Illuminating the Diffuse Radio Emission in Low-Mass Cluster: Abell 13

TL;DR

This paper investigates diffuse radio emission in the low-mass merging cluster Abell 13 using four-frequency, high-sensitivity radio data. The authors obtain a detailed morphological and spectral view, including the first spectral index and curvature maps for this emission, and find a largest linear size of about 521 kpc with a central origin and westward extension. The integrated spectrum is steep and curved (α ≈ −1.85 at 400 MHz and SC ≈ −0.93), consistent with re-energization of fossil AGN plasma via adiabatic compression, i.e., a radio phoenix. The results support a phoenix scenario associated with past AGN activity in the BCG and redistribution by ICM motions during the merger, highlighting the role of fossil plasma in shaping non-thermal phenomena in low-mass clusters and motivating polarization studies for further constraints on magnetic fields and dynamics.

Abstract

Recent advances in high-sensitivity radio observations have uncovered a population of faint, ultra-steep-spectrum sources in galaxy clusters, commonly known as radio phoenixes. However, their observational classification remains poorly constrained due to the limited number of confirmed detections. This study presents a detailed multi-frequency, high-sensitivity, and high-resolution analysis of diffuse radio emission in the merging galaxy cluster Abell 13. Using GMRT (147.5 MHz), uGMRT (400 MHz), ASKAP-low (887.5 MHz), and MGCLS (1284 MHz) images, we detect complex, filamentary diffuse emission with a largest linear extent of 521 kpc. This emission originates from the cluster center and extends westward, confined within the X-ray-emitting intra-cluster medium (ICM). Chandra X-ray data confirm that Abell 13 is undergoing a merger, and the radio morphology reflects signatures of this ongoing dynamical activity. We observed filamentary structures extending towards east-northeast and southwest directions. The spectral index across the emission appears irregular and lacks a coherent spatial gradient. The integrated spectrum reveals a steep spectral index of -1.85 +/- 0.05 and a spectral curvature of -0.93 +/- 0.21. These spectral properties, along with the observed morphology and brightness distribution, are consistent with a re-energization of a fossil radio plasma driven by adiabatic compression, supporting the classification of the emission as a radio phoenix.

Figures (8)

• Figure 1: Left: Chandra X-ray image in the 0.3–8.0 keV band (pixel size $\sim$ 2$^{\prime\prime}$), smoothed with a Gaussian kernel of $\sigma = 2$ pixels to highlight diffuse structures. The image is overlaid with uGMRT 400 MHz radio contours (red). The yellow 'X' denotes the center of the cluster, the cyan '+' is the BCG, and the yellow '+' is the position of the 2nd BCG, which is considered as the part of another sub-cluster according to juett2008chandra. Right: Pan-STARRS 'r' band optical image overlaid with uGMRT 400 MHz radio contours. The BCG is shown as 'H' and the 2nd BCG as 'F'. The labeling is done as in slee2001four. Contour levels in both the images corresponds to [-3, 3, 6, 12, 24, 48, 60, 75, 90] $\times$$\sigma_{\mathrm{rms}}$ of image IM4 (see Table \ref{['tab:complex_multicolumn_multirow']}). Negative contours are dashed.
• Figure 2: The uGMRT 400 MHz radio image same as in Fig. \ref{['fig: radio and optical']}. Contour levels are [-3, 3, 6, 12, 24, 48, 60, 75, 90] $\times$$\sigma_{\mathrm{rms}}$, where $\sigma_{\mathrm{rms}} = 191\,\mathrm{\mu Jy/beam}$ of image IM4 (see Table \ref{['tab:complex_multicolumn_multirow']}). The restoring beam is 16$^{\prime\prime}$$\times$ 13$^{\prime\prime}$. The important parts of diffuse emissions labelled are discussed in Sect. \ref{['results']}. The black colored star marks the position of the BCG, as identified from the optical image.
• Figure 3: The radio images at frequencies 147.5 MHz (GMRT), 400 MHz (uGMRT), 887.5 MHz (ASKAP-low), and 1284 MHz (MGCLS) with contour levels of [-3, 3, 6, 12, 24, 48, 50, 75, 90] $\times$$\sigma_{\mathrm{rms}}$. The restoring beam of all images are 25$^{\prime\prime}$$\times$ 25$^{\prime\prime}$. Negative contours are dashed. The left bottom corner of every image shows the restoring beam. The details of image is given in Table \ref{['tab:complex_multicolumn_multirow']} IM1, IM3, IM6, and IM7.
• Figure 4: Left: uGMRT 400 MHz radio image overlaid with GMRT 147.5 MHz contours. The contour levels of [-3, 3, 6, 12, 24, 48] $\times$$\sigma_{\mathrm{rms}}$, where $\sigma_{\mathrm{rms}}$= $1.05 \, \mathrm{mJy/Beam}$. Right: MGCLS 1.2 GHz radio image overlaid with uGMRT 400 MHz contours. The contour levels of [-3, 3, 6, 12, 24, 48] $\times$$\sigma_{\mathrm{rms}}$, where $\sigma_{\mathrm{rms}}$= $243 \,\mathrm{\mu Jy/Beam}$. Negative contours are dashed. The cyan contour within the magenta rectangular box is used to present the FRI remanent corresponding to the peak 1 in Fig. \ref{['fig:A13_patch_label']}. The images are corresponds to IM2, IM5 and IM8 in Table. \ref{['tab:complex_multicolumn_multirow']}
• Figure 5: Left: The surface brightness profile of the filament extending to BCG along its length. Right: The surface brightness profile of the bridge connection between the northwest filament and the main body. The black arrow indicates the plot direction. The profile is the normalized peak surface brightness enclosed within each bin. The bin width is 15$^{\prime\prime}$. The images used for the calculations are IM2, IM5, and IM8 in Table. \ref{['tab:complex_multicolumn_multirow']}