The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz

Abstract

Diffuse radio emission at the centre of galaxy clusters has been observed both in merging clusters on scales of Mpc, called giant radio haloes, and in relaxed systems with a cool-core on smaller scales, named mini haloes. Giant radio haloes and mini haloes are thought to be distinct classes of sources. However, recent observations have revealed the presence of diffuse radio emission on Mpc scales in clusters that do not show strong dynamical activity. RX J1720.1+2638 is a cool-core cluster, presenting both a bright central mini halo and a fainter diffuse, steep-spectrum emission extending beyond the cluster core that resembles giant radio halo emission. In this paper, we present new observations performed with the LOFAR Low Band Antennas (LBA) at 54 MHz. These observations, combined with data at higher frequencies, allow us to constrain the spectral properties of the radio emission. The large-scale emission presents an ultra-steep spectrum with $\alpha _{54}^{144}\sim 3.2$. The radio emission inside and outside the cluster core have strictly different properties, as there is a net change in spectral index and they follow different radio-X-ray surface brightness correlations. We argue that the large-scale diffuse emission is generated by particles re-acceleration after a minor merger. While for the central mini halo we suggest that it could be generated by secondary electrons and positrons from hadronic interactions of relativistic nuclei with the dense cool-core gas, as an alternative to re-acceleration models.

The Coma cluster at LOFAR frequencies II: the halo, relic, and a new accretion relic

“…We call this emission the "outer halo" and discuss in Secs. 4 and 6 whether the halo core and the outer halo show different properties, as recently proposed for some coolcore clusters (Savini et al 2018(Savini et al , 2019Biava et al 2021;Riseley et al 2021). The emission from the outer halo is only visible when baselines shorter than 80λ are included in the image, and hence it is filtered out in the LoTSS images.…”

“…Our analysis suggests that the point-to-point I R − I X correlation may be different in the halo central regions and in its outskirts. Recent low-frequency observations have found radio emission in galaxy clusters that can be interpreted as the coexistence of a mini halo in the cluster core and a giant halo on larger scales (Savini et al 2019, Biava et al 2021. A different I R − I X trend is found in the core and in the outer part of the halo of these clusters, with a super-linear scaling in the mini halo region and a sub-linear scaling in the outer part (Biava et al 2021, Lusetti et al, in prep).…”

“…(e.g. RXC J1720.1+2638, Biava et al 2021;Abell 1413, Lusetti et al, in prep). We note that this is the first time that a change in the I R − I X correlation has been investigated, hence it could be a common property of radio halos.…”

“…Hence, the advent of deep, low-frequency radio surveys, such as the LOFAR Two-metre Sky Survey (LoTSS; Shimwell et al 2017Shimwell et al , 2019 has both increased the number of new detections (see e.g. Biava et al 2021;Riseley et al 2021;Botteon et al, accepted;Hoang et al, submitted) and allowed the study of known objects with unprecedented sensitivity and detail. The Coma cluster hosts the most famous and best studied radio halo, as well as a radio relic and a radio bridge connecting the two (see Fig.…”

The Planck clusters in the LOFAR sky. I. LoTSS-DR2: new detections and sample overview

“…In this respect, LOFAR has recently enabled the first detailed observations of galaxy clusters at frequencies of <200 MHz thanks to the unprecedented high sensitivity and high resolution in its operational frequency range. This potential has already been demonstrated as LOFAR has proved to be very fruitful in investigating different aspects of nonthermal phenomena in the ICM, allowing us: to discover new instances of diffuse sources in clusters (e.g., Shimwell et al 2016;Savini et al 2018aSavini et al , 2019Wilber et al 2019), including ultra-steep spectrum emission (e.g., Brüggen et al 2018;Wilber et al 2018;Mandal et al 2020;Biava et al 2021b) and very large-scale emission outside the central cluster region (e.g., Govoni et al 2019;Botteon et al 2019aBotteon et al , 2020bBonafede et al 2021;Hoeft et al 2021;Hoang et al 2021b), as well as new faint halos and relics (e.g., Botteon et al 2019aBotteon et al , 2021aLocatelli et al 2020;Hoang et al 2021a) and high-z systems (e.g., Cassano et al 2019;Di Gennaro et al 2021a); to pinpoint the complex interplay between tailed cluster AGN and ICM motions (e.g., de Gasperin et al 2017;Clarke et al 2019;Hardcastle et al 2019;Botteon et al 2020aBotteon et al , 2021bIgnesti et al 2021); to study the central cluster AGN duty cycle, structure, and interaction with the hot ICM (e.g., Brienza et al 2020;Bîrzan et al 2020;Biava et al 2021a;Timmerman et al 2022); and to detect extended, extraplanar emission from star-forming galaxies infalling into clusters (e.g., Ignesti et al 2020Igne...…”