Spectral index image of the radio halo in the cluster Abell 520, which hosts the famous bow shock

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: vvacca@ira.inaf.it
² INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
³ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
⁴ National Radio Astronomy Observatory, Socorro, NM 87801, USA
⁵ Naval Research Laboratory Remote Sensing Division, Code 7213 4555 Overlook Ave SW, Washington, DC 20375, USA

Received: 19 August 2013
Accepted: 10 October 2013

Abstract

Context. Synchrotron-radio emission has been detected from an increasing number of galaxy clusters. Spectral index images are a powerful tool for investigating the origin and nature of these sources, and their connection with the dynamical state of the cluster.

Aims. The aim of this work is to investigate the spectral index distribution of the radio halo in the galaxy cluster A520, known to be a complex system from an optical, radio, and X-ray point of view.

Methods. We present deep Very Large Array observations in total intensity at 325 and 1400  MHz. We produced and analyzed spectral index images of the radio halo in this frequency range at a resolution of 39′′ and 60′′ and determined possible correlations with the thermal properties of the cluster.

Results. We found an integrated radio-halo spectral index α₃₂₅¹⁴⁰⁰ ~ 1.12. No strong radial steepening is present and the spectral index distribution is intrinsically complex with fluctuations only partially due to measurement errors. The radio-halo-integrated spectral index and the cluster temperature follow the general trend observed in other galaxy clusters, although a strong point-to-point correlation between the spectral index and the thermal gas temperature has not been observed.

Conclusions. The complex morphology in the spectral index image of the radio halo in A520 agrees with the primary models for radio-halo formation. The flatness of the radial profile suggests that the merger is still ongoing and is uniformly and continuously (re-) accelerating the population of relativistic electrons responsible of the radio emission even at large (~1 Mpc) distances from the cluster center.