Optimal strategy for polarization modulation in the LSPE-SWIPE
  experiment
  CMB B-mode experiments are required to control systematic effects with an
unprecedented level of accuracy. Polarization modulation by a half wave plate
(HWP) is a powerful technique able to mitigate a large number of the
instrumental systematics. Our goal is to optimize the polarization modulation
strategy of the upcoming LSPE-SWIPE balloon-borne experiment, devoted to the
accurate measurement of CMB polarization at large angular scales. We depart
from the nominal LSPE-SWIPE modulation strategy (HWP stepped every 60 s with a
telescope scanning at around 12 deg/s) and perform a thorough investigation of
a wide range of possible HWP schemes (either in stepped or continuously
spinning mode and at different azimuth telescope scan-speeds) in the frequency,
map and angular power spectrum domain. In addition, we probe the effect of
high-pass and band-pass filters of the data stream and explore the HWP response
in the minimal case of one detector for one operation day (critical for the
single-detector calibration process). We finally test the modulation
performance against typical HWP-induced systematics. Our analysis shows that
some stepped HWP schemes, either slowly rotating or combined with slow
telescope modulations, represent poor choices. Moreover, our results point out
that the nominal configuration may not be the most convenient choice. While a
large class of spinning designs provides comparable results in terms of pixel
angle coverage, map-making residuals and BB power spectrum standard deviations
with respect to the nominal strategy, we find that some specific configurations
(e.g., a rapidly spinning HWP with a slow gondola modulation) allow a more
efficient polarization recovery in more general real-case situations. Although
our simulations are specific to the LSPE-SWIPE mission, the general outcomes of
our analysis can be easily generalized to other CMB polarization experiments.
