Estimating extragalactic Faraday rotation

By N. Oppermann, H. Junklewitz, M. Greiner, Torsten A. Ensslin, T. Akahori, E. Carretti, B. Gaensler, A. Goobar, L. Harvey-Smith, M. Johnston-Hollitt and 4 more. 2014

147 citations recorded by the source corpus.

Observations of Faraday rotation for extragalactic sources probe magnetic fields both inside and outside the Milky Way. Building on our earlier estimate of the Galactic contribution, we set out to estimate the extragalactic contributions. We discuss the problems involved; in particular, we point out that taking the difference between the observed values and the Galactic foreground reconstruction is not a good estimate for the extragalactic contributions. We point out a degeneracy between the contributions to the observed values due to extragalactic magnetic fields and observational noise and comment on the dangers of over-interpreting an estimate without taking into account its uncertainty information. To overcome these difficulties, we develop an extended reconstruction algorithm based on the assumption that the observational uncertainties are accurately described for a subset of the data, which can overcome the degeneracy with the extragalactic contributions. We present a probabilistic derivation of the algorithm and demonstrate its performance using a simulation, yielding a high quality reconstruction of the Galactic Faraday rotation foreground, a precise estimate of the typical extragalactic contribution, and a well-defined probabilistic description of the extragalactic contribution for each data point. We then apply this reconstruction technique to a catalog of Faraday rotation observations for extragalactic sources. The analysis is done for several different scenarios, for which we consider the error bars of different subsets of the data to accurately describe the observational uncertainties. By comparing the results, we argue that a split that singles out only data near the Galactic poles is the most robust approach. We find that the dispersion of extragalactic contributions to observed Faraday depths is most likely lower than 7 rad/m(2), in agreement with earlier results, and that the extragalactic contribution to an individual data point is poorly constrained by the data in most cases.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/6d07f81c4c69cbed93abb3967651a448ebf090e8

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Nuclear and High Energy Physics

Citations in the corpus

CHIME/FRB Discovery of Eight New Repeating Fast Radio Burst Sources (2019, methodology): “The map presented by Oppermann et al. (2015) estimates a foreground Milky Way RM ≈ −72 ± 23 rad m−2 along this line-of-sight, based on Canadian Galactic Plane Survey (CGPS) and NVSS RMs.”
Nine New Repeating Fast Radio Burst Sources from CHIME/FRB (2020, methodology): “We used two methods to determine the Galactic contribution to RM: i) the constructed RM foreground map of Oppermann et al. (2015), previously applied in Paper I; and ii) the first Faraday moment of diffuse, polarized emission obtained from the Global Magneto-Ionic Medium Survey (GMIMS) (Dickey et…”
Planck 2015 results: XIX. Constraints on primordial magnetic fields (2015, unknown): “…the impact of the Galactic FR on the detection of primordial magnetic fields, we use Galactic observations of polarized synchrotron emission at 1.4 GHz and 23 GHz and the synthesized all-sky Faraday rotation map derived from extragalactic radio source emission provided by Oppermann et al. (2015).”