2013MNRAS.429.2773C

This paper presents optical BVRI photometry of the intermediate-energy-peaked BL Lac object ON 231 (W Comae). It was observed for five nights during 2010 March-April with the 80-cm Tsinghua-NAOC Telescope (TNT). Although no extremely large variations were seen on either intra- or internight time-scales, a flare-like event was recorded on March 17. On a time-scale of ∼ 5 h, the source exhibited a change of ∼ 0.15 mag in the B band and variations of ∼ 0.1 mag in V,R and I bands. On either intra- or internight time-scales, the variability amplitudes appear to be larger for shorter wavelengths. Cross-correlation analysis suggests that the R-band variations lagged the B-band ones by ∼ 1200 s on March 17. To our best knowledge, this is the first time that a time lag has been claimed for the optical variability of ON 231. Bluer-when-brighter trends were present for all intranight variations. On the plot of B - R colour indices against B magnitudes, however, the correlations between the two parameters shifted vertically relative to each other on different nights, indicating that the source slightly changed its optical spectral slope from night to night. The averaged spectral energy distributions of each night are similar, perhaps the consequence of low-amplitude variability during our observational season. The intranight optical variations of ON 231, along with those of other similar sources, e.g. BL Lacertae and S5 0716+714, are quite similar to those of high-energy-peaked BL Lac objects. Both can be ascribed to synchrotron emission produced by the most energetic relativistic electrons residing in the magnetized relativistic jet roughly aligned with our line of sight.