Impact of modulation on CMBB-mode polarization experiments

Abstract

We investigate the impact of both slow and fast polarization modulation strategies on the science return of upcoming ground‐based experiments aimed at measuring the B‐mode polarization of the cosmic microwave background. Using detailed simulations of the Cℓ OVER experiment, we compare the ability of modulated and un‐modulated observations to recover the signature of gravitational waves in the polarized CMB sky in the presence of a number of anticipated systematic effects. The general expectations that fast modulation is helpful in mitigating low‐frequency detector noise, and that the additional redundancy in the projection of the instrument's polarization sensitivity directions on to the sky when modulating reduces the impact of instrumental polarization, particularly for fast modulation, are borne out by our simulations. Neither low‐frequency polarized atmospheric fluctuations nor systematic errors in the polarization sensitivity directions are mitigated by modulation. Additionally, we find no significant reduction in the effect of pointing errors by modulation. For a Cℓ OVER‐like experiment, pointing jitter should be negligible but any systematic miscalibration of the polarization coordinate reference system results in significant E–B mixing on all angular scales and will require careful control. We also stress the importance of combining data from multiple detectors in order to remove the effects of common‐mode systematics (such as un‐polarized 1/f atmospheric noise) on the measured polarization signal. Finally we compare the performance of our simulated experiment with the predicted performance from a Fisher analysis. We find good agreement between the (optimal) Fisher predictions and the simulated experiment except for the very largest scales where the power spectrum estimator we have used introduces additional variance to the B‐mode signal recovered from our simulations. In terms of detecting the total B‐mode signal, including lensing, the Fisher analysis and the simulations are in excellent agreement. For a detection of the primordial B‐mode signal only, using an input tensor‐to‐scalar ratio of r= 0.026, the Fisher analysis predictions are ∼20 per cent better than the simulated performance.

Optimal strategy for polarization modulation in the LSPE-SWIPE experiment

“…We highlight that a HWP modulates only the polarization signal, while the telescope scan-speed affects both intensity and polarization. Intensity modulation can therefore be achieved only via telescope scanning and the (small) amount of sky rotation (Brown et al 2009). …”

“…At first order, this implies a miscalibration of Q and U and a leakage from I modulated at 2 f r (see Brown et al 2009). In Fig.…”

“…In particular, a HWP allows to (see, e.g., Brown et al 2009;MacTavish et al 2008): i) effectively mitigate calibration, beam and other instrumental systematics, as a HWP enables to perform the observation without differencing power from distinct orthogonal polarization-sensitive detectors and with no need to rotate the whole instrument; ii) reject the 1/ f noise at the hardware level, as the polarization signal is shifted to higher frequencies; iii) achieve a better angle coverage uniformity, since each pixel is observed over a wide range of orientations of the analyzer.…”

A large-diameter hollow-shaft cryogenic motor based on a superconducting magnetic bearing for millimeter-wave polarimetry

“…where τ drive is the applied torque and −τ loss is the right hand side of Equation 9. This indicates that the mechanical power input by the drive system at a given rotation frequency is equal to the power ultimately dissipated into the cryogenic system via τ loss .…”

The Primordial Inflation Explorer (PIXIE)

“…11,[22][23][24][25][26][27][28][29][30] These systematic errors may be grouped into several broad categories. Scan-synchronous effects, which accumulate coherently through multiple observations, are of particular concern for CMB polarimetry.…”