Nature of X-shaped sources

TL;DR

This study investigates the nature of X-shaped radio sources, testing whether they are due to binary AGNs or jet reorientation, through high-resolution radio observations and spectral analysis.

Contribution

The paper provides detailed morphological and spectral analysis of X-shaped sources using GMRT and VLA data, testing models of their origin and revealing unique spectral features.

Findings

Most sources show standard spectral steepening from hotspots.

One source, 3C 321, exhibits a flatter spectral index in low-surface-brightness extension.

Results support the binary AGN or jet reorientation models for X-shaped sources.

Abstract

The nature of X-shaped sources is a matter of considerable debate: it has even been proposed that they provide evidence for black hole mergers$ / $spin reorientation, and therefore constrain the rate of strong gravitational wave events (Merritt & Ekers 2002). Based on morphological and spectral characteristics of these sources, currently a strong contender to explain the nature of these sources is the `alternative' model of Lal & Rao (2007), in which these sources consist of two pairs of jets, which are associated with two unresolved AGNs. Detailed morphological and spectral results on milliarcsecond-scales (mas) provide a crucial test of this model, and hence these sources are excellent candidates to study on mas; {\it i.e.}, to detect he presence/absence of double nuclei/AGNs, signs of helical/disrupted jets, thereby, to investigate spatially resolved/unresolved binary AGN systems and providing clues to understanding the physics of merging of AGNs on mas. We conducted a systematic study of a large sample of known X-shaped, comparison FR II radio galaxies, and newly discovered X-shaped candidate sources using Giant Metrewave Radio Telescope and Very Large Array at several radio frequencies. In our new observations of `comparison' FR II radio galaxies we find that almost all of our targets show standard spectral steepening as a function of distance from the hotspot. However, one source, 3C 321, has a low-surface-brightness extension that shows a flatter spectral index than the high-surface-brightness hotspots$ / $lobes, as found in `known' X-shaped sources.