Simultaneous Swift and REM Monitoring of the Blazar PKS 0537–441 in 2005

The blazar PKS 0537-441 has been observed with all instruments of the Swift satellite between the end of 2004 and 2005 November. The BAT monitored it recurrently for a total of 2.7 Ms, and the XRT and UVOT pointed it on seven occasions for a total of 67 k. The automatic optical and near-infrared telescope REM has monitored simultaneously the source at all times. In 2005 January-February, PKS 0537-441 was detected at its brightest in optical and X-rays: more than a factor of 2 brighter in X-rays and about a factor of 60 brighter in the optical than observed in 2004 December. The 2005 July observation recorded a fainter X-ray state, albeit still brighter than the historical average. The simultaneous optical state is comparable to the one recorded in early 2005 January, before the outburst. In 2005 November, the source subsided both in X-rays and optical to a quiescent state. The optical and X-ray variations are well correlated, with no measurable time lag larger than about 1 month. On intraday timoscales there is no obvious correlation between X-ray and optical variations, but the former tend to be more pronounced, opposite to what is observed on monthly timescales. The widely different amplitude of the long-term variability in optical and X-rays is very unusual. The spectral energy distributions are interpreted in terms of the synchrotron and inverse Compton mechanisms within a jet where the plasma radiates via internal shocks and the dissipation depends on the distance of the emitting region from the central engine.