GMRT radio halo survey in galaxy clusters at z = 0.2–0.4

II. The eBCS clusters and analysis of the complete sample

¹ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy e-mail: tventuri@ira.inaf.it
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA02138, USA
³ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40126 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 20 February 2008
Accepted: 19 March 2008

Abstract

Aims. We present the results of the GMRT cluster radio halo survey. The main purposes of our observational project are to measure what fraction of massive galaxy clusters in the redshift range –0.4 host a radio halo, and to constrain the expectations of the particle re-acceleration model for the origin of the non-thermal radio emission.

Methods. We selected a complete sample of 50 clusters in the X-ray band from the REFLEX (27) and the eBCS (23) catalogues. We present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for all clusters still lacking high sensitivity radio information, i.e. 16 eBCS and 7 REFLEX clusters, thus completing the radio information for the whole sample. The typical sensitivity in our images is in the range 1–Jy b^-1.

Results. We found a radio halo in A 697, a diffuse peripheral source of unclear nature in A 781, a core-halo source in Z 7160, a candidate radio halo in A 1682 and “suspect” central emission in Z 2661. Including the literature information, a total of 10 clusters in the sample host a radio halo. A very important result of our work is that 25 out of the 34 clusters observed with the GMRT do not host extended central emission at the sensitivity level of our observations, and for 20 of them firm upper limits to the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey shows that radio halos are not common, and our findings on the fraction of giant radio halos in massive clusters are consistent with the statistical expectations based on the re-acceleration model. Our results favour primary to secondary electron models.