2. Light curves and time scales during the 1998 outburst
The historic light curve of ON 231 in the Johnson B band after 1970 is shown in Fig. 1: from these data the change of the source activity after 1995 and the extraordinary outburst of April-May 1998 are very evident. The lowest states of ON 231 were observed in 1972-74, when the flux was only 0.5 mJy (B=17.4) (Webb et al. 1988); afterwards the flux increased but always showing fluctuations with a typical amplitude of 1-1.5 mag. Since 1994 ON 231 was on the average brighter than in the past, but with the same fluctuation amplitude. Unfortunately, the lack of data in the period from 1986 to 1994 does not allow us to know exactly when the present active phase started on. In the very strong burst of 1998 ON 231 was about 3 times brighter than in the previous years and a factor of 60 than the minimum of 1972.
The light curves from December 1997 to May 1998 in the V and bands, which are the best sampled, are shown in Fig. 2: we have measurements in 71 days over a total span of 176 with a mean separation of about 2.5 days, which decreases to 1.9 after February 15th. Photometric measurements were performed with different telescopes equipped with CCD cameras; the standard stars used in the data reduction were the same and their calibration was accurately verified to make all the data sets homogeneous, as described by Tosti et al. (1998) for the earlier observations.
A mean increasing brightening trend is evident throughout all the period, particularly after February 1998, but from April to the end of May the mean luminosity of ON 231 increased by about 0.5 mag. In the same period the source continued its intense flaring activity and during one of these flares it reached the greatest brightness (April 23, ). Another prominent flare occurred on May 1, but it is not well described because we have only one photometric point in the V band. Typical durations of the rapid flares were not longer than two-three days and the amplitudes during the maximum phase were comparable to those of the previous periods. Notice also that the variation amplitude in the V band is systematically greater than in .
Flux changes on a few hour time scales were detected in some intranight observations. An example is shown in Fig. 3, where two simultaneous light curves in the and B bands, obtained with two telescopes are plotted in the same scale: a synchronous small flare around 23 h is clearly evident: again the amplitude is greater in the B band with variation rate of about 0.3 mag/hour. The same rate was again observed on April 23 when the source had the maximum luminosity.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999