Abstract
Pulse profile stability is a central assumption of standard pulsar timing
methods. Thus, it is important for pulsar timing array experiments such as the
North American Nanohertz Observatory for Gravitational Waves (NANOGrav) to
account for any pulse profile variability present in their data sets. We show
that in the NANOGrav 15-yr data set, the integrated pulse profile of PSR
J1022+1001 as seen by the Arecibo radio telescope at 430, 1380, and 2030 MHz
varies considerably in its shape from observation to observation. We
investigate the possibility that this is due to the "ideal feed assumption"
(IFA), on which NANOGrav's routine polarization calibration procedure relies.
PSR J1022+1001 is $\sim 90\%$ polarized in one pulse profile component, and
also has significant levels of circular polarization. Time-dependent deviations
in the feed's polarimetric response (PR) could cause mixing between the
intensity I and the other Stokes parameters, leading to the observed
variability. We calibrate the PR using a mixture of Measurement Equation
Modeling and Measurement Equation Template Matching techniques. The resulting
profiles are no less variable than those calibrated using the IFA method, nor
do they provide an improvement in the timing quality of this pulsar. We observe
the pulse shape in 25-MHz bandwidths to vary consistently across the band,
which cannot be explained by interstellar scintillation in combination with
profile evolution with frequency. Instead, we favor phenomena intrinsic to the
pulsar as the cause.