Astron. Astrophys. 342, L49-L52 (1999)

1. Introduction

ON 231 (1219+285) was one of the first radio sources classified as a BL Lacertae object (Biraud 1971; Browne 1971). The optical counterpart was already known as a variable star since the beginning of this century (Wolf 1916), when it was in a very bright state. The historic light curve (Tosti et al. 1999a) shows that the mean source luminosity is increasing after a minimum in the early seventies. Since 1994 we are carring out an optical monitoring whose results up to June 1997 are described by Tosti et al. (1998). In that period ON 231 was very active and showed in each season a bright maximum in which its (Cousins) magnitude reached the level of about 13.4. In 1998 the activity of ON 231 was even stronger than in the past years; in particular, the source had an extraordinary outburst in April 1998 and reached the highest brightness level ever recorded since the Wolf 's discovery.

We performed during this prominent burst many optical photometric and polarimetric measurements. The most relevant results are presented in this paper while all the data tables, the instruments and the data reduction techniques are described in a couple of separate papers (Efimov & Shakhovskoy 1998, Tosti et al. 1999b), retrivaible from an electronic data journal at the Web site http://bldata.pg.infn.it . Our observations triggered also two ToO pointings of the X-ray satellite BeppoSAX in May and June 1998: in the former ON 231 showed a very unusual spectrum with a clear excess at energies greater than 4 keV (Tagliaferri et al. 1998).

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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