QUBIC II: Spectro-Polarimetry with Bolometric Interferometry

Abstract

Bolometric Interferometry is a novel technique that has the ability to perform spectroimaging. A Bolometric Interferometer observes the sky in a wide frequency band and can reconstruct sky maps in several sub-bands within the physical band. This provides a powerful spectral method to discriminate between the Cosmic Microwave Background (CMB) and astrophysical foregrounds. In this paper, the methodology is illustrated with examples based on the Q & U Bolometric Interferometer for Cosmology (QUBIC) which is a ground-based instrument designed to measure the B-mode polarization of the sky at millimeter wavelengths. We consider the specific cases of point source reconstruction and Galactic dust mapping and we characterize the Point Spread Function as a function of frequency. We study the noise properties of spectro-imaging, especially the correlations between sub-bands, using end-toend simulations together with a fast noise simulator. We conclude showing that spectroimaging performance are nearly optimal up to five sub-bands in the case of QUBIC.

QUBIC III: Laboratory characterization

“…The secondary lobes are closer to centre for higher frequencies, as expected. This result is the first hardware demonstration of the capability of a bolometric interferometer to operate as a spectral imager (see Mousset et al [11] for a detailed analysis).…”

“…The spectral resolution improves with the number of baselines as the synthesized beam has finer secondary lobes with a larger cluster of horns. Spectral imaging is an innovative feature of bolometric interferometry which gives QUBIC an important advantage over other CMB imagers (see Hamilton et al [12] and Mousset et al [11] for details) and the expected frequency dependence of QUBIC's synthesized beam has been confirmed (see section 4.4).…”

“…Using the relative location and amplitude of all the peaks in the synthesized beam for each of the TES, we can now project the data onto the sky using optimal map-making to deconvolve from the effect of the multiple peaks. In addition, by exploiting the spatial separation of secondary peaks and the dependence of their location on the frequency of radiation, the analysis of the TOD leads to spectral imaging as described in detail in Mousset et al [11]. Spectral discrimination is necessary to disentangle foreground contamination.…”

Optimal method for reconstructing polychromatic maps from broadband observations with an aysmmetric antenna pattern

“…We emphasize that our goal is not to simulate actual instruments in full detail (although the 2-D interferometer may be considered "QUBIC-like" [8,9], and the great-circle pattern is "CHIME-like" [12]). Rather, our goal is to show illustrative examples of asymmetric antenna patterns of the sort to which the methods described in this paper are applicable.…”

“…When a telescope scans the sky with a broadband detector, each observation in the time-ordered data is a weighted average of signals at different spatial points and at different wavelengths. Because the antenna pattern is inevitably wavelength-dependent, it is in principle possible to reconstruct spectral information even though each individual observation lacks spectral resolution [6][7][8][9][10][11]. Because such spectral information can be extremely valuable, we develop in this paper a systematic approach to determining the amount and kind of information that can be reconstructed.…”