G. Sironi

Low-Frequency Measurements of the Cosmic Background Radiation Spectrum

The long-wavelength spectrum of the cosmic background radiation has been measured at five wavelengths (0.33, 0.9, 3.0, 6.3, and 12.0 cm). These measurements represent a continuation of the work reported by Smoot et al. (1983). The combine results have a weighted average of 2.73 {+-} 0.05 K and are consistent with past measurements. They limit the possible Compton distortion of the Cosmic Background Radiation spectrum to less than 8%.

The absolute temperature of the sky and the temperature of the cosmic background radiation at 600 MHz

The absolute temperature of the sky at delta = 46.5 deg has been measured at 600 MHz (50 cm wavelength) using a differential radiometer and a cold calibrator at liquid helium temperature. To separate the cosmic background radiation (CBR) from the remaining components of the sky temperature, observations at other frequencies, now under way, are necessary. A preliminary estimate of the CBR temperature at 600 MHz made using data of T(sky) at 600 MHz and maps of the sky at 408 MHz already in the literature gives T(cbr) = (3.0 + or - 1.2) K, in good agreement with previous measurements at nearby frequencies. 45 refs.

A 33 GHz polarimeter for observations of the Cosmic Microwave Background

Abstract Attempts carried out in the past to detect linear or circular polarization of the Cosmic Microwave Background (CMB) radiation have produced only upper limits to the degree of polarization. These limits are at least two orders of magnitude above the expected value of few times 10−6 or less, as suggested by models combined with the measured values of the CMB anisotropy. Because detection of polarization of the Cosmic Microwave Background will improve our knowledge of the phenomena which accompanied the evolution of the Universe, we built a polarimeter whose sensitivity, 2.1 t(S) mK, is sufficient to detect linear or circular polarization at level of few μK. Details of the system are presented. A testing campaign carried out with a partial prototype of our polarimeter, in a noisy environment, gave an upper limit to the degree of linear polarization of the CMB on angular scales of 14° of ∼8 · 10−5, comparable to the limits at similar angular scales so far published.

The brightness temperature of the south celestial pole and the temperature of the cosmic background radiation measured at 36.6 and 12 centimeter wavelength

The absolute temperature of the sky around the south celestial pole has been measured at ν=0.82 GHz (λ=36.6 cm) and ν=2.5 GHz (λ=12 cm) during an observing campaign from the Amundsen Scott Base at the South Pole in 1989 December. The antennas had an angular resolution of 18°× 23°. The measured temperatures of the sky have been used to determine the temperature of the cosmic background radiation in a frequency region where large deviations from a pure Planckian spectrum are expected.

A systematic search for radio pulses associated with gamma-ray bursts

An automated station for the detection of radio bursts at 151 and 408 MHz has been operating at Medicina (Italy) since 1976. Radio data associated with 32 gamma-ray bursts, detected between July 1976 and April 1979, have been analysed. The results will be discussed in the present paper.

CMB observations: improvements of the performance of correlation radiometers by signal modulation and synchronous detection

Abstract Observation of the fine structures (anisotropies, polarization, spectral distortions) of the Cosmic Microwave Background (CMB) is hampered by instabilities, 1/f noise and asymmetries of the radiometers used to carry on the measurements. Addition of modulation and synchronous detection allows to increase the overall stability and the noise rejection of the radiometers used for CMB studies. In this paper we discuss the advantages this technique has when we try to detect CMB polarization. The behaviour of a two channel correlation receiver to which phase modulation and synchronous detection have been added is examined. Practical formulae for evaluating the improvements are presented.

A research program to map the Galactic Emission at low frequencies

The need for accurate and complete maps of the galaxy at radio frequencies is made evident not only by the importance of the data in the study of galactic dynamics and structure formation but also in the analysis of cosmic background radiation data. A summary of the existing radio surveys of our galaxy is presented along with their limitations. A new program, the Galactic Emission Mapping (GEM) project, to obtain calibrated and complete maps of the galaxy in the 400 to 1400 MHz range, is outlined.

New multifrequency measurements of the spectrum of the cosmic background radiation

We have continued our program to measure the long-wavelength spectrum of the cosmic background radiation. Our previous observations were at five wavelengths--0.33, 0.9, 3.0, 6.3, and 12.0 cm--and had a weighted average value of 2.73 {+-} 0.05 K and deviated from a Planckian spectrum by less than 6%. In August 1984, we repeated our observations at 3.0, 0.9, and 0.33 cm and made new observations with a radiometer tunable from 1.7 to 15 cm. Preliminary analysis indicate that the new data are consistent with our previous results.

The SPOrt experiment

SPOrt is a space experimental aimed at studying the polarization of the CMB and of the diffused Galactic Background in the microwave range (22-90 GHz). Here we present the project as well as its main scientific goals.

High stability and sensitivity correlation polarimeters for CMB polarization measurements

The measure of the faint polarized signal of the Cosmic Microwave Background (few percent of the CMB Anisotropy) requires instruments with very low contamination from systematic effects, high stability and high sensitivity. The BaR-SPOrt experiment, in sharing with the SPOrt project on ISS, is based on analog correlation receivers with components custom designed to match all of these requirements. Here we present the architecture, the design analysis and the status of the realization of the 32 GHz receiver.