Microvariability in the southern gamma-ray blazar PKS 0537-441

The southern blazar PKS 0537-441 is one of the most variable active galactic nuclei. It has displayed extreme forms of variability in its emission from radio to gamma-ray wavelengths. Here we present results of new optical microvariability observations of this blazar in the V and R bands, obtained with high-quality CCD photometry made with a 2.15-m telescope at CASLEO, Argentina. We have found variability with amplitudes of more than 100% over timescales of ∼2 days, similar to what has been observed in gamma-rays by EGRET. The spectral index, contrarily to what is expected from microlensing, showed changes of ∼27% during the observing period, with a trend to steepen with decreasing flux. This seems to reflect an intrinsic mechanism behind the variability and, consequently, suggests that optical and gamma-ray emitting regions in this source can be co-extensive. If the most extreme manifestations of radio variability are also intrinsic, they might involve coherent emission processes. We have used our observations, along with EGRET and ROSAT data, to constrain the central black hole mass, which seems to be ∼8×107 M⊙ For such a mass the γ-ray variability is originated at ∼290 gravitational radii from the central source.