2017MNRAS.464.3972L

We present the results of simultaneous multiwavelength observations of the blazar PKS 0537-441 performed with the Rapid Eye Mount telescope (REM) and Swift in the period 2009-2011. During our campaign, PKS 0537-441 showed prominent activity with well-correlated light curves from near-infrared to X-rays in different time epochs. The multiwavelength light curves exhibit several significant flares, in which the 2009 November flare particularly presents a double-peak feature, while the 2010 February flare shows strong optical variability (up to a factor of ∼5). The REM data show smaller fractional variability amplitudes than the Swift data. There is a general correlation between optical and X-rays, time-lags have been determined with data correlation methods and their uncertainties with Monte Carlo runs, but they are either very uncertain or too long (of the order of 300 d) to be physically relevant. The X-ray spectra exhibit neither absorption nor emission features characteristically. A slight harder-when-brighter trend in the 2-10 keV energy band is detected. The optical/ultraviolet spectra suggest the presence of thermal emission. We find evidence that the broad-band spectrum is steeper when the degree of variability is larger.