1. “Astrophysics with E-LOFAR’’ 16-19 September 2008, Hamburg, Germany Istituto di Radioastronomia, INAF- Bologna, ITALY Cluster Radio Halos in the LOFAR era Rossella Cassano Credit: S. Giacintucci

2. Origin of giant radio halos in galaxy clusters GeV electrons on Mpc scales μG magnetic fields on Mpc scales T diff (~10 10 yr) >> T v (~10 8 yr) (Jaffe 1976) The Diffusion Problem: One possibility to explain RH is given by the re-acceleration model in which relic or secondary relativistic electrons are in situ re-accelerated due to the interection with MHD turbulence generated in the cluster volume during merger events (e.g., Brunetti et al. 2001, 2004; Petrosian 2001; Ohno et al. 2002; Fujita et al. 2003; Brunetti & Blasi 2005; Cassano & Brunetti 2005; Brunetti & Lazarian 2007; Petrosian & Bykov 2008) Govoni et al. 2004 “bullet” cluster

3. Basic expectations of the re-acceleration scenario RHs should be “transient” phenomena in merging GC, with a lifetime  1 Gyr derived from the combination of different timescales (crossing time between GC, dissipation timescale of the injected turbulence) => RH are not common in galaxy clusters. The synchrotron spectra of RHs should be caracterized by the presence of a cut-off at high frequency. The presence of the cut-off affects our ability to detect RH in the Universe, introducing a strong bias against observing them at frequency substantially larger than the cut-off frequency. Radio Power Frequency obs. ν not observable RH

4. Observations: statistics - 30% of GC with RH => RH not common - RH are only found in merging clusters - Bi-modality of GC=> empty region => fast evolution (in 10 8 yr) of GC from a “radio quiet” to a “radio loud” region! => T acc  10 8 yr Venturi et al. 2007, 2008; see also talk by S. Giacintucci GMRT observations of a complete sample of 50 galaxy clusters with similar L X and z (Venturi et al. 2007, 2008; see also talk by S. Giacintucci this conference) blue: GMRT GC magenta : other RH Brunetti et al. 2007

5. Statistical expectations for RHs in the framework of the re- acceleration scenario (Cassano & Brunetti 2005; Cassano et al. 2006) Turbulence merger trees χ -1 =τ acc γbχ/βγbχ/β ν b  B γ b 2

6. Fraction of galaxy clusters with radio halos Cassano et al. 2008 The expected fraction of galaxy clusters with RHs increases with cluster mass (and L X ) in line with present data. NVSS GMRT Observed Predicted 0.41  0.11 0.08  0.04

7. What about radio halos at low radio frequency? LWA LOFAR

8. Obs. frequency range Radio Power Frequency Acceleration efficiency Radio halos at lower radio frequencies Probability NOW NOW we see RH associated with the most energetic phenomena. LOFAR LOFAR should discover those RH associated with the most common and less energetic phenomena, caracterized by very steep radio spectra. USSRH “classical” RH

9. Fraction of galaxy clusters with radio halos at low ν 1.4 GHz 240 MHz 150 MHz 240 MHz  The expected fraction of clusters with radio halos increases at low ν.  This increase is even stronger for smaller clusters (M<10 15 M ⊙ ).  The increase of the fraction of galaxy clusters with radio halos as a function of the cluster mass become less striking at low radio frequency. Cassano et al. 2008

10. Number Counts of RHs at low radio frequency 1.4 GHz 150 MHz 240 MHz 1.4 GHz 240 MHz 150 MHz The expected number of RHs increases at lower frequencies by about a factor 10. LOFAR LOFAR should be able to detect the bulk of RHs. The bulk of RHs emitting at GHz freq. is expected at relatively low z: 0.1-0.3. At low radio frequency a number of RHs is expected to be discovered at relatively higher redshifts z>0.4. z

11. Radio Power Frequency The main expectation of the re-acceleration scenario is the presence of Ultra Steep Spectrum Radio Halos (USSRH) emerging at low υ 1 GHz 74 MHz USSRH “classical” RH An example

12. Ultra Steep Spectrum Radio halos (USSRH) at 74 MHz 74-600 MHz 600 MHz -1.4 GHz >1.4 or “classical” >1.4 600-1.4 74-600 600-1.4 At higher redshift USSRH start to become dominant with respect to “classical” RH also in massive clusters (M> 2·10 15 M ⊙ ). 74 MHz observations will reveal different populations of RH: USSRH + “classical RH”.  100 % of 74 MHz RH in cluster with M 2·10 15 M ⊙ would be “classical” RH. Cassano et al., in prep.

13. Summary giant radio halos Basic expectations of the re-acceleration scenario seem to be in agreement with present observations of giant radio halos in GC and some clear features of this model can be tested with LOFAR. A unique expectation of this scenario is the existence of a population of radio halos emerging only at low radio frequency (USSRH). These USSRH are –indeed- not expected in the framework of any other model. LOFAR The USSRH in A521 may be the first of many RH that are hidden to present observations but that would be discovered by LOFAR. These USSRH are expected to be ~10 times more common than classical RH being activated by less energetic merging events, which are more common in the Universe. A521 The prototype of these USSRH may be the halo in the galaxy cluster A521 discovered with GMRT observations at 330-240 MHz.

14. Thierbach +al. 2002 Schlickeiser et al. 1987 Evidence of break in the spectrum of the emitting electrons at energies of few GeV. Electron spectra losses F( ) Acceleration mechanism not efficient ! acceleration Observations: Spectral Cut-Off Coma Coma A 3562A 521 Evidences for a cut-off in the synchrotron spectra of some RHs: Coma (Schlickeiser et al. 1987) A 3562 (Giacintucci et al. 2005) A 521 (Brunetti et al. 2008)

15. Conclusions An increasing occurrence of giant RHs with cluster mass is expected in the framework of the re-acceleration model. This is in agreement with results of a statistical unbiased analisys of NVSS+GMRT observations for M>10 15 M ⊙ (L X > ~3·10 44 h 70 -2 erg/s) clusters. In this scenario the number of giant radio halos in the whole universe at 1.4 GHz is expected to be ~100. This number is expected to increases up to a factor of ~10 at low radio frequency (150 –74) MHz (LOFAR, LWA).  A new populations of USSRH is expected to be discovered at low radio frequency. A prototype of this new class of radio halos (USSRH) have been discovered at 610-330-235 MHz in the galaxy clusters A521!!!