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

¹ Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy e-mail: enrico.massaro@uniroma1.it
² IASF - Sez. di Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy
³ Istituto di Radioastronomia, viale P. Gobetti 101, 40129 Bologna, Italy
⁴ Dipartimento di Fisica, Università di Bologna, via Irnerio 46, 40126 Bologna, Italy
⁵ Stazione Astronomica di Vallinfreda, Vallinfreda (RM) Italy
⁶ Oss. Astronomico and Dipartimento di Fisica, Università di Perugia, via A. Pascoli, 06123 Perugia, Italy

Received: 27 February 2004
Accepted: 12 May 2004

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 . 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  Hz, and the extrapolation in the X-ray range gives a flux comparable to that observed with BeppoSAX.