J/AJ/149/60 Modeling Faraday structures. I. 1100-1400 MHz (Sun+, 2015)
Comparison of algorithms for determination of rotation measure and Faraday
structure. I. 1100-1400 MHz.
Sun 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.
<Astron. J., 149, 60 (2015)>
=2015AJ....149...60S 2015AJ....149...60S
ADC_Keywords: Magnetic fields ; Polarization
Keywords: ISM: magnetic fields - magnetic fields - polarization -
radio continuum: general - techniques: polarimetric
Abstract:
Faraday 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, RMwtd, (2) the
separation Δφ of two Faraday components, and (3) the
reduced chi-squared Χr2. 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 RMwtd but with significantly higher errors for Δφ.
All other methods, including standard Faraday synthesis, frequently
identify only one component when Δφ 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.
Description:
Motivated by the need for competitive algorithms to reconstruct
Faraday structures and the advent of many methods, we initiated a
Faraday structure determination data challenge aiming to benchmark all
the current methods (including Faraday synthesis, wavelet, compressive
sampling, and QU-fitting). In this first step, we focus on the
Polarization Sky Survey of the Universe's Magnetism (POSSUM; that will
be conducted with the Australian Square Kilometre Array Pathfinder)
and the Galactic Arecibo L-band Feeds Array Continuum Transit Survey
(GALFACTS) configurations, and use a similar frequency range covering
300MHz from 1.1 to 1.4GHz. The band is split into 300*1MHz channels.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 109 238 Input models and results for the data challenge
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See also:
VIII/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)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 5 A5 --- Comp Faraday component type (one, two, or thick) (1)
7- 16 A10 --- Model Algorithm description (Method-Name)
18- 25 A8 --- Meth Method (Wavelet, CS, FS, or QU) (2)
27- 29 A3 --- Name Abbreviation of the participant's name (3)
31- 36 F6.2 % pol1 ? Thin component 1 percent polarization
38- 44 F7.2 rad/m2 phi1 ? Thin component 1 Faraday depth φ1 (1)
46- 52 F7.2 deg chi1 ? Thin component 1 polarization angle χ1
54- 58 F5.2 % pol2 ? Thin component 2 percent polarization
60- 66 F7.2 rad/m2 phi2 ? Thin component 2 Faraday depth φ2 (1)
68- 74 F7.2 deg chi2 ? Thin component 2 polarization angle χ2
76- 81 F6.2 % polc ? Thick component percent polarization p0
83- 89 F7.2 rad/m2 phic ? Thick component central Faraday depth
φc (1)
91- 96 F6.2 rad/m2 phis ? Thick component Faraday depth extent
φs (1)
98-103 F6.1 rad/m2 RM ? Weighted average Faraday rotation measure (4)
105-109 F5.2 --- chi2r ? Reduced χ2 of fit
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Note (1): The three types of Faraday components are (see section 3):
One = One Faraday thin component (Faraday spectrum is a δ-function)
Two = Two Faraday thin components (Faraday spectrum is the sum of two
δ-functions)
Thick = Faraday thick component (Faraday spectrum is a boxcar function,
within φc-φs/2 and φc+φs/2)
Note (2): Methods used to reconstruct Faraday structures are:
Wavelet = wavelet decomposition (Section 2.1.2)
CS = Compressive Sampling (Section 2.1.3)
FS = Method based on Faraday Synthesis (previously called Faraday
rotation measure synthesis in the literature) and Faraday clean,
see Section 2.1.1 and Table 1);
QU = QU-fitting (Section 2.2)
Note (3): The abbreviations of the participant's name are:
AS = Anna Scaife;
JF = Jamie Farnes;
JS = Jeroen Stil;
KK = Kohei Kumazaki;
LR = Lawrence Rudnick;
MB = Michael Bell;
MBn = Not explained in the paper;
MW = Maik Wolleben;
RS = Rodion Stepanov;
RvW = Reinout van Weeren;
SOS = Shane O'Sullivan;
TOB = Tim O'Brien;
XS = Xiaohui Sun.
Note (4): polarization angle rotation χ(λ) from Eq.(1):
χ(λ) = χ0 + RM λ2
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 24-Feb-2015