Nature of X-shaped sources
  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.
