Letters to the Editor

A giant radio halo in the low luminosity X-ray cluster Abell 523

G. Giovannini^1,2, L. Feretti², M. Girardi³, F. Govoni⁴, M. Murgia⁴, V. Vacca^4,5 and J. Bagchi⁶

¹ Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
e-mail: ggiovann@ira.inaf.it
² Istituto di Radioastronomia-INAF, via P.Gobetti 101, 40129 Bologna, Italy
³ Dipartimento di Fisica-Sezione di Astronomia, via Tiepolo 11, 34143 Trieste, Italy
⁴ Osservatorio Astronomico di Cagliari-INAF, Strada 54, Loc. Poggio dei Pini, 09012 Capoterra (Ca), Italy
⁵ Dipartimento di Fisica, Università degli studi di Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy
⁶ Inter University Center for Astronomy and Astrophysics (IUCAA), Post Bag 4, Ganeshkhind, Pune 411 007, India

Received: 21 March 2011
Accepted: 14 April 2011

Abstract

Context. Radio halos are extended and diffuse non-thermal radio sources present at the cluster center, not obviously associated with any individual galaxy. A strong correlation has been found between the cluster X-ray luminosity and the halo radio power.

Aims. We observe and analyze the diffuse radio emission present in the complex merging structure Abell 523, classified as a low luminosity X-ray cluster, to discuss its properties in the context of the halo total radio power versus X-ray luminosity correlation.

Methods. We reduced VLA archive observations at 1.4 GHz to derive a deep radio image of the diffuse emission, and compared radio, optical, and X-ray data.

Results. Low-resolution VLA images detect a giant radio halo associated with a complex merging region. The properties of this new halo agree with those of radio halos in general discussed in the literature, but its radio power is about a factor of ten higher than expected on the basis of the cluster X-ray luminosity.

Conclusions. Our study of this giant radio source demonstrates that radio halos can also be present in clusters with a low X-ray luminosity. Only a few similar cases have so far been found. This result suggests that this source represents a new class of objects, that cannot be explained by classical radio halo models. We suggest that the particle reacceleration related to merging processes is very efficient and/or the X-ray luminosity is not a good indicator of the past merging activity of a cluster.