Microvariability in the southern g-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. Intraday variations in
the radio, optical, and gamma-ray flux have been reported with similar
timescales. This has led Romero et al. (1995) to postulate a superluminal
microlensing model based on previous reports of an interposed galaxy. Here
we present results of new optical microvariability observations 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 in 1996. The
spectral index, contrarily to what is expected from microlensing, showed
changes of more than 16 % during the observing period, with a trend to steepen
with decreasing flux. This seems to reflect an intrinsic mechanism and,
consequently, suggests that optical and gamma-ray emitting regions 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 the EGRET data,
to constrained the central black hole mass, which seems to be
~ 7×107 M\odot. For such a mass, the variability is
originated at ~ 320 gravitational radii.
File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:18.