J/AJ/149/60ivo://CDS.VizieR/J/AJ/149/60doi:10.26093/cds/vizier.51490060CDSSun X.H.Rudnick L.Akahori T.Anderson C.S.Bell M.R.Bray J.D.,Farnes J.S.Ideguchi S.Kumazaki K.O'Brien T.O'Sullivan S.P.,Scaife A.M.M.Stepanov R.Stil J.Takahashi K.van Weeren R.J.,Wolleben M.2015-05-10T14:43:44Z2015-04-16T12:46:43ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frmagnetic-fieldspolarimetryFaraday rotation measures (RMs) and more general Faraday structures are key parameters for studying cosmic magnetism and are also sensitive probes of faint ionized thermal gas. A definition of which derived quantities are required for various scientific studies is needed, as well as addressing the challenges in determining Faraday structures. A wide variety of algorithms has been proposed to reconstruct these structures. In preparation for the Polarization Sky Survey of the Universe's Magnetism (POSSUM) to be conducted with the Australian Square Kilometre Array Pathfinder and the ongoing Galactic Arecibo L-band Feeds Array Continuum Transit Survey (GALFACTS), we run a Faraday structure determination data challenge to benchmark the currently available algorithms, including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling, and QU-fitting. The input models include sources with one Faraday thin component, two Faraday thin components, and one Faraday thick component. The frequency set is similar to POSSUM/GALFACTS with a 300MHz bandwidth from 1.1 to 1.4GHz. We define three figures of merit motivated by the underlying science: (1) an average RM weighted by polarized intensity, RM_wtd_, (2) the separation {Delta}_{phi}_ of two Faraday components, and (3) the reduced chi-squared {Chi}_r_^2^. Based on the current test data with a signal-to-noise ratio of about 32, we find the following. (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found. (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for RM_wtd_ but with significantly higher errors for {Delta}_{phi}_. All other methods, including standard Faraday synthesis, frequently identify only one component when {Delta}_{phi}_ is below or near the width of the Faraday point-spread function. (3) No methods as currently implemented work well for Faraday thick components due to the narrow bandwidth. (4) There exist combinations of two Faraday components that produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q, U behavior, so no method can recover a unique input model. Further exploration of all these issues is required before upcoming surveys will be able to provide reliable results on Faraday structures.2015AJ....149...60Shttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/149/60CatalogResearchIsServedByTAP VizieR generic servicerelated-toVIII/65 : 1.4GHz NRAO VLA Sky Survey (NVSS) (Condon+ 1998)J/ApJS/212/15 : Polarized NVSS sources SEDs (Farnes+, 2014)J/ApJ/759/25 : Rotation measures at 1.4GHz toward the LMC (Mao+, 2012)J/ApJ/728/97 : VLA rotation measures in the Galactic plane (Van Eck+, 2011)J/ApJ/714/1170 : Faraday rotation at high Galactic latitude (Mao+, 2010)J/ApJ/702/1230 : Rotation measure image of the sky (Taylor+, 2009)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/AJ/149/60https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/AJ/149/60http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/AJ/149/60https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/AJ/149/60https://vizier.iucaa.in/viz-bin/votable?-source=J/AJ/149/60http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/AJ/149/60GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapdefaultJ/AJ/149/60/table3Input models and results for the data challengerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintCompFaraday component type (one, two, or thick) (1)meta.id;stat.fitcharModelAlgorithm description (Method-Name)meta.id;stat.fitcharMethMethod (Wavelet, CS, FS, or QU) (2)meta.notecharNameAbbreviation of the participant's name (3)obs.observercharpol1? Thin component 1 percent polarization%phys.polarizationfloatnullablephi1? Thin component 1 Faraday depth {phi}_1_ (1)rad.m**-2phys.absorption.opticalDepthfloatnullablechi1? Thin component 1 polarization angle {chi}_1_degphys.polarization.rotMeasurefloatnullablepol2? Thin component 2 percent polarization%phys.polarizationfloatnullablephi2? Thin component 2 Faraday depth {phi}_2_ (1)rad.m**-2phys.absorption.opticalDepthfloatnullablechi2? Thin component 2 polarization angle {chi}_2_degphys.polarization.rotMeasurefloatnullablepolc? Thick component percent polarization p_0_%phys.polarizationfloatnullablephic? Thick component central Faraday depth {phi}_c_ (1)rad.m**-2phys.absorption.opticalDepthfloatnullablephis? Thick component Faraday depth extent {phi}_s_ (1)rad.m**-2phys.absorption.opticalDepthfloatnullableRM? Weighted average Faraday rotation measure (4)rad.m**-2phys.polarization.rotMeasure;stat.meanfloatnullablechi2r? Reduced {chi}^2^ of fitstat.fit.chi2floatnullable