Deep 1.4 GHZ Follow Up of the Steep Spectrum Radio Halo in Abell 521

TL;DR

This paper confirms the presence of a very steep spectrum radio halo in galaxy cluster Abell 521 at 1.4 GHz, supporting turbulent acceleration models and highlighting a potentially large population of similar halos detectable by future low-frequency surveys.

Contribution

First detection of the 1.4 GHz flux density of the steep spectrum radio halo in Abell 521 using deep VLA observations, confirming its steep spectrum and supporting turbulence-based models.

Findings

Confirmed steep spectrum of the radio halo at 1.4 GHz

Supported turbulent acceleration as the origin of relativistic electrons

Identified Abell 521 as a prototype for a class of steep spectrum halos

Abstract

In a recent paper we reported on the discovery of a radio halo with very steep spectrum in the merging galaxy cluster Abell 521 through observations with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons and supports a turbulent acceleration scenario. At that time, due to the steep spectrum, the available observations at 1.4 GHz (archival NRAO - Very Large Array - VLA CnB-configuration data) were not adequate to accurately determine the flux density associated with the radio halo. In this paper we report the detection at 1.4 GHz of the radio halo in Abell 521 using deep VLA observations in the D-configuration. We use these new data to confirm the steep-spectrum of the object. We consider Abell 521 the prototype of a population of very-steep spectrum halos. This population is predicted assuming that turbulence plays an important role in the acceleration of relativistic particles in galaxy clusters, and we expect it will be unveiled by future surveys at low frequencies with the LOFAR and LWA radio telescopes.