# Bandwidth in bolometric interferometry

## TL;DR

This paper analyzes how bandwidth affects the performance and sensitivity of bolometric interferometers designed for detecting B-mode polarization in the CMB, providing analytical and numerical insights.

## Contribution

It offers analytical expressions and sensitivity estimates for broadband bolometric interferometers, addressing bandwidth smearing effects and hardware dependencies.

## Key findings

- Broadband bolometric interferometers can reconstruct visibilities regardless of phase shifter type.
- Bandwidth smearing causes a sensitivity loss of about a factor of 2 for typical 20% bandwidths.
- Numerical simulations confirm analytical predictions about sensitivity and bandwidth effects.

## Abstract

Bolometric Interferometry is a technology currently under development that will be first dedicated to the detection of B-mode polarization fluctuations in the Cosmic Microwave Background. A bolometric interferometer will have to take advantage of the wide spectral detection band of its bolometers in order to be competitive with imaging experiments. A crucial concern is that interferometers are presumed to be importantly affected by a spoiling effect known as bandwidth smearing. In this paper, we investigate how the bandwidth modifies the work principle of a bolometric interferometer and how it affects its sensitivity to the CMB angular power spectra. We obtain analytical expressions for the broadband visibilities measured by broadband heterodyne and bolometric interferometers. We investigate how the visibilities must be reconstructed in a broadband bolometric interferometer and show that this critically depends on hardware properties of the modulation phase shifters. Using an angular power spectrum estimator accounting for the bandwidth, we finally calculate the sensitivity of a broadband bolometric interferometer. A numerical simulation has been performed and confirms the analytical results. We conclude (i) that broadband bolometric interferometers allow broadband visibilities to be reconstructed whatever the kind of phase shifters used and (ii) that for dedicated B-mode bolometric interferometers, the sensitivity loss due to bandwidth smearing is quite acceptable, even for wideband instruments (a factor 2 loss for a typical 20% bandwidth experiment).