Removing non-physical structure in fitted Faraday rotated signals: non-parametric QU-fitting. (arXiv:2010.07932v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pratley_L/0/1/0/all/0/1">Luke Pratley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Johnston_Hollitt_M/0/1/0/all/0/1">Melanie Johnston-Hollitt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gaensler_B/0/1/0/all/0/1">Bryan M. Gaensler</a>
Next generation spectro-polarimetric broadband surveys will probe cosmic
magnetic fields in unprecedented detail, using the magneto-optical effect known
as Faraday rotation. However, non-parametric methods such as RM CLEAN can
introduce non-physical linearly polarized flux. This non-physical flux is
introduced into the fitted model across negative wavelengths squared and leads
to potentially incorrect and non-verifiable Faraday rotation structures that
are consistent with the observed data but violate conservation of energy or are
difficult to detect. We introduce a novel method called non-parametric
$QU$-fitting and use it to show that constraining the non-observable flux in
the set of solutions to zero provides a model Faraday depth spectrum that
avoids non-physical structures in Faraday depth, a property that is needed when
comparing theoretical magnetic field models to fitted Faraday rotation signals.
We verify this approach both on simulated and observed broadband data sets,
demonstrating that the avoidance of non-physical linear polarized flux in model
fitting is a critical requirement in future broadband polarimetric observations
and surveys.
Next generation spectro-polarimetric broadband surveys will probe cosmic
magnetic fields in unprecedented detail, using the magneto-optical effect known
as Faraday rotation. However, non-parametric methods such as RM CLEAN can
introduce non-physical linearly polarized flux. This non-physical flux is
introduced into the fitted model across negative wavelengths squared and leads
to potentially incorrect and non-verifiable Faraday rotation structures that
are consistent with the observed data but violate conservation of energy or are
difficult to detect. We introduce a novel method called non-parametric
$QU$-fitting and use it to show that constraining the non-observable flux in
the set of solutions to zero provides a model Faraday depth spectrum that
avoids non-physical structures in Faraday depth, a property that is needed when
comparing theoretical magnetic field models to fitted Faraday rotation signals.
We verify this approach both on simulated and observed broadband data sets,
demonstrating that the avoidance of non-physical linear polarized flux in model
fitting is a critical requirement in future broadband polarimetric observations
and surveys.
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