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Antenna array design, beam calibration of the CHIME to measure the late-time cosmic acceleration and mapping of the north celestial cap

University of British Columbia

Measurements indicate that at present the expansion of the universe is accelerating even though the long-distance gravitational force is attractive. To help understand this late-time cosmic acceleration, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is designed to measure the Baryon Acoustic Oscillations (BAO) scale, therefore the expansion history of the universe in the redshift range 0.8 < z < 2.5. CHIME is a transit interferometer consisting of linear antenna arrays at the focal line of each of the four north-south aligned cylindrical reflectors. As the earth rotates, CHIME maps half of the sky every day. A dual-polarized cloverleaf antenna array illuminates the full reflector and is designed to match the LNA so that its effective noise is about 25 Kelvin across both the frequency domain and the scanning angle domain for both polarizations. To separate the cosmic 21cm signal from the 10⁵ times brighter Galactic foreground, we need to calibrate the CHIME beam to 0.1%. We have developed both measurement programs and simulation programs for this goal. With limitations found on both, the crosstalk approach to combine measurements and simulations together has been successful to calibrate the CHIME beam's north-south profile to 5%. We are still working on improving this approach for higher accuracy and larger coverage. As a pathfinder to map the whole northern hemisphere for better understanding and removal of the CHIME foreground and for galactic science, the north celestial cap has been mapped from 400 MHz to 800 MHz in the declination range [75 degree, 87.25 degree] using the John A. Galt telescope and the CHIME receiver with gated noise source. The intensity maps at 408 MHz are found to be consistent with the Haslam map, except with fluctuations up to 3 kelvins. Because this fluctuation does not generally decrease with frequency while the sky signal does, the intensity map quality at the higher band is degraded. The spectrum of the north celestial cap is measured from intensity maps with fitted spectral index varying from -2.8 to -3.1. The polarization maps are not available for science yet and the telescope instrumental polarization needs to be calibrated in the future.

Text

2021-01-06

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76972

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/