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Register a new userHWP non-idealities and their impact on future measurements of CMB polarization anisotropies from space: a detailed study
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We study the propagation of a specific class of instrumental systematics to the reconstruction of the B-mode power spectrum of the cosmic microwave background (CMB). We focus on non-idealities of the half-wave plate (HWP), a polarization modulator that will be deployed by future CMB experiments, such as the phase-A satellite mission LiteBIRD. More in details, we study the effects of non-ideal HWP properties, such as transmittance, phase shift and cross-polarization. To this purpose, we develop a simple, yet stand-alone end-to-end simulation pipeline adapted to LiteBIRD. Through the latter, we analyze the effects of a possible mismatch between the measured frequency profiles of HWP properties (used in the mapmaking stage of the pipeline) and the actual profiles (used in the sky-scanning step). We simulate single-frequency, CMB-only observations to emphasize the effects of non-idealities on the BB power spectrum. We also consider multi-frequency observations to account for the frequency dependence of HWP properties and the contribution of foreground emission. We quantify the systematics effects in terms of a bias $Delta r$ on the tensor-to-scalar ratio $r$ with respect to the ideal case of no-systematics. We derive the accuracy requirements on the measurements of HWP properties by requiring $Delta r < 10^{-5}$ (1% of the expected LiteBIRD sensitivity on $r$). The analysis is introduced by a detailed presentation of the mathematical formalism employed in this work, including the use of the Jones and Mueller matrix representations.
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Searching for the signal of primordial gravitational waves in the B-modes (BB) power spectrum is one of the key scientific aims of the cosmic microwave background (CMB) polarization experiments. However, this could be easily contaminated by several foreground issues, such as the thermal dust emission. In this paper we study another mechanism, the cosmic birefringence, which can be introduced by a CPT-violating interaction between CMB photons and an external scalar field. Such kind of interaction could give rise to the rotation of the linear polarization state of CMB photons, and consequently induce the CMB BB power spectrum, which could mimic the signal of primordial gravitational waves at large scales. With the recent polarization data of BICEP2 and the joint analysis data of BICEP2/Keck Array and Planck, we perform a global fitting analysis on constraining the tensor-to-scalar ratio $r$ by considering the polarization rotation angle which can be separated into a background isotropic part and a small anisotropic part. Since the data of BICEP2 and Keck Array experiments have already been corrected by using the self-calibration method, here we mainly focus on the effects from the anisotropies of CMB polarization rotation angle. We find that including the anisotropies in the analysis could slightly weaken the constraints on $r$, when using current CMB polarization measurements. We also simulate the mock CMB data with the BICEP3-like sensitivity. Very interestingly, we find that if the effects of the anisotropic polarization rotation angle can not be taken into account properly in the analysis, the constraints on $r$ will be dramatically biased. This implies that we need to break the degeneracy between the anisotropies of the CMB polarization rotation angle and the CMB primordial tensor perturbations, in order to measure the signal of primordial gravitational waves accurately.
We have studied the implications of high sensitivity polarization measurements of objects from the WMAP point source catalogue made using the VLA at 8.4, 22 and 43 GHz. The fractional polarization of sources is almost independent of frequency with a median of ~2 per cent and an average, for detected sources, of ~3.5 per cent. These values are also independent of the total intensity over the narrow range of intensity we sample. Using a contemporaneous sample of 105 sources detected at all 3 VLA frequencies, we have investigated the spectral behaviour as a function of frequency by means of a 2-colour diagram. Most sources have power-law spectra in total intensity, as expected. On the other hand they appear to be almost randomly distributed in the polarized intensity 2-colour diagram. This is compatible with the polarized spectra being much less smooth than those in intensity and we speculate on the physical origins of this. We have performed an analysis of the correlations between the fractional polarization and spectral indices including computation of the principal components. We find that there is little correlation between the fractional polarization and the intensity spectral indices. This is also the case when we include polarization measurements at 1.4 GHz from the NVSS. In addition we compute 45 rotation measures from polarization position angles which are compatible with a lambda^2 law. We use our results to predict the level of point source confusion noise that contaminates CMB polarization measurements aimed at detecting primordial gravitational waves from inflation. We conclude that some level of source subtraction will be necessary to detect r~0.1 below 100 GHz and at all frequencies to detect r~0.01. We present estimates of the level of contamination expected and the number of sources which need to be subtracted as a function of the imposed cut flux density and frequency.
BICEP1 is a millimeter-wavelength telescope designed specifically to measure the inflationary B-mode polarization of the Cosmic Microwave Background (CMB) at degree angular scales. We present results from an analysis of the data acquired during three seasons of observations at the South Pole (2006 to 2008). This work extends the two-year result published in Chiang et al. (2010), with additional data from the third season and relaxed detector-selection criteria. This analysis also introduces a more comprehensive estimation of band-power window functions, improved likelihood estimation methods and a new technique for deprojecting monopole temperature-to-polarization leakage which reduces this class of systematic uncertainty to a negligible level. We present maps of temperature, E- and B-mode polarization, and their associated angular power spectra. The improvement in the map noise level and polarization spectra error bars are consistent with the 52% increase in integration time relative to Chiang et al. (2010). We confirm both self-consistency of the polarization data and consistency with the two-year results. We measure the angular power spectra at 21 <= l <= 335 and find that the EE spectrum is consistent with Lambda Cold Dark Matter (LCDM) cosmology, with the first acoustic peak of the EE spectrum now detected at 15sigma. The BB spectrum remains consistent with zero. From B-modes only, we constrain the tensor-to-scalar ratio to r = 0.03+0.27-0.23, or r < 0.70 at 95% confidence level.
We present a new upper limit on CMB circular polarization from the 2015 flight of SPIDER, a balloon-borne telescope designed to search for $B$-mode linear polarization from cosmic inflation. Although the level of circular polarization in the CMB is predicted to be very small, experimental limits provide a valuable test of the underlying models. By exploiting the non-zero circular-to-linear polarization coupling of the HWP polarization modulators, data from SPIDERs 2015 Antarctic flight provide a constraint on Stokes $V$ at 95 and 150 GHz from $33<ell<307$. No other limits exist over this full range of angular scales, and SPIDER improves upon the previous limit by several orders of magnitude, providing 95% C.L. constraints on $ell (ell+1)C_{ell}^{VV}/(2pi)$ ranging from 141 $mu K ^2$ to 255 $mu K ^2$ at 150 GHz for a thermal CMB spectrum. As linear CMB polarization experiments become increasingly sensitive, the techniques described in this paper can be applied to obtain even stronger constraints on circular polarization.
Recent polarimetric surveys of extragalactic radio sources (ERS) at frequencies u>1GHz are reviewed. By exploiting all the most relevant data on the polarized emission of ERS we study the frequency dependence of polarization properties of ERS between 1.4 and 86GHz. For flat-spectrum sources the median (mean) fractional polarization increases from 1.5% (2-2.5%) at 1.4GHz to 2.5-3% (3-3.5%) at u>10GHz. Steep-spectrum sources are typically more polarized, especially at high frequencies where Faraday depolarization is less relevant. As a general result, we do not find that the fractional polarization of ERS depends on the total flux density at high radio frequencies, i.e >20GHz. Moreover, in this frequency range, current data suggest a moderate increase of the fractional polarization of ERS with frequency. A formalism to estimate ERS number counts in polarization and the contribution of unresolved polarized ERS to angular power spectra at Cosmic Microwave Background (CMB) frequencies is also developed and discussed. As a first application, we present original predictions for the Planck satellite mission. Our current results show that only a dozen polarized ERS will be detected by the Planck Low Frequency Instrument (LFI), and a few tens by the High Frequency Instrument (HFI). As for CMB power spectra, ERS should not be a strong contaminant to the CMB E-mode polarization at frequencies u>70GHz. On the contrary, they can become a relevant constraint for the detection of the cosmological B--mode polarization if the tensor-to-scalar ratio is <0.01.
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