1. Annalisa Bonafede PhD student at IRA Radio Astronomy Institute Bologna (Italy) With: L. Feretti, G. Giovannini, M. Murgia, F. Govoni, G. B.Taylor, H. Ebeling, S. Allen, G. Gentile, Y. Philstrom Ascona, 1/06/2009 Cosmological magnetic field

2. O U T LI N E O U T LI N E Magnetic Field in Galaxy Clusters The interesting case of MACS J0717+3745 Radio observations: discovery of the most distant radio halo Polarization properties Constrain on the magnetic field strength and structure from polarization and under equipartition assumption conclusions

3. Galaxy Clusters: Intra Cluster Medium : Non thermal component  magnetic field +relativistic particles Radio Diffuse EmissionPolarization properties of radio sources Abell 2163 Feretti et al. (2001), Govoni et al. (2004) Feretti et al. (2001), Govoni et al. (2004) Abell 1240 Bonafede et al (2009) Bonafede et al (2009)

4. Optical emission: Ma et al (2008) Edge et al (2003) “A prime example of a complex major cluster merger ” (Ma + 2009) Z=0.551 arsec  6.394 kpc 07h17m25s30s35s 40s 37°43’30” 44’23” 45’00” 46’00” 30” HST F814w filter X-ray emission : Ma et al (2008) Chandra 60 ksec ACIS-I L =2.74±0.03 10 45 erg/s [0.1 – 2.4]keV =11 kev T going from 5 to 20 keV

5. Joint Optical – X-ray analysis 4 distinct sub-clusters ongoing triple merger + 6 Mpc long filament, source of continuous and discrete accretion of matter by the cluster Z=0.551 arsec  6.394 kpc What happens to the non- thermal component of the ICM? Edge et al. 2003

6. VLA – B-array obs 4 freq in the 20 cm band tos ~ 2h per freq. C-array obs 2 freq in the 6 cm band tos ~ 2h per freq Radio observations: In full polarization mode + VLA Archive obs at 8.5 GHz C array obs at 4.8 GHz D-array θ~ 5’’ x 5” θ~ 2’’ x 2” VLA – C-array obs 1.4 GHz, tos~ 2h θ~ 18’’ x15”

7. Observing the compact features A B C Filamentary structure θ~ 5’’ x 5” θ~ 2’’ x 2” Chandra + VLA 1.4 GHZ HST + VLA 8.5 GHZ 260 kpc

8. Discovery of the most distant & most powerful radio halo P 1.4 GHz ~1.6 10 26 W/Hz ; also emitting at 4.9 GHz 1.5 Mpc Observing the extended emission θ~ 18’’ x20” Chandra [0.5-7keV] + VLA 1.4 GHZ 1.5 Mpc

9. Polarized emission from the radio halo 2nd case after Abell 2255 (Govoni e al. 2005) 1.4 GHZ 5% mean Pol Max ~ 24% Min ~ 0.6% θ~ 18’’ x20” Upper limits 4% con beam 45% quidi non è ‘’strano’’

10. 1.4 GHZ4.9 GHZ What are we seeing at high resolution? 8% mean Pol17% mean Pol 9% pol flux 16% pol flux 20% pol flux 0.4 % pol flux

11. d H E E λ  RM Faraday rotation: IF the cluster acts like an external screen:

12. But... Faraday rotation internal to the emiting source Strong beam depolarization RM...we can verify the fit of Ψ versus λ 2 Foreground structure RM due to our galaxy λ 2 law Background structure RM due to the cluster λ 2 law Within the cluster icm structure Internal rotation NO λ 2 law

13. RM fit with PACERMAN algorithm (Polarization Angle CorrRecting Rotation Measure ANalisys ) Dolag et al. (2005) Filamentary part of the radio halo

14. % Polarization: -Increases with increasing n -Decreases toward the cluster center Numerical Simulations by murgia et al. 2004 Vectorial form n ≥3 Λ max 100s kpc The magnetic field power spectrum

15. if E min is constat with r Using the deprojected brightness profile Factor 2.5 Under the equipartition assumption: L computed in a fixed energy range:E min E max Beta-model from x-ray analysis (Ma et al. 2008)

16. Under the equipartition assumption: L computed in a fixed energy range:E min E max E min =100 and E max >> E min

17. Conclusions Discovery of a giant radio halo emitting from 74 MHZ – 4.9 GHz z=0.55  the most distant one P 1.4 GHz ~1.6 10 26 W/Hz  the most powerful one Polarized emission from the radio halo 0.6 % – 24% second case after A2255 (Govoni et al. 2005)  Magnetic field power spectrum n> 3, Λ max ≥ 100s kpc B eq ~ 1.2 µG, B 0 ~3 µG, B decreases as the gas density Under equipartition assumption Bonafede et al, arXiv:0905.3552. arXiv:0905.3552