Spectral index variation across X-shaped radio galaxies

Abstract: The formation mechanism of the enigmatic subclass of radio galaxies, called 'X-shaped radio galaxies' (XRGs), or 'winged' radio galaxies, which account for $\sim 10\%$ of the radio galaxy population, can be effectively constrained using the radio spectral-index distribution across their twin pairs of radio lobes. If indeed, the existing claims of no systematic spectral index difference between the wing and the associated primary lobe are valid in general, this would provide impetus to the XRG model attributing their origin to an unresolved binary of active supermassive black holes within the nucleus of the host galaxy. To investigate this interesting possibility, we have mapped spatial variation of spectral index for a well-defined sample of 25 XRGs, by combining their 1.4 GHz VLA (FIRST survey)/uGMRT maps with their 144 MHz maps (LoTSS-DR2). This has yielded the best available combination of sensitivity, angular resolution, frequency range and sample size, for spectral mapping of an XRG sample. A rich diversity of spectral index patterns is thus revealed in our XRG sample, but we find at most one case where a secondary lobe (wing) exhibits a flatter spectrum compared to its associated primary lobe. We conclude that such a spectral pattern is exceedingly rare and by no means a common trait of XRGs.