Intra- and inter-night optical photopolarimetric variations in ON 231(W Com)

ATel #2581; K S Baliyan, S Chandra, S Ganesh, U C Joshi (PRL)
on 23 Apr 2010; 11:45 UT
Credential Certification: Kiran S Baliyan (baliyan@prl.res.in)

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ON 231 (W Com) was one of the first radio sources classified as a BL Lac. It shows considerable variation in almost all energy regimes. We monitored it for linear Optical Polarization using 1.2m Telescope at Mt Abu IR Observatory, India, mounted with two-channel PRL Photo-Polarimeter. W Com showed interesting behavior in optical polarization during April 14-16, 2010. On April 14, linear polarization slowly increased from 7.4(+/-0.7)% to 9.7% during UT 19.5 to 21.0 and then dropped sharply to about 5.8% within half an hour. The magnitude (white light) decreased by about 0.5 during this period. The linear polarization increased slowly again to reach about 9.5% in about two hrs. The polarization position angle remained almost un-changed during this period. On April 15, degree of polarization showed U-shaped structure, decreasing from about 8.5% (UT=16.5) to ~6.1%(UT=17.5), then rising (UT=20.2) to about 8% at UT=21.7 Hrs while position angle again remained almost constant with WCom showing marginal brightness (by 0.1mag). On April 16, W Com linear polarization increased from about 6.5% to about 11.4%(Errors +/-1%) during UT=16.8 to UT=22.6 Hrs - a change of about 4.6% in ~ 6 hours(>0.7%/hour). On April 16, while WCom brightness decreased by about 0.2 mag, position angle of polarization did not change.

The average degree of polarization changed from night to night by about 1.5% to 2.5% and brightness by about 0.2 mag. Relatively consistent position angle over night might be indicative of either a stable magnetic field orientation in the jet or the enhancement in the polarization due to synchrotron flares in the accretion disk. The relationship between degree of polarization and total intensity variations are rather complicated to make any meaningful comment here.