Radio relics and magnetic field amplification in the Intra-cluster Medium

Transcription

1 Radio relics and magnetic field amplification in the Intra-cluster Medium Annalisa Annalisa Bonafede Bonafede Hamburger Hamburg Sternwarte University Hamburg University Jacobs University Bremen Collaborators: F. Vazza, M. Brüggen (Hamburg University) L. Feretti, G.Giovannini, (INAF - Bologna) F.Govoni, M. Murgia (INAF - Cagliari)

2 Outline: - Observational properties of radio relics - Relics and shocks in the Intra Cluster Medium - Magnetic field, Shock Mach number, efficiency of e acceleration -Magnetic field in relics preliminary results on the Coma cluster relic

3 Radio relics: observational properties Cluster: MACSJ X-ray XMM-Newton (Ebeling et al. In prep) Optical emission from SDSS Radio 325 Mhz from GMRT (Bonafede et al. 2012)

4 Radio relics: observational properties X-ray, Optical, Radio - Extended radio sources - Low radio brightness - Steep Spectrum α > 1 Annalisa Bonafede - Cluster peripheral ~ ~1.1 Mp c Cluster: MACSJ regions pc M Hamburger Sternwarte Hamburg University

5 Radio relics: observational properties Cluster: MACSJ X-ray, Optical, Radio - Extended radio sources - Low radio brightness - Steep Spectrum α > 1 Annalisa Bonafede - Cluster peripheral regions Hamburger Sternwarte Hamburg University - Polarized 20-30%

6 Radio relics: observational properties Cluster: MACSJ X-ray, Optical, Radio - Extended radio sources - Low radio brightness - Steep Spectrum α > 1 Annalisa Bonafede - Cluster peripheral regions Hamburger Sternwarte Hamburg University - Polarized 20-30% Host cluster: Minor or major merger

7 Radio relics: observational properties Cluster: MACSJ X-ray, Optical, Radio - Extended radio sources - Low radio brightness - Steep Spectrum α > 1 Annalisa Bonafede - Cluster peripheral regions Hamburger Sternwarte Hamburg University - Polarized 20-30% Host cluster: Minor or major merger Double-relics in a few cases

8 Radio relics: morphologies Abell 115 (Govoni et al. 2001) Abell 521 (Giacintucci et al. 2008) Abell 1664 (Govoni et al. 2001) ZwCl (van Weeren et al. 2011) Abell 1240 (Bonafede et al. 2009)

9 Radio Relics from observations to theory So far, detected in ~ 50 clusters Life time of particles << diffusion time Polarization detected Transient phenomena (re)acceleration required Magnetic field Amplified / ordered on Mpc scale Radio Relics are powered by Shock waves which form in the Intra-cluster Medium during mergers (low Mach numbers 2-4)

10 Radio emission and shock properties DSA: Mach number radio spectral index Particle acceleration efficiency at low Mach numbers Magnetic field amplification Courtesy of F. Vazza

11 Mach number - radio spectral index Van Weeren et al Spectral steepening particle aging Bonafede et al Diffusive Shock Acceleration regime Blandford & Eichler 87 Mach numbers ~ 2-4

12 Radial distribution of Radio Relics Mach number Mach number vs radial distance from cluster center Vazza et al. 2010, 2012 Radial trend of dissipated kinetic energy in shocks

13 Radial distribution of Radio Relics Mach number Vazza et al. 2010, 2012 Relics are Preferentially found in the cluster outskirts

14 Cosmological simulations Recipies to translate E injected by shocks into observable radio emission Low Mach Number low electron efficiency acceleration (?) - Diffusive Shock Acceleration, Re-acceleration of relativistic e,... (Hoef & Brueggen 2007, Pfrommer et al. 2008, Kang et al. 2012, Skillmann et al. 2011,.) Skilmann et al. 2011

15 Acceleration or re-acceleration? Cluster A754: Mach number ~ 1.6 direct re-acceleration would require E acceleration efficiency ~ 1 Macario et al Synctrothron profiles of the relic CIZA direct acceleration (M ~4.5?) -re-acceleration (M ~ 2) Kang et al. 2012

16 Constraining the efficiency of shock acceleration Simulation (Vazza et al. 2009) MACS J1752 XMM-Newton obs. (courtesy of Ebeling)

17 Constraining the efficiency of shock acceleration Observations (X-ray +radio contours) Simulation (Vazza et al. 2009) (Bonafede et al. 2012)

18 Constraining the efficiency of shock acceleration Observations (X-ray +radio contours) Simulation (Bonafede et al. 2012)

19 Constraining the efficiency of shock acceleration Observations (X-ray +radio contours) Simulation Electron to proton acceleration efficiency (e.g. Hoeft & Brϋggen 2007) (Bonafede et al. 2012)

20 Magnetic field in Radio relics Polarization of synchrotron emission: Van Weeren et al Bonafede et al Bonafede et al The magnetic field is aligned Along the relics' main axes: Compression? Amplification?

21 Magnetic field - Ampification in Radio relics small scale dynamo compressional amplification baroclinic amplification interaction with density inhomogenities Iapichino & Brueggen (2012) turbulent amplification is inefficient compressional amplification is the dominant one Litle is knnwn about MF in the outskirts-little can be deduced about MF amplification (Iapichino & Brüggen 12) Maximum amplification M~3 R =3 Rankine Hugoniot jump conditions

22 Magnetic field amplification: Magnetic field estimates in radio relics: B ~ µg. - Radio equipartition estimate (Giovannini et al. 1999, Govoni et al. 2001, Bonafede et al. 2009, van Weeren et al ) - Inverse Compton/ radio B > 1 3 µg (Finoguenov et al 2010, Feretti & Neumann 2006) - relic-width method assuming the shock scenario So far only for CIZA 2022 cluster (van Weeren et al. 2010) B ~ 6 µg. Magnetic field in the cluster outskirts? Little is known about MF in the outskirts- little can be deduced about MF amplification

23 Magnetic field amplification in the Coma relic Coma cluster NGC4839 Sub-group accreating ROSAT image kev Briel et al. 1992

24 Magnetic field amplification in the Coma relic Radio halo NGC4839 Sub-group accreating Radio relic LOFAR image at 146 MHz A. Bonafede & LOFAR survey key project

25 The standard picture for the Coma relic Merger/ accretion process Shock wave B is amplified Particles (re)accelerated n and T jump Feretti & Neumann 2006

26 The standard picture for the Coma relic Merger/ accretion process Shock wave NOT DETECTED B is amplified Particles (re)accelerated n and T jump NOT DETECTED B > 1.05 µg. Feretti & Neumann 2006

27 Magnetic field in the Coma cluster from Faraday Rotation Measures B( r )=B 0 ne n0 ( ) B0=4.7 µg, η=0.5 Bonafede et al η

28 Magnetic field in the Coma cluster from Faraday Rotation Measures B( r )=B 0 ne n0 ( ) B0=4.7 µg, η=0.5 In the relic region (~ 2 Mpc from the cluster center) B~0.1 µg Bonafede et al η

29 Magnetic field in the Coma relic Rotation Measures Central sources Rotation Measures Across the relic B0=4.7 µg, η=0.5 Coma WSRT 320 Mhz, Venturi et al. 93

30 5C4.31 5C4.24 5C4.51 5C4.20 5C4.43 5C4.29 5C4.20a

31 Central sources Sources in the direction of the Relic Compatible with model derived from central sources? Is there need for shock amplification? How much?

32 Mock RM observations 3dim simulations d ψ RM 2 n e H cos ϑ dl λ 0 Observed quantity 3D Model for the magnetic field in the cluster Model for the gas distribution

33 Mock RM observations The magnetic field model at low resolution: 2 ζ A k k the same approach of the Faraday code (Murgia et al. 2004) B k =ik A k B( r )=B 0 ne n0 η ( ) The gas component: Double beta cosmological simulation Vazza et al. 2010

34 Mock RM observations: At high resolution: New approach The MIRO' code B0 5 μg n=0.5 Pixel column resampled at higher resolution - Power spectrum modes added (A vector in Fourier space) new resolution 1 kpc Low resolution RM map 512 cube box Pixel resolution 16 kpc Bonafede et al., in prep RM mock observation

35 Preliminary results Magnetic field: Model 1: Double Beta, B0 4.7 μg, n=0.5 res 1 kpc MiRo' code The total profile Zoom in the relic region

36 Preliminary results Model 2: cosmological simulation, The total profile Magnetic field: B0 4.7 μg, n=0.5 MiRo' code Zoom in the relic region

37 Summary - Radio relics are likely tracing shock waves in the Intra- cluster Medium, BUT open issues: - Low Mach numbers (2-4) low acceleration efficiency? re-acceleration? - Magnetic field compressed/amplified along the shock surface - Preliminary results in the Coma relic: RM show little evidence for shock amplification.