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

Abstract

We report evidence for in-situ acceleration/re-acceleration 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 afore-mentioned 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 bending of 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.

Figures (3)

• Figure 1: Radio maps showing the chain of contiguous circles of 6" diameter marking the ridge-line along each lobe and its associated wing. For each map, the spectral indices measured at the locations of the circles are shown in the panel directly below; the zero point on the horizontal axis corresponds to the position of the brightness peak in the primary lobe (hot spot), marked by a red circle in the lobe's map. The top two rows display the 5 XRGs from the present work and the lower two rows are for the 5 XRGs belonging to Paper I (see Sect. 4).
• Figure A1: The radio contour maps (VLASS, FIRST, and LoTSS-DR2), spectral-index (144 MHz -- 1.4 GHz) image, spectral-index error map, and the corresponding spectral index profile measured along the ridge-line (s) marked in the radio contour maps, are displayed row-wise for each XRG in our sample of 15 XRGs. The name of the source is mentioned at the top of the first panel in each row. The last panel in each row displays spectral index at several points along the ridge-line(s). These values represent average taken over a circular regions of 6 arcsec diameter (the beamsize) as shown along the corresponding ridge-line displayed in one of the three radio maps deemed best suited for identifying the wing component.
• Figure A1: Continued