Abstract
In a companion paper we have reported a $>5\sigma$ detection of degree scale $B $-mode polarization at 150 GHz by the BICEP2 experiment. Here we provide a detailed study of potential instrumental systematic contamination to that measurement. We focus extensively on spurious polarization that can potentially arise from beam imperfections. We present a heuristic classification of beam imperfections according to their symmetries and uniformities, and discuss how resulting contamination adds or cancels in maps that combine observations made at multiple orientations of the telescope about its boresight axis. We introduce a technique, which we call "deprojection", for filtering the leading order beam-induced contamination from time ordered data, and show that it removes power from BICEP2's $BB$ spectrum consistent with predictions using high signal-to-noise beam shape measurements. We detail the simulation pipeline that we use to directly simulate instrumental systematics and the calibration data used as input to that pipeline. Finally, we present the constraints on $BB$ contamination from individual sources of potential systematics. We find that systematics contribute $BB$ power that is a factor $\sim10\times$ below BICEP2's 3-year statistical uncertainty, and negligible compared to the observed $BB$ signal. The contribution to the best-fit tensor/scalar ratio is at a level equivalent to $r=(3-6)\times10^{-3}$.
Highlights
Since the the discovery of the 2.7 K cosmic microwave background (CMB) by Penzias & Wilson (1965), rapid progress in instrumental sensitivity has permitted the detection of progressively subtler effects
BICEP2 employs a combination of high magnetic permeability and superconducting shielding to block external magnetic fields, and its scan strategy allows for nearly perfect filtering (“ground subtraction”) of pickup that is constant in time and a function of telescope pointing direction, as is expected of most magnetic fields
We have extended our pipeline to optionally incorporate the effects of various instrumental systematics into these simulated data, which allows us to model their effects on the final power spectra and r estimate
Summary
Since the the discovery of the 2.7 K cosmic microwave background (CMB) by Penzias & Wilson (1965), rapid progress in instrumental sensitivity has permitted the detection of progressively subtler effects. The degree scale primary CMB temperature anisotropies are polarized at the ∼1% level (Kovac et al 2002), with fluctuations of the order of 1 μK. This polarization, which arises as a natural consequence of the same acoustic oscillations that source the temperature anisotropies (Bond & Efstathiou 1984), is curl-free (E-mode) and its angular power spectrum is uniquely predicted given the temperature (T) spectrum with the addition of no additional cosmological parameters. Effects that convert CMB temperature anisotropy into a false polarization signal are of particular importance This is especially true for B-mode measurements because both the temperature and the expected inflationary Bmode spectra peak at similar angular scales. In a series of four appendices we provide the formal definition of our elliptical Gaussian beam parametrization (Appendix A), an expanded discussion of beam shape mismatch (Appendix B), the mathematical and practical details of deprojection (Appendix C), and a discussion of the uncertainties in the beam mismatch simulations (Appendix D)
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