The GMRT Radio Halo survey Results and implications for LOFAR

1. 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)

2. Cluster scale diffuse radio emission Abell 2256 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… RADIO HALOS:centrally located, regular structure similar to the X-ray morphology, unpolarized 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,…) VLA 1.4 GHz on Chandra(discrete radio galaxies subtracted) Clarke & Ensslin 2006

3. Statistical expectations for radio halos 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 Most of radio halos are expected in massive and luminous (M ≥ 2 x 1015 Mo , Lx ≥ 5 x 1044 erg s-1 ) clusters in the redshift range z = 0.1 - 0.4 Need for statistical information for z > 0.2 Agreement with the observed statistics at z ≤ 0.2 (Giovannini et al. 1999)

4. The GMRT Radio halo survey • Aims • discovery new radio halos (and relics) • measure for the first time the occurrence of radio halos at z = 0.2 - 0.4 • 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)

5. 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 ; Lx ≥ 5 x 1044 erg s-1 -30°< 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) 7 known radio halos EXPECTED 5- 8 NEW DETECTIONS • 2 – 3 hrs observation at 610 MHz for each cluster (USB+LSB: 32 MHz tot. band) • <rms> ~ 60 µJy/beam (35 – 100 μJy/beam)

6. RESULTS I. Detections and non-detections 7 known radio halos from the literature (A2744, A1300, A2163, A773, A1758a, A2219, A2390) 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) Evidence of merger in all these clusters Halos and relics are rare 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 Venturi et al. 2007 & 2008

7. 0 < z < 0.4 : GMRT + literature Increase of the fraction of clusters with radio halo with the X-ray luminosity (mass) Bimodal distribution of clusters with and without radio halo Brunetti et al. 2007, ApJ 670L, 5 Cassano et al., 2008, A&A, 480, 327

8. Results II. Halos, relics and cluster mergers All new halos/relics are in merging custers Venturi et al., 2007 & 2008 RXCJ2003-2323 RXCJ1314.4-2515 MAJOR MERGERS 1 Mpc 1 Mpc GMRT 610 MHz + XMM-Newton + optical GMRT 610 MHz on Chandra

9. Results II. Halos, relics and cluster mergers Cluster dynamical state and presence of a radio halo/relic. A209 - GMRT 610 MHz on Chandra Multiple peaks 1 Mpc ellipticity Multiple-moment power ratio analysis of the 2-dimensional potential (Buote & Tsai 1995; Hart 2008) Venturi et al., 2007 & 2008

10. Abell 521: Relic + first Ultra Steep Radio Halo GMRT 610 MHz on Chandra GMRT 327 MHz HALO Res. 35” (point sources subtracted out) RELIC RELIC Res. 13” - rms = 0.1 mJy/b Res. 13” - rms = 0.04 mJy/b 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)

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

12. 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