The very steep spectrum radio halo in Abell 697

TL;DR

This study analyzes the steep-spectrum giant radio halo in Abell 697 using multifrequency radio observations to understand its origin and relation to cluster dynamics, finding a very steep spectrum inconsistent with hadronic models.

Contribution

It provides the first detailed multifrequency analysis of Abell 697's radio halo, revealing its steep spectrum and challenging hadronic origin models.

Findings

Radio halo is brighter and larger at 325 MHz.

Spectrum is very steep with α ≈ 1.7-1.8.

Hadronic models are disfavoured due to steep spectrum.

Abstract

In this paper we present a detailed study of the giant radio halo in the galaxy cluster Abell 697, with the aim to constrain its origin and connection with the cluster dynamics. We performed high sensitivity GMRT observations at 325 MHz, which showed that the radio halo is much brighter and larger at this frequency, compared to previous 610 MHz observations. In order to derive the integrated spectrum in the frequency range 325 MHz--1.4 GHz, we re--analysed archival VLA data at 1.4 GHz and made use of proprietary GMRT data at 610 MHz. {Our multifrequency analysis shows that the total radio spectrum of the giant radio halo in A\,697 is very steep, with $\alpha_{\rm~325 MHz}^{\rm~1.4 GHz} \approx 1.7-1.8$. %\pm0.1$. Due to energy arguments, a hadronic origin of the halo is disfavoured by such steep spectrum. Very steep spectrum halos in merging clusters are predicted in the case that the emitting electrons are accelerated by turbulence, observations with the upcoming low frequency arrays will be able to test these expectations.}