Magnetic power spectra from Faraday rotation maps

REALMAF and its use on Hydra A

Max Planck Institute for Astrophysics, Karl-Schwartzschildstr.1, 85741 Garching, Germany
e-mail: ensslin@mpa-garching.mpg.de

Received: 19 December 2009
Accepted: 29 January 2011

Abstract

We develop a novel maximum-a-posteriori method to measure magnetic power spectra from Faraday rotation data and implement it in the REALMAF code. A sophisticated model for the magnetic autocorrelation in real space permits us to alleviate previously required simplifying assumptions in the processing. We also introduce a way to treat the divergence relation of the magnetic field with a multiplicative factor in Fourier space, with which we can model the magnetic autocorrelation as a spherically symmetric function. Applied to the dataset of Hydra A north, we find a power law power spectrum on spatial scales between 0.3 kpc and 8 kpc, with no visible turnover at large scales within this range and a spectral index consistent with a Kolmogorov-like power law regime. The magnetic field strength profile seems to follow the electron density profile with an index α = 1. A variation of α from 0.5 to 1.5 would lead to a spectral index between 1.55 and 2.05. The extrapolated magnetic field strength in the cluster centre highly depends on the assumed projection angle of the jet. For an angle of 45° we derive extrapolated 36 μG in the centre and directly probed 16 μG at 50 kpc radius.