Project concept 

Polarized microwave source in space

We propose to build an artificial polarized microwave source and install it in space to be in the far-field of large ground-based telescopes in Europe and Chile. The easiest step into space would be to launch the source on a nano-satellite into a low-earth orbit (LEO) as originally proposed by Johnson et al. (2015). Unfortunately, on such an orbit the source would move too quickly in the sky (several degrees per second) to be tracked by large microwave telescopes on Earth.  Our baseline plan is to install the payload in geostationary orbit on a commercial platform from the telecommunications operator Eutelsat Group. In this configuration, COSMOCal would be used to calibrate polarization observations from telescopes in Europe and Chile. In parallel, we are also considering to study a second configuration, with COSMOCal in a circumpolar orbit, enabling direct calibration of the large 10m telescope at the South Pole, SPT. 

Microwave telescopes

In our nominal plan, COSMOCal will be used to calibrate polarization observations from a complementary set of ground-based telescopes that comprises the IRAM 30m and the Sardinia radio 64 m telescope (SRT) in Europe, and the large aperture telescope of 6 meter diameter (LAT) of the Simons Observatory (SO) in Chile.  The IRAM and SRT telescopes are equipped with instruments dedicated to Galactic and extragalactic astrophysics while the SO observatory is dedicated to CMB science. Notice that the SO LAT will survey 40% of the sky with arc-minute resolution. The CMB lensing maps used to remove lensing B-modes will be constructed with LAT data.  The SO LAT will also be a prime telescope to search for cosmic birefringence (Pogosian et al. 2019), which relies on the ability to disentangle the cosmic birefringence angle from the instrumental one. The IRAM 30m and SRT telescopes offer the unique opportunity to have very well calibrated polarization data at higher resolution (~12 arcseconds). Once precisely calibrated with COSMOCal these telescopes will allow us to map astrophysical sources with very high precision and then offer well determined reference for any other polarization experiment, including a CMB space mission such as LiteBIRD. These first three telescopes have been privilegied by the expertise of the COSMOCal collaboration with these facilities. We are currently investigating the opportunity to serve a wider range of observatories. 

The COSMOCal source is designed to match three main atmospheric windows best suited to map the CMB. The SO LAT is equipped with a multi-frequency camera operating at frequency channels centered at 90, 150, 220 and 280 GHz, while the NIKA2 camera on IRAM 30m observes polarization in a band centered at 260 GHz and SRT is equipped with a multi-beam instrument observing in polarization within the frequency range (75-116 GHz). The COSMOCal source will allow us to calibrate the polarization angles of these observatories/experiments to an accuracy of at least of 0.1 deg covering the entire range of frequencies.

Deliverables

• The absolute calibration of the selected telescopes at an unprecedented accuracy.

• Polarization Stokes Q and U maps of a list of polarized celestial sources within the frequency range 90-280 GHz, such as Tau A (the Crab supernova remnant). These maps will become references for use to calibrate microwave polarization observations of all telescopes observing at mm and submm wavelengths, including the telescopes at the South Pole and LiteBIRD, which cannot directly calibrate their observations observing the COSMOCal source.