A high-resolution study of the double radio relic system in MACS J1752.0+4440

Abstract

Radio relics are diffuse, extended synchrotron sources located at the outskirts of merging galaxy clusters. Their origin has been linked with shock waves injected into the intracluster medium, but the acceleration mechanism at the shock front is still under debate. Some clusters, like MACS J1752.0+4440, host a double relic system, with two relics found on opposite sides with respect to the cluster center. To investigate the acceleration mechanism that generates radio relics, we study the morphological and spectral properties of the double relic system in MACS J1752. We present new wideband radio continuum observations made with uGMRT and JVLA, and LOFAR data. We perform a detailed, high-resolution spectral analysis of the double relic system in MACS J1752, observing and characterizing substructures, particularly for the brighter relic. We find a double-peaked surface brightness and spectral index profile for the NE relic and identify a "bright bar" substructure. Moreover, we observed surprisingly flat integrated spectral indices for both relics, at $α_{\mathrm{int}}^{\mathrm{NE}} = -0.91 \pm 0.06$ and $α_{\mathrm{int}}^{\mathrm{SW}} = -0.83 \pm 0.05$. We study the spatial variation of the spectral index, observing a coherent trend with the observed substructures. We estimate an injection Mach number of $\mathcal{M}_{\mathrm{NE}} = 3.1^{+0.1}_{-0.1}$ and $\mathcal{M}_{\mathrm{SW}} = 3.2^{+0.1}_{-0.1}$. By performing a spectral curvature analysis for both relics, generating color-color plots and a spectral curvature maps, we observe two "concave" spectra represented by positive spectral curvature, in contrast with particle population ageing models. The observed properties of the NE relic are not consistent with a simple scenario with a single shock front. Multiple shock surfaces, re-acceleration, and projection effects likely play a role in shaping the morphology of the relic.

Figures (10)

• Figure 1: Composite image of MACS J1752.0+4440. The LOFAR 144 MHz contours (in white) are overlaid onto an optical image from Pan-STARRS (in grey) and an X-ray image from XMM-Newton (in red) from Botteon2022Zhang2023.
• Figure 2: uGMRT Band 4 ($650\,$MHz) image of MACS J1752+4440. This image, at a resolution of $3$"$\times 3$", is the highest resolution image ever obtained for this cluster. In white, the different structures are labelled. The outer edge component outlines the possible position of the merger shock that generated the NE relic, the bright bar is a substructure present in the downstream region of the first component. The SW relic shows no substructures at this resolution. Contours are drawn starting at $3\sigma_{rms}$ and are separated by a factor 2. Beam and scalebar are found at the bottom and top of the image, respectively.
• Figure 3: $7$" resolution radio images for the MACS J1752+4440 cluster. Top Left: LOFAR image at $144\,$MHz with noise of $\sigma_{rms,144}=280\,$$\mu$Jy $\text{beam}^{-1}$. Top Right: uGMRT image at $416\,$MHz with noise of $\sigma_{rms,416}=70\,$$\mu$Jy $\text{beam}^{-1}$. Bottom Left: uGMRT image at $650\,$MHz with noise of $\sigma_{rms,650}=23\,$$\mu$Jy $\text{beam}^{-1}$. Bottom Right: JVLA image at $1.6\,$GHz with noise of $\sigma_{rms,1600}=15\,$$\mu$Jy $\text{beam}^{-1}$. Images have a common angular resolution of $7^{\text{"}} \times 7^{\text{"}}$ and contour levels are drawn at $[-3, 3, 9, 18, 36, 72] \times \sigma_{rms}$ (the negative contour is in dashed). The color scale is logarithmic and beam and scalebar are found at the bottom of the images.
• Figure 4: Left: Spectra for NE (red) and SW (blue) relic, fitted by two power-laws. Right: LOFAR high-resolution continuum image at $144\,$MHz with the regions used to estimate the flux density of the two relics over-imposed in yellow. Additionally, the regions used to produce the brightness and spectral index profiles are overlaid in red along the relics.
• Figure 5: Surface brightness profiles for both radio relics. Left: Profile of the NE relic. The profile presents the peak brightness in region 5 and 6, with a second peak in region 3. This profile is consistent with what can be seen in the spectral index profiles, with two peaks in the position of the two substructures of the relic. Right: Profile of the SW relic. The profile shows the peak brightness in region 2, followed by a fast decline in the downstream region.