High significance detection at 4.8 GHz of the radio halo in the Coma galaxy cluster with the Sardinia Radio Telescope

Abstract

We present the results of observations of the radio halo in the Coma galaxy cluster at 4.8 GHz performed with the Sardinia Radio Telescope. The radio halo in this cluster is detected for the first time at this frequency with a statistical significance higher than $3σ$. After the removal of the Radio Frequency Interference and of the discrete sources contribution, and after the correction for the Sunyaev-Zel'dovich effect, we estimate a flux density of $61\pm11$ mJy, higher than the value previously reported in literature at this frequency. By using the value we obtained, it is possible to estimate an integrated spectral index between 4.8 and 6.6 GHz of $α\sim1.17$, where $F(ν)\propto ν^{-α}$, indicating a possible higher-frequency slowdown of the spectral steepening observed between 1.4 and 4.8 GHz. Such a spectral behavior is compatible with turbulent re-acceleration if the seed electrons have a spectrum extending up to high energies, as in the case of continuous injection by hadronic interactions or dark matter annihilation. We also report the detection at 4.8 GHz of a polarized spot inside the halo, without an evident counterpart, already detected at 6.6 GHz.

Figures (5)

• Figure 1: Left panel: 0.1--2.4 keV RASS image, smoothed with a 5-pixel (corresponding to 225 arcsec) Gaussian beam, with SRT contours at 4.8 GHz overlapped. The horizontal bar indicates a length of 500 kpc at the cluster distance. SRT contours are traced at levels of $-3\sigma$ (dotted lines), $3\sigma$, and scale with a factor of two (solid lines), with $\sigma=1$ mJy/beam. The SRT beam (FWHM 3.9 arcmin) is plotted in the top-left corner. Right panel: SRT image at 4.8 GHz of the cluster radio halo region with overlapped contours at 4.8 GHz (black lines) and 6.6 GHz (orange lines, from Murgia et al. 2024). Contour levels start at $3\sigma$, with $\sigma=1$ mJy/beam at 4.8 GHz and $\sigma=0.33$ mJy/beam at 6.6 GHz, and scale with a factor of two. The horizontal bar indicates a length of 100 kpc at the cluster distance. The SRT beam at 4.8 GHz is plotted in the bottom-right corner.
• Figure 2: VLA C-configuration image at 1.49 GHz with overlapped the 4.8 GHz SRT contours drawn as in right panel of Fig.\ref{['fig.radio_maps']}; sources labels are as in fig.5 in Murgia et al. (2024), with the addition of the K source (1257+28W06). The F source is not well visible in the image because it is covered by a contour line. The horizontal bar indicates a length of 100 kpc at the cluster distance. The VLA beam (13 arcsec) is shown in the bottom-right corner.
• Figure 3: Left panel: Spectrum of the Coma radio halo produced by secondary electrons of hadronic origin for a spectral index of non-thermal protons $s_p=2.7$ and a central density normalization of $N_{p,0}=5.5\times10^{-9}$ cm$^{-3}$, in absence of turbulent re-acceleration. Right panel: As in the left panel, but for $s_p=2.3$, $N_{p,0}=1.5\times10^{-10}$ cm$^{-3}$, and for re-acceleration with $\chi=5\times10^{-17}$ s$^{-1}$ and $T_{acc}=3\times10^8$ yr (solid line) and without re-acceleration (dashed line). Empty circles are data taken from the compilation in Murgia et al. (2024), while the filled red circle is from this paper.
• Figure 4: Linearly polarized intensity map, interpolated with the Laplace-Everett method, at 4.8 GHz obtained with SRT of the halo region in Coma; overlapped are the contours of the total intensity map, starting at $3\sigma_I$, with $\sigma_I=1$ mJy/beam, and scaling with a factor of two. The polarized map is cut at $3\sigma_P$, with $\sigma_P=0.5$ mJy/beam. The vectors refer to the radio wave E-field and are traced only where the fractional polarization $FPOL=P/I$ is detected with signal/noise higher than 3. The SRT beam at 4.8 GHz is plotted in the bottom-left corner.
• Figure 5: Upper panel: fractional polarization of the polarized spot as a function of the wavelength, fitted with an internal depolarization model. Lower panel: polarization angle of the polarized spot as a function of the wavelength, fitted with a rotation measure model.