The GMRT Radio Halo survey Results and implications for LOFAR Simona Giacintucci Harvard-Smithsonian CfA, Cambridge, USA INAF-IRA, Bologna, Italy T. Venturi, R. Cassano, G. Brunetti, D. Dallacasa, G. Setti (INAF-IRA, Bologna) S. Bardelli (OAB, Bologna)

Cluster scale diffuse radio emission Abell 2256 RADIO HALOS: centrally located, regular structure similar to the X-ray morphology, unpolarized VLA 1.4 GHz on Chandra (discrete radio galaxies subtracted ) Cl arke & Ensslin 2006 RADIO RELICS: cluster outskirts, elongated morphology, polarized up to 30% Origin: shock (re)-acceleration of relativistic electrons or shock adiabatic compression of fossil radio plasma ? e.g., Ensslin et al. 1998; Rottgering et al. 1997; Ensslin & Gopal-Krishna 2001; Markevitch et al. 2005; Hoeft and Bruggen 2007… Origin: a promising possibility is the (re)- acceleration of relativistic electrons by merger driven turbulence ( Brunetti et al. 2001, Petrosian 2001, Fujita et al. 2003,…)

Statistical calculations in the framework of the re-acceleration scenario (Cassano & Brunetti 2005; Cassano, Brunetti & Setti 2006) allow to derive the probability to form a radio halo as function of the cluster mass and redshift Statistical expectations for radio halos Most of radio halos are expected in massive and luminous (M ≥ 2 x M o, L x ≥ 5 x erg s -1 ) clusters in the redshift range z = Agreement with the observed statistics at z ≤ 0.2 (Giovannini et al. 1999) Need for statistical information for z > 0.2

Aims discovery new radio halos (and relics) measure for the first time the occurrence of radio halos at z = constrain the dependence of their occurrence on the cluster mass combine the results with the statistics at z ≤ 0.2 and test the predictions of the statistical calculations verify the connection between radio halos/relics and cluster mergers WHY GMRT at 610 MHz? Radio halos/relics have steep radio spectra ( α ≥ 1 ) and low surface brigthness → GMRT is an ideal instrument for our goal, since it is capable of very high sensitivity at low radio frequencies We asked for 610 MHz observations, since this frequency offers the best compromise among sensitivity, confusion and range of resolutions (from 5” to “tapered” images with 20”-30” resolution) The GMRT Radio halo survey

Sample selection & observations From the X-ray catalogues REFLEX, BCS and eBCS we extracted a complete sample of 50 clusters (27 REFLEX e 23 BCS/eBCS) with: 0.2 < z < 0.4 ; L x ≥ 5 x erg s °< δ < 2.5° (REFLEX) +15° < δ < 60° (BCS/eBCS) 16 clusters: literature + VLA archive + GMRT Cluster Key Project 34 observed with the GMRT (Jan 2005 – Aug 2006)   2 – 3 hrs observation at 610 MHz for each cluster (USB+LSB: 32 MHz tot. band)   ~ 60 µJy/beam (35 – 100 μ Jy/beam) 7 known radio halos EXPECTED 5- 8 NEW DETECTIONS

34 clusters observed with the GMRT: - 4 new halos: 3 giant, 1 halo with LLS ~ 500 kpc - 1 candidate radio halo - 1 cluster with possible diffuse emission - 1 relic + 1 double relic - 1 mini-halo (cool core) - 1 candidate mini-halo (candidate cool core) 7 known radio halos from the literature (A2744, A1300, A2163, A773, A1758a, A2219, A2390) 26 non-detections (mergers and relaxed) 9 remaining clusters (literature, VLA archive, GMRT cluster key project): 3 undetections, 1 deserves further investigation, 5 without information RESULTS I. Detections and non-detections Halos and relics are rare Evidence of merger in all these clusters Venturi et al & 2008

Brunetti et al. 2007, ApJ 670L, 5Cassano et al., 2008, A&A, 480, 327 Bimodal distribution of clusters with and without radio halo Increase of the fraction of clusters with radio halo with the X-ray luminosity (mass) 0 < z < 0.4 : GMRT + literature

Results II. Halos, relics and cluster mergers RXCJ RXCJ GMRT 610 MHz on Chandra All new halos/relics are in merging custers Venturi et al., 2007 & 2008 GMRT 610 MHz + XMM-Newton + optical 1 Mpc MAJOR MERGERS

Results II. Halos, relics and cluster mergers A209 - GMRT 610 MHz on Chandra Cluster dynamical state and presence of a radio halo/relic. Venturi et al., 2007 & Mpc Multiple-moment power ratio analysis of the 2-dimensional potential (Buote & Tsai 1995; Hart 2008) ellipticity Multiple peaks

Abell 521: Relic + first Ultra Steep Radio Halo Follow up at 327 MHz (GMRT) to study the relic revealed the existence of a central radio halo with a very steep spectrum ( α ~ 2) GMRT 610 MHz on ChandraGMRT 327 MHz RELIC HALO RELIC Res. 13” - rms = 0.04 mJy/b Res. 13” - rms = 0.1 mJy/b Res. 35” (point sources subtracted out)

RELIC Abell 521: Relic + first Ultra Steep Radio Halo Resolution 35” - rms = 0.2 mJy/b α ~1.5 Electron acceleration by a shock with Mach number ~ 2.2 Spectral index steepening across the relic RELIC

Conclusions   The GMRT Radio Halo Survey provided support to the re-acceleration scenario, contributing to our understanding of the origin of diffuse radio emission in galaxy clusters and its connection with the large scale structure formation.   The GMRT Radio Halo Survey revelead the existence of ultra steep radio halos which emerge only at low frequency ( < 1 GHz)  merging events less energetic than those producing the standard “high frequency” radio halos (~ GHz ) LOFAR will be important for: - the study of the low-frequency spectrum (total and local) of radio relics - the discovery of other Ultra Steep Spectrum Radio Halos