Abstract

We report new measurements of the BL Lac object ON 231 (W Com) from radio to optical wavelengths. This source was found to be at its highest brightness in the near IR and optical bands for many years.

Introduction

ON 231 is one of the targets in the observational programme in the near IR that we have been carrying out since 1986 at the 1.5 m Italian IR Telescope at Gornergrat (TIRGO, 3150 m a.s.l.). The results of the measurements performed up to the spring of 1988 are given in. At the beginning of February 1992 we found the source in the highest state for about 20 years and therefore, in addition to the IR and optical measurements, we observed ON 231 at radio frequencies to obtain a more complete picture of its spectral distribution. Radio observations were carried out with the 32 m dish of Istituto di Radioastronomia (CNR - Bologna, Medicina) at a frequency of 22.2 GHz.

Results

The results of the new measurements are presented in Fig. 1 (open circles) together with those of Landau et al. performed in the spring 1983 (crosses). Some remarkable facts are evident: i) the flux in the near IR and optical bands in 1992 is generally higher than in 1983; ii) at variance with the above, the flux at 22.2 GHz does not show a significant change with respect to the previous measurement; iii) the maximum power emitted by ON 231 in the high state lies between 250 and 500 THz, while in 1983 it was at a lower frequency, likely below 100 THz; iv) the spectral slopes in the optical and near IR bands are flatter when the luminosity is higher.