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

4. Polarimetry

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 35^o range: on April 23 the PA was 1622, after two days 1282 and on May 26 it was 142 1.

Fig. 5. Frequency dependent linear polarisation of ON 231 observed in April and May 1998.

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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