Multiband polarimetric observations were performed just before and during the brightest phase (April 17-25) and after one month when the luminosity of ON 231 was about a factor of 3 weaker than the maximum. On April 17 (V=13.6) the polarisation percentage was very low in all the bands: between 2.30.4% and 3.60.5% in and practically not detected (i.e. less than 0.4%) in and . Three days after, when the V magnitude of ON 231 was 13.10, we measured the mean polarisation around 8% and a comparable level was also found on April 23 (V=12.64). Higher levels were measured on April 25, when the source brightness decreased to V=13.42 and one month later (May 26), when it was at the level of V=13.71.
A clear dependence of the polarisation fraction upon the frequency was also evident: it was greatest in the U band, ranging during the outburst from 9.90.5% to 12.90.7%, and lowest in (from 6.60.3% to 9.00.3%). These results are shown in Fig. 5. Polarisation angles were the same in all the five photometric bands and changed with time, but only in a 35o range: on April 23 the PA was 1622, after two days 1282 and on May 26 it was 142 1.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999