Optical spectral behaviour of the blazar PKS 0537--441

Abstract

We present the long-term light curves of the blazar PKS 0537--441 in the optical B and R bands during the period from January 2011 to May 2015 on the basis of the photometric data provided by the SMARTS Consortium, and calculate the corresponding colour indices. The linear regression and discrete correlation function methods have been performed on the light curves and the colour indices curve. The results can be summarized as follows: (1) the light curves in the B and R bands show large variabilities on various timescales, while the colour indices curve reveals significant fluctuations which indicates the variation of spectra. A general BWB (bluer-when-brighter) trend is confirmed in long-term timescales and in the ascent and descent states during flares; (2) the linear regression shows that the light curves in the B and R bands are well correlated. The results obtained by the discrete correlation function method which reveals that the flux variation in the B band led that in the R band by $(35.62\pm8.63)$ d; (3) the analysis of periodicity suggests that $(53.06\pm6.62)$ d and $(273.88\pm20.69)$ d are the possible periods in the R-band light curve.