Correlations between the peak flux density and the position angle of inner-jet in three blazars

We aim to investigate the relation between the long-term flux density and the position angle (PA) evolution of inner-jet in blazars. We have carried out the elliptic Gaussian model-fit to the `core' of 50 blazars from 15 GHz VLBA data, and analyzed the variability properties of three blazars from the model-fit results. Diverse correlations between the long-term peak flux density and the PA evolution of the major axis of the `core' have been found in $\sim$ 20% of the 50 sources. Of them, three typical blazars have been analyzed, which also show quasi-periodic flux variations of a few years (T). The correlation between the peak flux density and the PA of inner-jet is positive for S5~0716+714, and negative for S4~1807+698. The two sources cannot be explained with the ballistic jet models, the non-ballistic models have been analyzed to explain the two sub-luminal blazars. A correlation between the peak flux density and the PA (with a T/4 time lag) of inner-jet is found in [HB89]~1823+568, this correlation can be explained with a ballistic precession jet model. All the explanations are based mainly on the geometric beaming effect; physical flux density variations from the jet base would be considered for more complicated situations in future, which could account for the no or less significance of the correlation between the peak flux density and the PA of inner-jet in the majority blazars of our sample.Comment: 6 pages, 7 figures, accepted for publication in Astrophysics and Space Scienc

Are there two types of BL Lac objects?

ROSAT data as well as archival radio and optical data are used for a broad band study of a large group of blazars with positive detection in the soft X-ray band. It is found that X-ray selected BL Lacs differ in their properties from radio selected BL Lacs which, in turn, merge smoothly into the group of highly polarized quasars in many respects. The average soft X-ray spectra of all three object classes differ slightly and X-ray selected BL Lacs seem to require absorption in excess of the Galactic N_H-values. We purpose that the two types of BL Lacs are intrinsically different and either originate from different parent populations or have emission conditions with characteristically distinct physical parameters. Radio selected BL Lacs and highly polarized quasars exhibit nearly identical continuum emission properties but differ in their redshift distribution and intrinsic luminosities. They are thus objects with physically similar parsec-scale jets and might even represent different evolutionary or environmental stages of the same class of highly beamed objects. X-ray selected and radio selected BL Lacs populate distinctly different phase space regions in flux - ratio diagrams. The corresponding values might thus serve as a unambiguous class - definition for the BL Lac population. (orig.)Available from TIB Hannover: RN 9303(360) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Radio-loud active galaxies in the Northern ROSAT all-sky survey. Pt. 2 Multi-frequency properties of unidentified sources

We present the broad band, radio - to - X-ray, properties of a large sample of mostly previously optically unidentified radio-loud X-ray sources from the correlation of a ROSAT All-Sky Survey source list with the 5 GHz Green Bank Survey of the northern sky (RGB sample) which is one of the largest well-defined flux-limited surveys of AGN ever obtained. Further, the RGB pushes 1-2 orders of magnitude deeper in both X-ray and radio flux compared to previous unbiased wide-area AGN surveys. Follow up VLA observations of the candidate objects yielded positions with arcsec accuracy which were used to find optical counterparts to the sources from digitized POSS plates. The sources are divided into three classes according to the positional offset between the X-ray and radio candidates and the spatial resolution of the radio observations, reflecting the various degrees of confidence about the correctness of the proposed association. Although the nature of the sources as well as their red-shifts remain to be determined in spectroscopic follow up observations, the derived flux ratios lead to the conclusion that the majority of them are quasars. Hardly any correlations could be found between different source parameters, possibly due to the fact that most of the objects are found in a relatively small flux range near the sensitivity limit of the radio catalogue. The majority of the new RGB sources have broad-band properties between those of traditional radio-selected and X-ray selected AGN. There is no bimodal distribution in the radio-loudness distribution, and the traditional division between radio-quiet and radio-loud AGN may not be warranted. (orig.)Available from TIB Hannover: RN 9303(389) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman