Wide-field GMRT imaging of X-shaped Radio-Galaxies: Spectral properties of 4C32.25 and 4C61.23

TL;DR

The paper presents wide-field 400 MHz uGMRT imaging of three X-shaped radio galaxies using a modified extreme-peeling direction-dependent calibration to correct field-wide DD effects. By combining new uGMRT data with archival LOFAR, VLA, and RACS observations, it derives integrated spectra, spectral-index maps, and spectral ages, finding that the wings are older than the primary lobes and that spectral gradients support precession for 4C32.25 and backflow for 4C61.23. The results strengthen the case for distinct formation channels for XRGs and demonstrate the efficacy of the DD calibration approach, while also identifying serendipitous radio sources in the fields. These methods and findings have implications for low-frequency, wide-field studies of diffuse AGN activity and can be extended to other bands and surveys.

Abstract

We present wide-field upgraded Giant Metrewave Radio Telescope (uGMRT) images of the fields around the X-shaped radio-galaxies (XRGs) 4C32.25, 4C61.23, and MRC 2011-298 obtained at 400 MHz. The observations are calibrated using the extreme peeling method to account for direction-dependent effects across the field of view, as previously applied to Low-frequency array (LOFAR) data. Our 400 MHz images capture in fine detail the radio-morphology of the XRGs, as well as other serendipitous radio-sources located in these fields. We use these images along with archival low-frequencyand high-frequency radio data to investigate the spectral properties of the XRGs 4C32.25 and 4C61.23. Under the assumption of conditions corresponding to the maximum radio-source age, we estimate the spectral ages of both the primary lobes and the wings. These ages indicate that the wings are the oldest component of the XRGs and are a product of past radio activity. Moreover, we have used the radio images available to derive high-resolution spectral index maps for these two XRGs. We find that the spectral index steepens from the primary lobes towards the wings, consistent with our spectral age estimates. These results suggest that precessional and backflow models explain the X-shaped radio-morphology of 4C32.25 and 4C61.23, respectively. Finally, taking advantage of our wide-area images, we identify several serendipitous diffuse radio-sources located in our XRG fields and cross-reference them with previous surveys.

Figures (14)

• Figure 1: Fitted clock differences using the gain solutions from 3C147. The values show a bimodal distribution between core and arm antennas (mainly from the Southern arm). The colors represent different antennas.
• Figure 2: Schematic view of the direction-independent calibration steps for uGMRT Band 3 data.
• Figure 3: Schematic view of the direction-dependent calibration steps for uGMRT Band 3 data.
• Figure 4: The spatial distribution of facets in the 4C61.23, 4C25.25, and MRC 2011-298 fields. The blue circle denotes angular distance where the uGMRT beam is 50 percent of that at the pointing center, that is about $0.74$ degrees.
• Figure 5: Images showing a direction-dependent self-calibration sequence for facet calibrators from the different fields. The corresponding field name is indicated in the images of the first column. The first column displays the images made with only the DI self-calibration solutions. The central column displays the improvements after two iterations of fast phase (TEC and phase-offset)-only DDE calibration. The right-hand column shows the improvement after two further iterations of fast phase (TEC+phase offset) and slow phase and amplitude (XX and YY gain) DDE calibration. The scale is the same for all images of the self-calibration sequence.