The radio structure of BL Lac object S5 2007+777

Abstract

BL Lac object S5 2007+777 has been classified as a HYMOR (Hybrid Morphology radio source) for its hybrid FR I and FR II radio morphology. It is one of four BL Lac objects with a kpc scale extended X-ray jet. In this paper, we searched the unpublished European VLBI Network (EVN) archived high resolution data for this source, and present the high resolution radio structure. The EVN images all show a core-jet structure in 1.6, 5 and 8 GHz. In combination with the VLA data in the literature, we found position angle differences (∆PA) of about 10° -20° between pc and kpc scale jet structures. The brightness temperature estimated from multi-band EVN images are around 10 11.2 -10 11.8 K, from which the Doppler factor ranges from 3.2 to 12.0. We found significant flux variations of the radio core in 1.6 and 5 GHz, from which we derived the variability in brightness temperature T var to be 10 13.6 K and 10 14.0 K, corresponding to the Doppler factors of 9.3 and 12.4, respectively.active galactic nuclei, VLBI, BL Lac objects S5 2007+777 was identified as a BL Lac object owing to its featureless optical spectrum [1] with redshift of z =0.342. It is an Intraday Variable (IDV) source showing variations in total and in polarized intensity on time scales of 2 to 6 days at cm wavelengths, as well as at optical wavelengths, with amplitudes of 2% to 15% on a time scale of a few hours to a few days [2]. The multi-band Spectral Energy Distribution (SED) of this source shows that it can be classified as a low frequency peaked BL Lac object [3].X-ray jets in AGNs are commonly observed in FR II and FR I radio galaxies, but rarely in BL Lac objects. Recently, evidence was found of an extended X-ray jet in S5 2007+777; only three BL Lac objects are known so far to contain a kpc-scale X-ray jet [4]. To better understand its jet properties and the reason for detecting the large scale X-ray jet, we investigated the radio jet properties of this source. This source is strong in radio, and is frequently used as a fringe-finding calibrator in VLBI observations. We searched the EVN archive for this source. The organization of this paper is as follows: in sec. 1, we list the VLBI observations we used, and also present the procedure of data reduction and model fitting, and results and discussions are finally given in sec. 2.

Investigation of stronger diurnal ERP signals in summer derived from the VLBI CONT08 campaign

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