GMRT study of X-shaped radio sources
  Context. The nature of X-shaped sources is a matter of considerable debate in the literature: 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. Aim. To explore the nature of these X-shaped radio galaxies. Method. We conduct a systematic study of a large sample of known and newly discovered X-shaped sources along with a comparison sample. We used the Giant Metrewave Radio Telescope with resolution of $\sim6^{\prime\prime}$ to $\sim15^{\prime\prime}$ at 610 MHz and 240 MHz in the dual-frequency mode. Preliminary Result. Based on our careful analysis and estimation of the possible systematic errors, the known X-shaped sources divide into the following three categories: (i) the wings have flatter spectral indices than the active lobes, (ii) the wings and the active lobes have comparable spectral indices, and (iii) the wings have steeper spectral indices than the active lobes. In addition, based on our preliminary analysis, one sample source from our comparison sample shows a spectral index result belonging to category (i). Future. Milliarcsecond scale imaging will be conducted on some of these X-shaped sources to investigate if they are examples of binary AGN systems and thereby understand the nature of these sources. 
Dharam Vir Lal, A. Pramesh Rao, Martin J. Hardcastle, C.C. Cheung,   Sanjay Bhatnagar, Ralph P. Kraft, Andrei P. Lobanov and...