S. Spinelli

QUBIC: The QU Bolometric Interferometer For Cosmology

The primordial B-mode polarisation of the Cosmic Microwave Background is the imprints of the gravitational wave background generated by inflation. Observing the B-mode is up to now the most direct way to constrain the physics of the primordial Universe, especially inflation. To detect these B-modes, high sensitivity is required as well as an exquisite control of systematics effects. To comply with these requirements, we propose a new instrument called QUBIC (Q and U Bolometric Interferometer for Cosmology) based on bolometric interferometry. The control of systematics is obtained with a close-packed interferometer while bolometers cooled to very low temperature allow for high sensitivity. We present the architecture of this new instrument, the status of the project and the self-calibration technique which allows accurate measurement of the instrumental systematic effects.

Intensity and polarization of the atmospheric emission at millimetric wavelengths at Dome Concordia

Atmospheric emission is a dominant source of disturbance in ground-based astronomy at millimetric wavelengths. The Antarctic plateau is recognized as an ideal site for millimetric and submillimetric observations, and the French/Italian base of Dome Concordia (Dome C) is among the best sites on Earth for these observations. In this paper, we present measurements at Dome C of the atmospheric emission in intensity and polarization at a 2-mm wavelength. This is one of the best observational frequencies for cosmic microwave background (CMB) observations when considering cosmic signal intensity, atmospheric transmission, detector sensitivity and foreground removal. Using the B-mode radiation interferometer (BRAIN)-pathfinder experiment, we have performed measurements of the atmospheric emission at 150 GHz. Careful characterization of the airmass synchronous emission has been performed, acquiring more than 380 elevation scans (i.e. skydip) during the third BRAIN-pathfinder summer campaign in 2009 December/2010 January. The extremely high transparency of the Antarctic atmosphere over Dome C is proven by the very low measured optical depth, = 0.050 ± 0.003 ± 0.011, where the first error is statistical and the second is the systematic error. Mid-term stability, over the summer campaign, of the atmosphere emission has also been studied. Adapting the radiative transfer atmosphere emission model am to the particular conditions found at Dome C, we also infer the level of the precipitable water vapor (PWV) content of the atmosphere, which is notoriously the main source of disturbance in millimetric astronomy (? mm). Upper limits on the airmass correlated polarized signal are also placed for the first time. The degree of circular polarization of atmospheric emission is found to be lower than 0.2 per cent [95 per cent confidence level (CL)], while the degree of linear polarization is found to be lower than 0.1 per cent (95 per cent CL). These limits include signal-correlated instrumental spurious polarization.

A template of atmospheric O2 circularly polarized emission for cosmic microwave background experiments

We compute the circularly polarized signal from atmospheric molecular oxygen. The polarization of O 2 rotational lines is caused by the Zeeman effect in the Earths magnetic field. We evaluate the circularly polarized emission for various sites suitable for cosmic microwave background (CMB) measurements: the South Pole and Dome C (Antarctica), Atacama (Chile) and Testa Grigia (Italy). We present and discuss an analysis of the polarized signal within the framework of future CMB polarization experiments. We find a typical circularly polarized signal (V Stokes parameter) of ∼50―300 μK at 90 GHz looking at the zenith. Among the sites, Atacama shows a lower polarized signal at the zenith. We present maps of this signal for the various sites and we show typical elevation and azimuth scans. We find that Dome C presents the lowest gradient in polarized temperature: ~0.3 μK deg ―1 at 90 GHz. We also study the frequency bands of observation: around ν ≃ 100 GHz and ν ≃ 160 GHz, we find the best conditions because the polarized signal vanishes. Finally, we evaluate the accuracy of the templates and the signal variability in relation to our knowledge of and the variability of the Earths magnetic field and atmospheric parameters.

A template of atmospheric

We compute the circularly polarized signal from atmospheric molecular oxygen. The polarization of O 2 rotational lines is caused by the Zeeman effect in the Earths magnetic field. We evaluate the circularly polarized emission for various sites suitable for cosmic microwave background (CMB) measurements: the South Pole and Dome C (Antarctica), Atacama (Chile) and Testa Grigia (Italy). We present and discuss an analysis of the polarized signal within the framework of future CMB polarization experiments. We find a typical circularly polarized signal (V Stokes parameter) of ∼50―300 μK at 90 GHz looking at the zenith. Among the sites, Atacama shows a lower polarized signal at the zenith. We present maps of this signal for the various sites and we show typical elevation and azimuth scans. We find that Dome C presents the lowest gradient in polarized temperature: ~0.3 μK deg ―1 at 90 GHz. We also study the frequency bands of observation: around ν ≃ 100 GHz and ν ≃ 160 GHz, we find the best conditions because the polarized signal vanishes. Finally, we evaluate the accuracy of the templates and the signal variability in relation to our knowledge of and the variability of the Earths magnetic field and atmospheric parameters.

O_2

Room temperature VNA calibration to measure cryogenic devices can be inadequate when the loss of the unavoidable thermal decoupling line is order of magnitude higher than the DUT one. We present a cryogenic calibration setup with an accuracy at the level of some tens of milli-dB for S21 parameter.

circularly polarized emission for CMB experiments

We present a concept for the payload SAGACE, the Spectroscopic Active Galaxies And Cluster Explorer, devoted to study the evolution of Universe structures using different observables, all of them in the mm∕submm wavelength. The SAGACE payload is made of a passively cooled 3 m telescope, a cryogenic Fourier Transform Spectrometer (FTS) and detector arrays to be operated at 0.3 K by a 3He fridge. The detectors are Ti∕Au Transition Edge Sensor (TES) bolometers with a NEP<10−17 W/Hz½. A phase‐A study has been recently completed for this experiment, in the framework of the call for small missions of the Italian Space Agency.

Measurement accuracy of S-parameters in W band at cryogenic temperature

The rotational component of the CMB polarization, the so-called B-modes, is one of the major topic for next generation CMB experiments. This signal traces the effect on the CMB due to primordial gravitational waves produced during the inflationary epoch, probing the physics of the very early universe at GUT energy scales. This is a challenge, being the expected amplitude of B-mode polarization ~ 0.1μK. In this paper we describe the BRAIN experiment, a bolometric interferometer which combines high sensitivity bolometric detectors with the excellent control of systematic effects proper of interferometers. Being a ground based experiment, we identified Dome Charlie in Antarctica as the best site for such measurements. In order to validate the goodness of the site, as well as some of the implemented technical solutions, we built a pathfinder experiment which has been successfully operated during last Antarctic summer, and we report about preliminary results obtained.