Evidence for in-situ acceleration of relativistic particles in the wings of X-shaped radio galaxies

Abstract

We report evidence for in-situ acceleration/reacceleration of relativistic particles in 11 radio wings out of a total of 68 wings sufficiently well-resolved for spectral mapping, which belong to our sample of 40 X-shaped radio galaxies (XRGs). This representative XRG sample includes 15 XRGs newly reported here, which we selected from the LOTSS-DR2 survey, following well-defined criteria. The evidence for in-situ particle acceleration comes from the observed cessation of steepening, or even flattening (i.e., gradient reversal) of the spectral index profile along the lobe into the associated wing, as determined here by combining the LoTSS-DR2 (144 MHz) and FIRST (1.4 GHz) maps. Interestingly, the aforementioned trends in spectral gradient, indicative of in-situ particle acceleration, are mostly found to set in near the region where the lobe plasma stream bends to connect to the wing. Such a spatial coincidence with the bending of the radio lobe/tail has been noticed in recent years for just a couple of radio galaxies. The large increase in such examples, as reported here, is expected to give a fillip to observational, theoretical, and numerical simulation follow-up investigations of this important clue about the occurrence of in-situ particle acceleration in lobes of radio galaxies.