A Broadband Spectro-polarimetric View of the NVSS Rotation Measure Catalogue I: Breaking the n{pi}-ambiguity. (arXiv:1905.04313v1 [astro-ph.GA])

A Broadband Spectro-polarimetric View of the NVSS Rotation Measure Catalogue I: Breaking the n{pi}-ambiguity. (arXiv:1905.04313v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ma_Y/0/1/0/all/0/1">Yik Ki Ma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mao_S/0/1/0/all/0/1">S. A. Mao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stil_J/0/1/0/all/0/1">Jeroen Stil</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Basu_A/0/1/0/all/0/1">Aritra Basu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+West_J/0/1/0/all/0/1">Jennifer West</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Heiles_C/0/1/0/all/0/1">Carl Heiles</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hill_A/0/1/0/all/0/1">Alex S. Hill</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Betti_S/0/1/0/all/0/1">S. K. Betti</a>

The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue is invaluable
for the study of cosmic magnetism. However, the RM values reported in it can be
affected by n$pi$-ambiguity, resulting in deviations of the reported RM from
the true values by multiples of +-652.9 rad m-2. We therefore set off to
observationally constrain the fraction of sources in the RM catalogue affected
by this ambiguity. New broadband spectro-polarimetric observations were
performed with the Karl G. Jansky Very Large Array (VLA) at 1–2 GHz, with 23
n$pi$-ambiguity candidates selected by their peculiarly high |RM| values. We
identified nine sources with erroneous RM values due to n$pi$-ambiguity and 11
with reliable RM values. In addition, we found two sources to be unpolarised
and one source to be inconsistent with neither n$pi$-ambiguity nor reliable RM
cases. By comparing the statistical distributions of the above two main
classes, we devised a measure of how much a source’s RM deviates from that of
its neighbours: $Delta/sigma$, which we found to be a good diagnostic of
n$pi$-ambiguity. With this, we estimate that there are at least 50 sources
affected by n$pi$-ambiguity among the 37,543 sources in the catalogue.
Finally, we explored the Faraday complexities of our sources revealed by our
broadband observations.

The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue is invaluable
for the study of cosmic magnetism. However, the RM values reported in it can be
affected by n$pi$-ambiguity, resulting in deviations of the reported RM from
the true values by multiples of +-652.9 rad m-2. We therefore set off to
observationally constrain the fraction of sources in the RM catalogue affected
by this ambiguity. New broadband spectro-polarimetric observations were
performed with the Karl G. Jansky Very Large Array (VLA) at 1–2 GHz, with 23
n$pi$-ambiguity candidates selected by their peculiarly high |RM| values. We
identified nine sources with erroneous RM values due to n$pi$-ambiguity and 11
with reliable RM values. In addition, we found two sources to be unpolarised
and one source to be inconsistent with neither n$pi$-ambiguity nor reliable RM
cases. By comparing the statistical distributions of the above two main
classes, we devised a measure of how much a source’s RM deviates from that of
its neighbours: $Delta/sigma$, which we found to be a good diagnostic of
n$pi$-ambiguity. With this, we estimate that there are at least 50 sources
affected by n$pi$-ambiguity among the 37,543 sources in the catalogue.
Finally, we explored the Faraday complexities of our sources revealed by our
broadband observations.

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