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

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

Non-thermal components Abell 3376 Bagchi et al & cluster mergers Radio Halos and Radio Relics are only found in non-relaxed clusters with recent /ongoing cluster mergers (e.g. Buote 2001) Abell 754 Henry et al Both Halos & Relics have steep spectrum, F( )=F o - , with   1.3 Abell 2163 Feretti et al Radio Relics Radio Halos

The general picture merger history clusters increase their mass via merger with smaller subclusters e , p TURBULENCE reaccelerates fossil e  and secondaries e  on Mpc scales B SHOCKS accelerate e , p cr ±  e ±  e p cr p th   0   rays

The general picture merger history clusters increase their mass via merger with smaller subclusters e , p TURBULENCE reaccelerates fossil e  and secondaries e  on Mpc scales B SHOCKS accelerate e , p cr ±  e ±  e p cr p th   0   rays ? ? (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)

Why low frequency ? 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. Ensslin & Roettgering 2002

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

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

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

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

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

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 ⊙ ) Cassano et al Radio Power Frequency Acc loss

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

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

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

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

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

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

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

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

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

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

Radio Relics as tracers of shocks at LSS 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 to unveil the evolution of radio plasma (& its sources) in the ICM Abell 3376 Bagchi et al Abell 3667 Roettiger et al 1999

Shocks in Galaxy Clusters Vazza, Brunetti, Gheller 2008

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

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

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

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

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

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

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

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