1. measuring magnetic power spectra of Hydra A Petr Kuchar, Torsten Ensslin

2. Hydra A opticalx-ray NASA/CXC/SAO La Palma/B. McNamara

3. Hydra A radio RM map Greg Taylor, NRAO pacerman algorithm Dolag & Vogt & Ensslin

4. we measure this rotation measure we want that

5. previous works Laing 2008 compare observed and computed structure function Vogt & Ensslin 2005 Bayesian

6. Bayesian approach given RM data d wanted power spectrum p P(p|d)~P(d|p)P(p) Likelihood Jeffrey's Prior

7. correlation matrix C electron density magnetic density cavity geometry projection angle of jet powerspectrum

8. maximize the posterior P(p|d) analytical expressions for gradient and hessian

9. power spectrum

10. spectral index

12. projection angle

13. characteristics of magnetic field magnetic density: autocorrelation lengths spectral index

14. conclusion maximum a posteriori method to retrieve magnetic power spectra re-analysis of Hydra A data (as Vogt & Ensslin) now including cavities, noise, Galactic foreground, beam discussion, increased internal accuracy robust results: peaked spectrum with 3 kpc correlation length, indication of power-law inertia range on smaller scales required: better understanding of RM noise & beam effects, window effects on large scales