X-shaped radio sources as parent population of core-dominated triple blazars

Abstract

There are a number of theories explaining the nature of the so-called X-shaped radio sources. According to one of them, an X-shaped source is indeed a cross whose one arm is associated with a double radio source that has changed its orientation in space, while the other arm is associated with relic lobes and its position indicates the former direction of the jets. Here, I present two new arguments in favour of this conjecture. Firstly, it is obvious that shortly after the repositioning, the pair of the new lobes must be very compact. To illustrate such a possibility, I show an EVN image of the central component of a triple source J1625+2712. When resolved, it appears as a compact double that is not aligned with the outer double so the whole source is indeed X-shaped. Secondly, I consider the situation when one of the arms of an X-shaped source is not intrinsically short but foreshortened by projection. I show two examples of triple sources whose central component is a blazar and the span of the lobes that straddle it amounts to more than 6\times10^5 pc. An assumption that sources of this kind have one axis, and so the lobes are beamed in the same way the core is, would require unrealistically huge deprojected linear sizes. Therefore, I claim that core-dominated triples (CDT) like these two have two axes: the one pertinent to the relic lobes is not pointed towards us so they are not beamed/foreshortened, whereas the axis pertinent to the jets makes a small angle with the line of sight so that a blazar is observed. It follows that X-shaped sources must be actual crosses and they are the parent (unbeamed) population of at least some CDT blazars, particularly those with large overall sizes.