TY - JOUR
T1 - A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue - I. Breaking the nπ-ambiguity
AU - Ma, Yik Ki
AU - Mao, S. A.
AU - Stil, Jeroen
AU - Basu, Aritra
AU - West, Jennifer
AU - Heiles, Carl
AU - Hill, Alex S.
AU - Betti, S. K.
N1 - Publisher Copyright:
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2019/8/11
Y1 - 2019/8/11
N2 - 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 nπ-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 broad-band spectro-polarimetric observations were performed with the Karl G. Jansky Very Large Array (VLA) at 1-2, GHz, with 23 nπ-ambiguity candidates selected by their peculiarly high |RM| values. We identified nine sources with erroneous RM values due to nπ-ambiguity and 11 with reliable RM values. In addition, we found two sources to be unpolarized and one source to be inconsistent with neither nπ-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: Δ/σ, which we found to be a good diagnostic of nπ-ambiguity. With this, we estimate that there are at least 50 sources affected by nπ-ambiguity among the 37 543 sources in the catalogue. Finally, we explored the Faraday complexities of our sources revealed by our broad-band observations.
AB - 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 nπ-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 broad-band spectro-polarimetric observations were performed with the Karl G. Jansky Very Large Array (VLA) at 1-2, GHz, with 23 nπ-ambiguity candidates selected by their peculiarly high |RM| values. We identified nine sources with erroneous RM values due to nπ-ambiguity and 11 with reliable RM values. In addition, we found two sources to be unpolarized and one source to be inconsistent with neither nπ-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: Δ/σ, which we found to be a good diagnostic of nπ-ambiguity. With this, we estimate that there are at least 50 sources affected by nπ-ambiguity among the 37 543 sources in the catalogue. Finally, we explored the Faraday complexities of our sources revealed by our broad-band observations.
KW - ISM: magnetic fields
KW - galaxies: active
KW - galaxies: magnetic fields
KW - radio continuum: galaxies
UR - http://www.scopus.com/inward/record.url?scp=85072253588&partnerID=8YFLogxK
U2 - 10.1093/mnras/stz1325
DO - 10.1093/mnras/stz1325
M3 - Article
SN - 0035-8711
VL - 487
SP - 3432
EP - 3453
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 3
ER -