Combined LOFAR-uGMRT analysis of the diffuse radio emission in the massive clusters Abell 773 and Abell 1351

TL;DR

This study presents a joint LOFAR-uGMRT analysis of diffuse radio halos in two massive merging clusters, A773 and A1351, using 144 MHz and 650 MHz data plus XMM-Newton imaging. The halos extend to about 2 Mpc, with integrated spectral indices near $\alpha_{144}^{650} \sim -1.0$; A773 resembles a classical halo, while A1351 exhibits complex morphology due to embedded sources and a ridge. A773 shows a sublinear radio–X-ray correlation consistent with turbulent re-acceleration, whereas A1351 shows a weak or highly scattered relation, likely driven by contaminating radio sources that obscure the halo signal. A LOFAR model injection technique is introduced to quantify flux losses in the uGMRT data, yielding robust spectral measurements and highlighting the importance of careful source subtraction in complex halos. Overall, the results support turbulent re-acceleration in massive clusters and demonstrate the utility of joint low- and mid-frequency radio analyses for disentangling diffuse emission from contaminating sources.

Abstract

Radio halos are megaparsec-scale diffuse, non-thermal radio sources located at the centers of galaxy clusters, tracing relativistic particles and magnetic fields in the intra-cluster medium. Their origin is generally attributed to cluster mergers that generate turbulence and re-accelerate aged electrons. We study the diffuse radio emission, spectral properties, and the connection between thermal and non-thermal components in the massive galaxy clusters Abell 773 and Abell 1351 ($M_{500} \sim 7 \times 10^{14}\,M_{\odot}$), both of which are dynamically disturbed. We combine LOFAR LoTSS-DR2 observations at 144 MHz with uGMRT observations at 650 MHz, supplemented by archival XMM-Newton X-ray imaging. We confirm that both clusters host radio halos extending up to a largest linear size of $\sim 2$ Mpc. We measure an integrated spectral index $α_{144}^{650} \sim -1.0$ for both clusters. The radio halo in Abell 773 resembles a classical halo and follows a sublinear radio--X-ray surface brightness relation. In contrast, Abell 1351 shows a more complex and asymmetric morphology, influenced by embedded radio sources including the brightest cluster galaxy, a tailed radio galaxy, and a ridge-like feature. These contaminating sources lead to deviations from the sublinear trend in the point-to-point radio--X-ray analysis of Abell 1351.

Figures (11)

• Figure 1: Left: High resolution LOFAR image of A773 at 144 MHz. Right: High resolution uGMRT image of A773 at 650 MHz. Both images are at a resolution of $10" \times 10"$, with $M_{500}$ in units of $10^{14} M_\odot$ and rms in mJy/beam. The drawn region represents the $r_{500}$ scale of the cluster.
• Figure 2: High-resolution LOFAR image of A1351 at 144 MHz with a resolution of $8.64" \times 4.32"$ and an rms noise of 68 $\mu$Jy beam$^{-1}$. The black circle represents the $r_{500}$ scale of the cluster. Inset top: Zoom-in of the cluster center at uGMRT 650 MHz at a resolution of $4.68" \times 3.34"$. The rms of the uGMRT image is 13 $\mu$Jy beam$^{-1}$. Inset bottom: Optical Pan-STARRS (g,r,i) image of the cluster center overlaid with uGMRT radio contours at levels of (9, 18, 27, 36, 45, 52) $\times ~\sigma_{rms}$. $M_{500}$ is given in units of $10^{14}\,M_\odot$.
• Figure 3: Left: Low resolution source subtracted LOFAR image of A773 at 144 MHz overlaid with LOFAR 2$\sigma$ contours. Middle: Low resolution source subtracted uGMRT image of A773 at 650 MHz overlaid with uGMRT 2$\sigma$ contours. Right: XMM image of A773 overlaid with LOFAR source subtracted contours. The contour levels are spaced as $(-2, 2, 4, 8, 16, 32) \times \sigma_{rms}$. LOFAR and uGMRT source subtracted images have a resolution of $50" \times 50"$. The region drawn around the halo represents the $r_{500}$ scale of the cluster.
• Figure 4: Left: Low resolution source subtracted LOFAR image of A1351 at 144 MHz overlaid with LOFAR 2$\sigma$ contours. Middle: Low resolution source subtracted uGMRT image of A1351 at 650 MHz overlaid with uGMRT 2$\sigma$ contours. Right: XMM image of A1351 overlaid with LOFAR source subtracted contours. The contour levels are spaced as $(-2, 2, 4, 8, 16, 32) \times \sigma_{rms}$. LOFAR and uGMRT source subtracted images have a resolution of $33" \times 33"$. The region drawn around the halo represents the $r_{500}$ scale of the cluster.
• Figure 5: From left: Spectral index map between the source subtracted images of LOFAR at 144 MHz and uGMRT at 650 MHz of A773 and its error map, with a resolution of $50" \times 50"$, overlaid with 3 sigma contours of the LOFAR image. The contour levels for LOFAR are set at $(3, 6, 9, 12, 27, 41) \times \sigma_{\text{rms}}$, where $\sigma_{\text{rms}}$ is 0.30 mJy/beam.