best represents the 800 flares recorded for the years 1966-1971 (Kunkel, 1975) gives U_0 13.35 +- 0.20. This activity is nearly double that for the 1966-1971 period, for which U_0 = 14.0. TABLE 1 FLARE ABSTRACT ------------------------------------------------------------------------------ Monitoring times Date Intervals ------------------------------------------------------------------------------ 75 01 05 01h21.4m- 02h11.0m 75 01 06 01 49.6 - 02 27.3 75 01 07 01 05.9 - 02 16.4 75 01 08 01 09.0 - 02 15.8 ------------------------------------------------------------------------------ FLARES OBSERVED ------------------------------------------------------------------------------ Durations Date U.T. Airmass. U T_.5 T_.2 T_.1 Notes (Peak) ------------------------------------------------------------------------------ 75 01 05 01h26.95m 1.16 14.77 0.90 3.0 - 01 53.32 1.25 15.32 0.16 - 75 01 06 01 50.5 1.23 12.16 0.26 4.3 6.8 01 56.3 1.26 13.97 0.06 - 02 04.81 1.29 14.80 0.8 - 02 09.26 1.31 14.78 0.11 0.45 - 02 13.30 1.32 14.08 0.47C 1.50C - 02 14.81 1.33 14.71 0.07 0.20U - 02 23.18 1.42 14.06 0.27C 0.40 - 02 24.54 1.42 <12.4 0.2 U - - Peak lost 02 25.02 1.42 13.18 0.32 1.16 1.19 750107 01 21.08 1.17 14.50 0.05 0.07 1.0U 01 39.66 1.22 14.73 0.18 0.08 - 01 55.53 1.26 15.21 0.30 1.00U - 01 58.14 1.28 15.40 1.6 2.0 - 02 05.65 1.31 14.92 0.1 0.3 0.4 02 09.09 1.33 14.08 0.08 0.19 0.26 750108 01 17.56 1.17 < 9.9 6.80U 11.12U Peak lost 01 55.69 1.26 14.14 0.16C 0.18C - 02 08.23 1.33 13.97 0.09 0.14 - ------------------------------------------------------------------------------ Note: C signifies complex flare form U signifies uncertainty greater than ten percent. As the data of Table 1 show, the bulk of the flares here reported occurred on the nights of January 6 and January 7. On the remaining nights, which account for about half of the total observing time, only five flares were recorded. It seems probable, therefore, that the enhanced activity noted on two nights represents a sporadic fluctuation, and we feel that our data warrant no conclusion regarding possible long term changes in the level of flare activity in UV Ceti. W.E. KUNKEL Cerro Tololo Interamerican Observatory La Serena, Chile N. ZARATE Cerro Calan Santiago, Chile References: Grindlay, J.E., 1974, private communication. Kunkel, W.E., 1968, I.B.V.S. No. 315. 1973, Ap.J.Suppl. 25, 1. [BIBCODE 1973ApJS...25....1K ] 1975, I.A.U. Symposium No. 67, in press.