LOFAR & Particle acceleration in Galaxy Clusters

1. LOFAR & Particle acceleration in Galaxy Clusters Gianfranco Brunetti Institute of Radioastronomy –INAF, Bologna, ITALY

2. Outline • - Galaxy Clusters & LS radio emission • Radio Halos (turbulence?) • - Why low frequency ? • - Ultra steep spectrum halos • - Evolution of radio halos (and LS magnetic fields) • Radio Relics (shock acceleration?) • - Tracing formation of LSS • - Constraining particle acceleration at weak shocks

3. Non-thermal components & cluster mergers Both Halos & Relics have steep spectrum, F()=Fo-,with1.3 Radio HalosandRadio Relicsare only found in non-relaxedclusters withrecent /ongoing cluster mergers (e.g. Buote 2001) Abell 3376 Bagchi et al. 2005 Abell 754 Henry et al. 2004 Abell 2163 Feretti et al. 2001 Radio Halos Radio Relics

4. The general picture merger history SHOCKSaccelerate e , pcr B e, p pcr pth  0  rays clusters increase their mass via merger with smallersubclusters ±e TURBULENCEreaccelerates fossil eand secondarieseon Mpc scales

5. The general picture ? merger history SHOCKSaccelerate e , pcr B e, p pcr pth  0  rays clusters increase their mass via merger with smallersubclusters ±e TURBULENCEreaccelerates fossil eand secondarieseon Mpc scales (eg., Brunetti et al. 2001, 2004, 2009; Petrosian 2001; Miniati et al. 2001; Fujita et al. 2003; Ryu et al. 2003; Pfrommer & Ensslin 2004; Brunetti & Blasi 2005; Cassano & Brunetti 2005; Cassano et al. 2006; Brunetti & Lazarian 2007; Hoeft & Bruggen 2007; Pfrommer et al. 2008; Petrosian & Bykov 2008) ?

6. Why low frequency ? Ensslin & Roettgering 2002 Regardless of the origin of Radio Halos, extrapolations of their number counts at 1.4 GHz based on the Radio-X ray correlation observed for Radio Halos suggest that a large fraction of these Halos is at faint fluxes. Due to their steep synchrotron spectrum, faint Radio Halos should appear more luminous at low frequencies and thus LOFAR and LWA are expected to discover a large number of these objects.

7. Are we biased (high freq) ?(Brunetti +al. 2008, Nature 455,944) Low frequency High frequency Ferrari et al.2003,06 Radio Power Frequency

8. Are we biased (high freq) ?(Brunetti +al. 2008, Nature 455,944) =1.9 N(E)=k E-4.8 =1.5 Dallacasa, GB, et al. 2009 losses acceleration Radio Power Frequency

9. The case of the “ultra steep” spectrm radio halos (turbulent acceleration model) Radio Power Energy release Acceleration efficiency Rare events More common events Frequency

10. The case of the “ultra steep” spectrm radio halos Radio Power Frequency

11. The case of the “ultra steep” spectrm radio halos Radio Power We expect a populations of radio halos with steeper spectrum that is (better) visible at low frequencies (Cassano, Brunetti, Setti 2006) Frequency

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

13. Montecarlo calculations ( Cassano, GB, et al in prep ) 600 MHz Radio Power Frequency z=0-0.1 z=0.5-0.6

14. Expected number counts ( Cassano, GB, et al in prep ) Mpc scale beam=20x20 arcsec 120 MHz Total b < 600 MHz

15. Evolution of Radio Halos Brunetti et al 2007++ Which is the difference between RH-clusters and ULimits ? Does non thermal emission evolve ? Which time-scale ?

16. Evolution of Radio Halos Radio Emitting GC Brunetti et al 2007++ Which is the difference between RH-clusters and ULimits ? Does non thermal emission evolve ? Which time-scale ? Radio Quiet GC

17. Evolution of Radio Halos Radio Emitting GC Brunetti et al 2007++ Which is the difference between RH-clusters and ULimits ? Does non thermal emission evolve ? Which time-scale ? Radio Quiet GC Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004, Venturi et al. 2008)

18. Evolution of Radio Halos Radio Emitting GC Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004) Brunetti et al 2007++ Which is the difference between RH-clusters and ULimits ? Does non thermal emission evolve ? Which time-scale ? B+δB Brunetti et al. 2009 B Radio Quiet GC Magnetic field dissipation ?

19. Evolution of Radio Halos Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004) Which is the difference between RH-clusters and ULimits ? Does non thermal emission evolve ? Which time-scale ? B+δB 0.4 Gyr Brunetti et al. 2009 0.6 Gyr B 1 Gyr 1.4 Gyr Magnetic field dissipation ? diss<< 1 Gyr

20. Evolution of Radio Halos Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004) Which is the difference between RH-clusters and ULimits ? Does non thermal emission evolve ? Which time-scale ? B+δB B 1 Gyr Magnetic field dissipation ? Subramanian et al.2006

21. Evolution of Radio Halos 1/2 Gyr The acceleration of emitting particles must be “transient” and particle cooling drives the transition … consistent with turbulent acceleration(Brunetti et al. 2007, 2009)

22. Evolution of Radio Halos 1/2 Gyr The acceleration of emitting particles must be “transient” and particle cooling drives the transition … consistent with turbulent acceleration(Brunetti et al 2007, 2009) Cluster “bi-modality” is expected less important at lower frequencies. Also the spread of the correlation must increase at lower frequencies

23. Radio Relics as tracers of shocks at LSS Abell 3667 Roettiger et al 1999 Abell 3376 Bagchi et al. 2005 Shock acceleration thermal (ICM) particles (Ensslin et al.1998; Roettiger et al.1999; Sarazin 1999; Miniati et al.2001; ..) Compression of ghost radio plasma (by shocks) (Ensslin & Gopal-Krishna 2001; Bruggen & Ensslin 2002;..) The potential is to trace LS shocks where thermal emission is fading away & to unveil the evolution of radio plasma (& its sources) in the ICM

24. Shocks in Galaxy Clusters Vazza, Brunetti, Gheller 2008

25. Shocks in Galaxy Clusters Miniati et al. 2001; Ryu et al. 2003; Pfrommer et al. 2006,08; Hoeft & Bruggen 2007; Skillman et al. 2008 Vazza, Brunetti, Gheller 2008

26. First attempts to radio images from Cosmological simulations Hoeft & Bruggen 2007 also Miniati et al 2001; Pfrommer et al 2008; Donnert et al 2009

27. Hoeft & Bruggen 2007 Magnetic field is compressed (and amplified) at shocks, consequently Radio Relics should be polarised (relatively large polarisation)

28. Complex situations … van Weeren et al. 2009 Bonafede et al. 2009 Hoeft & Bruggen 2007 Brentjens 2008

29. Uncertainties in CR acceleration Vazza, Brunetti, Gheller 2008 Kang & Jones 2007 Pfrommer et al. 2008

30. Uncertainties in CR acceleration Vazza, Brunetti, Gheller 2008 Kang & Jones 2007 Kang & Jones 2002 Pfrommer et al. 2008 Kang & Jones 2002

31. Uncertainties in CR acceleration Vazza, Brunetti, Gheller 2008 Kang & Jones 2007 Kang & Jones 2002 Pfrommer et al. 2008 Galaxy clusters are unique labs to study particle acceleration at weak & LS shocks Kang & Jones 2002

32. Conclusions • A fraction of the energy dissipated during cluster • formation is channelled into non thermal components • Radio Halos (turbulence?) • - Calculations suggest that we are missing the bulk of Halos ! • - Calculations suggest that LOFAR will detect several 100+ • Radio Halos (depending on rms….) • - Radio Halos are “transient” sources connected with mergers • and we claim that the synchrotron emission in “radio quiet” • clusters is suppressed by particle cooling (test by LOFAR) • Radio Relics (shock acceleration?) • - Allow tracing formation of LSS… better(?) than X-rays • - Unique: constraining particle acceleration at weak shocks