“ Astrophysics with E-LOFAR ’’ 16-19 September 2008, Hamburg, Germany

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

2. Origin of giant radio halos in galaxy clusters Govoni et al. 2004 GeV electrons on Mpc scales μG magnetic fields on Mpc scales The Diffusion Problem: Tdiff (~1010 yr) >> Tv (~108 yr) “bullet” cluster (Jaffe 1976) One possibility to explain RH is given by the re-acceleration modelin 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)

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 commonin galaxy clusters. not observable RH • 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. obs. ν Radio Power Frequency

4. Observations: statistics GMRTobservations of a complete sampleof 50galaxy clusters with similarLXandz(Venturi et al. 2007, 2008; see also talk by S. Giacintuccithis conference) Brunetti et al. 2007 blue: GMRT GC • 30% of GC with RH => RH notcommon • RH are only found in merging clusters • -Bi-modality of GC=> empty region => fast evolution (in 108 yr) of GC from a “radio quiet” to a “radio loud” region! => Tacc108 yr magenta : other RH

5. Statistical expectations for RHs in the framework of the re-acceleration scenario(Cassano & Brunetti 2005; Cassano et al. 2006) Turbulence χ-1 =τacc merger trees γbχ/β νbB γb2

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

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

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

9. Fraction of galaxy clusters with radio halos at low ν Cassano et al. 2008 150 MHz 150 MHz 240 MHz 240 MHz 1.4 GHz 1.4 GHz • The expected fraction of clusters with radio halos increases at low ν. • This increase is even stronger for smaller clusters (M<1015 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.

10. Number Counts of RHs at low radio frequency 150 MHz 150 MHz 240 MHz 240 MHz 1.4 GHz 1.4 GHz z The expected number of RHs increases at lower frequencies by about a factor10. LOFARshould be able to detect the bulk of RHs. The bulk of RHs emitting atGHz freq.is expected at relatively low z:0.1-0.3.Atlow radio frequencya number of RHs is expected to be discovered at relatively higher redshiftsz>0.4.

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

12. Ultra Steep Spectrum Radio halos (USSRH) at 74 MHz Cassano et al., in prep. 74-600 MHz 74-600 74-600 600-1.4 600-1.4 600 MHz -1.4 GHz >1.4 >1.4 >1.4 or “classical” 74 MHz observations will reveal different populations of RH: USSRH + “classical RH”. 100 % of 74 MHz RH in cluster with M < 1015 M⊙would be USSRH, while 50 % of 74 MHz RH in clusters with M > 2·1015 M⊙would be “classical” RH. At higher redshift USSRH start to become dominant with respect to “classical” RH also in massive clusters (M> 2·1015 M⊙).

13. Summary 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 apopulation of radio halosemerging only at low radio frequency (USSRH). These USSRH are –indeed- not expected in the framework of any other model. The prototype of these USSRHmay bethe halo in the galaxy cluster A521 discovered with GMRT observations at 330-240MHz . The USSRH in A521 may be the first of many RH that are hidden to present observations but that would be discovered byLOFAR. These USSRH are expected to be ~10times more common than classical RH being activated by less energetic merging events, which are more common in the Universe.

14. Observations: Spectral Cut-Off Coma Schlickeiser et al. 1987 Electron spectra F() acceleration losses Thierbach +al. 2002 Evidence of break in thespectrum of the emittingelectrons at energies offew GeV. Evidences for acut-offin the synchrotron spectra of some RHs: Coma (Schlickeiser et al. 1987) A 3562 (Giacintucci et al. 2005) A 521 (Brunetti et al. 2008) Acceleration mechanism not efficient !

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>1015 M⊙ (LX> ~3·1044 h70-2 erg/s) clusters. In this scenariothe numberof giant radio halosin the whole universeat 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!!!