Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523

Abstract

The galaxy cluster Abell 523 (A523) hosts an extended diffuse synchrotron source historically classified as a radio halo. Its radio power at 1.4 GHz makes it one of the most significant outliers in the scaling relations between observables derived from multi-wavelength observations of galaxy clusters: it has a morphology that is different and offset from the thermal gas, and it has polarized emission at 1.4 GHz typically difficult to observe for this class of sources. A magnetic field fluctuating on large spatial scales (∼1 Mpc) can explain these peculiarities but the formation mechanism for this source is not yet completely clear. To investigate its formation mechanism, we present new observations obtained with the LOw Frequency ARray at 120-168 MHz and the Jansky Very Large Array at 1-2 GHz, which allow us to study the spectral index distribution of this source. According to our data the source is observed to be more extended at 144 MHz than previously inferred at 1.4 GHz, with a total size of about 1.8 Mpc and a flux density S144 MHz = (1.52 ± 0.31) Jy. The spectral index distribution of the source is patchy with an average spectral index α ∼ 1.2 between 144 MHz and 1.410 GHz, while an integrated spectral index α ∼ 2.1 has been obtained between 1.410 GHz and 1.782 GHz. A previously unseen patch of steep spectrum emission is clearly detected at 144 MHz in the south of the cluster. Overall, our findings suggest that we are observing an overlapping of different structures, powered by the turbulence associated with the primary and a possible secondary merger.

Puzzling large-scale polarization in the galaxy cluster Abell 523

“…From the point of view of the radio data, at 1.4-GHz the bulk of the emission sits in the north of the system (Giovannini et al 2011). Observations at 144-MHz reveal a more extended source characterized by a complex morphology consisting of two bright filaments in the north and a further extension towards the south consisting of an additional filament and a roundish patch of emission (see Vacca et al 2022) following the optical overdensity and the X-rays elongation presented respectively by Girardi et al (2016) andCova et al (2019). On the basis of the multi-frequency data presently available, as already discussed in Vacca et al (2022), a possibility is that we are looking at the superposition of different sources.…”

“…The wave-number 𝑘 is linked to the spatial scale of fluctuation Λ by Λ = 𝑁 pix × Δ/𝑘, where 𝑁 pix is the size of the computational grid in pixels and Δ is the size of a pixel in kpc, assumed here to be respectively 512-pixels and 4-kpc. Moreover, we assume 𝛿 = 3.4 in agreement with the spectral index 𝛼 = 1.2 (𝛿 = 2𝛼 + 1) derived by Vacca et al (2022). A summary of the parameters adopted for the numerical simulations is given in Table 5.…”

“…In Fig. 7, we show the emission at the centre of the galaxy cluster A523 as observed from 231-MHz down to 72-MHz by LOFAR (Vacca et al 2022) and the Murchison Widefield Array (MWA, Wayth et al 2015;Hurley-Walker et al 2017). At the location of the polarization patches hints of a total intensity signal are present especially at 144-MHz with LOFAR data (central panel of Fig.…”

“…The mean of the means is -0.26 mJy beam −1 and is characterized by a standard deviation of 0.18 mJy beam −1 , indicating that the signal detected at the location of P1 is significant. By inspecting the LOFAR image at higher resolution (9-arcsec, Vacca et al 2022), no compact source is identified at the location of the LOFAR radio brightness peak in the region corresponding to P1 at the sensitivity and resolution of the available data. For P2 we measure a mean radio brightness of 1.8 mJy beam −1 and a total flux density of 14.5 ± 4.7 mJy, and for P3 a mean radio brightness of 3.1 mJy beam −1 and a total flux density of 32.9 ± 7.8 mJy.…”

“…Recently, Vacca et al (2022) found that the radio halo emission at 144-MHz of A523 is more extended than at 1.4-GHz up to a size of 1.8-Mpc, with an overall spectral index 𝛼 1.410 GHz 144 MHz = 1.2 ± 0.1. By locally comparing the radio emission at 144-MHz and the X-ray emission in the energy band 0.5-2.5 keV, they infer the presence of two components, one brighter in X-ray and dimmer in radio and the second the other way around, likely due to the complex dynamical state of the system.…”