VLBI imaging and optical variability of the BL Lac object OQ 530 (B1418+546)

Abstract

Abstract.Results of VLBI and optical observations are presented for the BL Lac object OQ 530 (B1418+546), whose long term optical variability is characterised by a decreasing mean luminosity trend. EVN images at 1.6 GHz and 5 GHz of OQ 530 show a very bright core with a much weaker jet extending to about 35-40 mas. The radio flux density from the core in June 2001 was much higher than that measured in February 1999. A similar increase was also found from nearly simultaneous optical photometry, while the total flux density of the jet remained unchanged. The structure of the jet shows several knots which move outward with an apparent superluminal motion with a β app ≥ 3.5h −1 . The region within about 3 mas from the core contains a component not well resolved in our images and emerging from it in the jet direction. That component was also detected in images taken from 1990 to 1997 and it seems to be a rather stable structural feature. Using nearly simultaneous optical and radio data, we can describe the SED of the synchrotron peak with a simple formula, approximating a power law at low frequencies and a log-parabola at the high ones, whose maximum lies in the range 3.5−5 × 10 13 Hz, and the extrapolation in the X-ray range gives a flux comparable to that observed with BeppoSAX.

Exploring the bulk of the BL Lacertae object population

“…It is an extremely peculiar (see Sect. 5) and variable, both on long and short timescales (Massaro et al 2004) and at all frequencies. Variability in the optical and radio bands has shown to be correlated (Massaro et al 2004), as has polarization (Gabuzda et al 1996).…”

“…5) and variable, both on long and short timescales (Massaro et al 2004) and at all frequencies. Variability in the optical and radio bands has shown to be correlated (Massaro et al 2004), as has polarization (Gabuzda et al 1996). Multi-epoch studies have shown that the flux density variability affects are related to the core component, rather than the extended south-east jet: for example, an increase of a factor 3 was observed in the core flux density in the period 1999-2001 (Massaro et al 2004).…”

Optical follow-up of BL Lac candidates in the 2BL sample

“…We tried to establish what the SED of a source should look like to be properly considered as a "radioquiet" BL Lac. It is known that the synchrotron emission in many LBL objects covers the frequency interval from radio to the optical-UV: the energy distribution is generally well represented by a log-parabola (Landau et al 1986;Massaro et al 2004a), also combined with a power law on the low frequency side (Massaro et al 2004b):…”

BL Lac objects

“…These campaigns took great advantage of the Whole-Earth Blazar Telescope (WEBT) collaboration, a network of optical and radio telescopes that, together, are able to obtain high temporal density monitoring of blazars. A common result of these studies was that, while the low-level optical variability was not well correlated with that in the radio domain, the major optical bursts were followed by radio fluxdensity and polarisation variations in the VLBI core, with no apparent changes in the VLBI jet (see also [24]). …”