Changing the de-biasing in a POL2 vector catalogue

The new poledit command in the Starlink POLPACK package (see a previous blog post) has an option to recalculate the PI (polarised intensity) and P (percentage polarisation) values using a specified form of de-biasing. If the existing catalogue is in file “mycat.FIT“, you can do:

% polpack
% poledit mycat newcat mode=debias debiastype=mas

This will create a new file “newcat.FIT” containing a copy of “mycat.FIT“, but with new P and PI columns re-calculated from the existing Q, U, I and DPI values using the “Modified Asymptotic” (MAS) bias estimator (any de-biasing used to create the original catalogue is ignored). The options for the DEBIASTYPE parameter are:

  • “MAS”: De-bias using the modified asymptotic estimator
  • “AS”: De-bias using the asymptotic estimator
  • “None”: Do not include any de-biasing

See a previous blog post for more on these types of de-biasing.

The following plot shows the results of using the above command. The horizontal axis is PI/DPI (polarised intensity signal to noise ratio) with no de-biasing. The vertical axis is the new PI/DPI value created with each of the DEBIASTYPE options listed above (red is “AS”, blue is “MAS” and green is “None”)

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