[daip] CXPOLN and CXCLN: missing linear polarization?

[Uwe Bach]

Dear AIPS group, dear Bill Cotton,

recently I reduced some VSOP observations of 0716+714 at 5GHz in full 
polarization together with Thomas Krichbaum at the MPI in Bonn. We did 
the polarization calibration (D-terms) following the AIPS Memo by Bill 
Cotton with total intensity models and LPCAL.  The d-terms were 
comparable to previous observation and were also reproducible on 
different calibrators.

The polarization maps of the ground array were produced with IMAGR, but 
the Q and U images of the observations including HALCA had to be imaged 
using CXPOLN and CXCLN, since HALCA recorded only left circular 
polarization. From the comparison of the ground  array only images with 
the images including HALCA we found that the core of 0716+714 (which is 
very compact) had about 40% less polarized flux in the VSOP images then 
in the ground array images and that the E-vectors were rotated by about 
24 deg. This happened in all 3 epochs.

To test if this is due to the higher resolution of the VSOP images we 
also imaged the ground array data using CXPOLN and CXCLN. The resulting 
maps also contain 40% less linear polarization and show E-vectors 
rotated by about 24 deg with respect to the IMAGR maps, although the 
number of antennas, resolution and weighting are completely the same. Do 
you have any idea where this comes from? Is it a bug in CXPOLN or CXCLN 
or is it just due to the fact that a complex clean uses only one of the 
cross polarizations (LR or RL)? But the last point might only explain 
the rotated E-vector and the higher noise, but not why there is always 
less linear polarization. We observed this also in another VSOP 
experiment of 0954+62.

Thanks for your help and kind regards
Uwe

-- 
_________________________________________
Uwe Bach
INAF - Osservatorio Astronomico di Torino
Via Osservatorio 20
10025 Pino Torinese (TO), ITALY
http://www.to.astro.it/blazars/bach
phone: +39 011 8101912
fax: +39 011 8101930