M. Bensadoun

A Determination of the Spectral Index of Galactic Synchrotron Emission in the 1-10 GHz Range

We present an analysis of simultaneous multifrequency measurements of the Galactic emission in the 1-10 GHz range with 18° angular resolution taken from a high-altitude site. Our data yield a determination of the synchrotron spectral index between 1.4 and 7.5 GHz of βsyn = 2.81 ± 0.16. Combining our data with maps made by Haslam et al. and Reich & Reich, we find βsyn = 2.76 ± 0.11 in the 0.4-7.5 GHz range. These results are in agreement with the few previously published measurements. The variation of βsyn with frequency based on our results and compared with other data found in the literature suggests a steepening of the synchrotron spectrum toward high frequencies, as expected from theory because of the steepening of the parent cosmic-ray electron energy spectrum. Comparison between the Haslam data and the 19 GHz map of Cottingham also indicates a spatial variation of the spectral index on large angular scales. Additional high-quality data are necessary to provide a serious study of these effects.

THE GEM PROJECT: AN INTERNATIONAL COLLABORATION TO SURVEY GALACTIC RADIATION EMISSION

The GEM (Galactic Emission Mapping) project is an international collaboration established with the aim of surveying the full sky at long wavelengths with a multi-frequency radio telescope. A total of 745 hours of observation at 408 MHz were completed from an Equatorial site in Colombia. The observations cover the celestial band 0h<α<24h, and −24° 22′<δ<+35° 37′. Preliminary results of this partial survey will be discussed. A review of the instrumental setup and a ∼10° resolution sky map at 408 MHz is presented.

A Measurement of the Cosmic Microwave Background Temperature at 7.5 GHz

The temperature of the cosmic microwave background (CMB) radiation at a frequency of 7.5 GHz (4 cm wavelength) is measured, obtaining a brightness temperature of T(CMB) = 2.70 +/- 0.08 K (68 percent confidence level). The measurement was made from a site near the geographical South Pole during the austral spring of 1989 and was part of an international collaboration to measure the CMB spectrum at low frequencies with a variety of radiometers from several different sites. This recent result is in agreement with the 1988 measurement at the same frequency, which was made from a different site with significantly different systematic errors. The combined result of the 1988 and 1989 measurements is 2.64 +/- 0.06 K. 11 refs.

The Temperature of the Cosmic Microwave Background Radiation at 3.8 GHz; Results of a Measurement from the South Pole Site

As part of an international collaboration to measure the low-frequency spectrum of the cosmic microwave background (CMB) radiation, its temperature was measured at a frequency of 3.8 GHz (7.9 cm wavelength), during the austral spring of 1989, obtaining a brightness temperature of 2.64 ± 0.07 K (68% confidence level). The new result is in agreement with previous measurements at the same frequency obtained in 1986-88 from a very different site and has comparable error bars. Combining measurements from all years, a value of 2.64 ± 0.06 K is obtained for the brightness temperature

A measurement of the temperature of the cosmic microwave background at a frequency of 7.5 GHz

LBL-27451 Preprint Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA Physics Division For Reference R r- z \ v E D Not to be Submitted to Astrophysical Journal isk^al&afaeMm OCT 0 1909 U3KARY AND DOCUMENTS SECTION A Measurement of the Temperature of the Cosmic Microwave Background at a Frequency of 7.5 GHz A. Kogut, M . Bensadoun, G. De Amici, S. Levin, G.F. Smoot, and C. Witebsky June 1989 Prepared for the U.S. Department of Energy under Contract Number DE-AC03-76SF00098.

Effects of Atmospheric Emission on Ground-based Microwave Background Measurements

We present an analysis of multifrequency measurements ofatmospheric emission in the Rayleigh-Jeans portion of the cosmicmicrowave background spectrum (1-90 GHz) taken since 1986 from WhiteMountain, CA, and from the South Pole. Correlations of simultaneous dataat 10 and 90 GHz and accurate low-frequency measurements show goodagreement with model predictions for both sites. Our data from the SouthPole 1989 campaign combined with real-time measurements of the localatmospheric profiles provide accurate verification of the expectedindependent contributions of H2O and O2 emission. We show that variationson the order of 10 percent of the oxygen emission (both resonant andnonresonant components) are present on timescales of hours to days,mainly due to the evolution of the atmospheric pressure profile. Oxygenemission fluctuations appear larger than previously expected and may havesignificant consequences for ground-based cosmic microwave backgroundexperiments.

The temperature of the cosmic background radiation : results from the 1987 and 1988 measurements at 3.8 GHz

The low-frequency (Rayleigh-Jeans) spectrum of the cosmic background radiation (CBR) is expected to contain information relevant to the physical processes that occurred in the early universe. In this paper, the authors have measured the temperature of the cosmic background radiation (CBR) at a frequency of 3.8 GHz (7.9 cm wavelength), during two consecutive summers, obtaining a brightness temperature, T{sub CBR}, of 2.56 {plus minus} 0.08 K in 1987 and 2.71 {plus minus} 0.07 K in 1988 (68% confidence level). The new results are in agreement with their previous measurement at 3.7 GHz obtained in 1986 and have smaller error bars. Combining measurements from all 3 years, they obtain T{sub CBR} = 2.64 {plus minus} 0.07 K.

A large L-band rectangular corrugated horn

A lightweight, rectangular, corrugated-horn antenna constructed from sheet metal is described. Over a 1.3-1.7 GHz operating band, its half-power beamwidth is approximately 20\deg in the E -plane and varies from 17\deg to 13\deg in the H -plane. Quarter-wave choke slots at the aperture help to reduce the E -plane sidelobes below -55 dB at angles greater than 90\deg , while the H -plane sidelobes lie in that range both with and without choke slots. Return loss throughout the operating band is -25 dB or below. Critical dimensions are provided, together with useful guidelines for designing similar antennas.

Spillover and diffraction sidelobe contamination in a double-shielded experiment for mapping Galactic synchrotron emission

We have analyzed observations from a radioas- tronomical experiment to survey the sky at decimetric wavelengths along with feed pattern measurements in or- der to account for the level of ground contamination enter- ing the sidelobes. A major asset of the experiment is the use of a wire mesh fence around the rim-halo shielded an- tenna with the purpose of levelling out and reducing this source of stray radiation for zenith-centered 1-rpm circu- lar scans. We investigate the shielding performance of the experiment by means of a geometric diraction model in order to predict the level of the spillover and diraction sidelobes in the direction of the ground. Using 408 MHz and 1465 MHz feed measurements, the model shows how a weakly-diracting and unshielded antenna conguration becomes strongly-diracting and double-shielded as far- eld diraction eects give way to near-eld ones. Due to the asymmetric response of the feeds, the orientation of their radiation elds with respect to the secondary must be known a priori before comparing model predictions with observational data. By adjusting the attenuation coe- cient of the wire mesh the model is able to reproduce the amount of dierential ground pick-up observed during test measurements at 1465 MHz.

A Liquid-Helium-Cooled Absolute Reference Cold Load for Long-Wavelength Radiometric Calibration

We describe a large (78-cm) diameter liquid-helium-cooled black-body absolute reference cold load for the calibration of microwave radiometers. The load provides an absolute calibration near the liquid helium (LHe) boiling point, accurate to better than 30 mK for wavelengths from 2.5 to 25 cm (12-1.2 GHz). The emission (from non-LHe temperature parts of the cold load) and reflection are small and well determined. Total corrections to the LHe boiling point temperature are {le} 50 mK over the operating range. This cold load has been used at several wavelengths at the South Pole and at the White Mountain Research Station. In operation, the average LHe loss rate was {le} 4.4 l/hr. Design considerations, radiometric and thermal performance and operational aspects are discussed. A comparison with other LHe-cooled reference loads including the predecessor of this cold load is given.