Quantification of Uncertainty in the Measurement of Magnetic Fields in Clusters of Galaxies

William I. Newman, Alice L. Newman, Yoel Rephaeli

Published 2002-04-26Version 1

We assess the principal statistical and physical uncertainties associated with the determination of magnetic field strengths in clusters of galaxies from measurements of Faraday rotation (FR) and Compton-synchrotron emissions. In the former case a basic limitation is noted, that the relative uncertainty in the estimation of the mean-squared FR will generally be at least one third. Even greater uncertainty stems from the crucial dependence of the Faraday-deduced field on the coherence length scale characterizing its random orientation; we further elaborate this dependence, and argue that previous estimates of the field are likely to be too high by a factor of a few. Lack of detailed spatial information on the radio emission--and the recently deduced nonthermal X-ray emission in four clusters--has led to an underestimation of the mean value of the field in cluster cores. We conclude therefore that it is premature to draw definite quantitative conclusions from the previously-claimed seemingly-discrepant values of the field determined by these two methods.