The Cosmic Twilight Polarimeter (CTP) is the first ground-based proof-of-concept experiment being developed to measure the low-frequency foreground spectrum with polarimetry. The CTP consists of a pair of broadband (60-120 MHz) sleeved dipoles on a finite ground plane with the antenna pointing at the North Celestial Pole. The CTP does not measure the intrinsic linear polarization from the synchrotron emission as does traditional polarimetric measurements in radio astronomy. Instead, the experiment focuses on the projection-induced polarization arising from anisotropy in the foreground source distribution. The spatial variations in the foreground emission, contrasting to the relatively isotropic cosmological 21-cm background, induce a non-zero net composite linear polarization onto the broad antenna beam fixed on the ground.
The CTP is designed to measure this net composite polarization by sampling the Stokes parameters, which are used to parametrize different orientations of the polarization. As the foreground revolves continuously about the celestial pole, as observed by the sleeved dipoles, a unique sinusoidal signature (modulation) with angular frequency consistent to twice the sky’s revolution rate is imprinted on the polarization measurement as a function of time.
This dynamical signature in the Stokes measurement results in a second-order harmonic when the Stokes parameters are Fourier decomposed from the time space to frequency space. The magnitude at the second-order harmonic represents intensity of the measured projection-induced foreground polarization. By compiling the second harmonics, channel by channel across the frequency band, a duplicate of the foreground spectrum is constructed, which is free from the background 21-cm signal. Meanwhile, the total-power, consisting of the foreground and background signals, is also measured through Stokes I. Thus, two separate spectra are measured simultaneously yet distinctly from the same set of Stokes parameters. One has both the background and foreground signal, whereas the second-harmonic spectrum only captures the foreground information. In principle, the isolated foreground spectrum can then be used to subtract out the foreground portion in the total-power spectrum and reveal the embedded 21-cm spectrum.
This non-parametric strategy for estimating the contribution from unpolarized diffuse foregrounds to the cosmological 21-cm measurement is described in more detail in Nhan, Bradley, & Burns (2017).
The CTP is planned to be deployed in the Green Bank Observatory located in the National Radio Quiet Zone for preliminary testing and early observation. Careful analysis and thorough evaluation of the CTP is being carried out. Eventually, the CTP will be relocated to a more isolated site, such as the Karoo Region in South Africa or the Murchison area in Western Australia for better RFI prevention. Once in the southern hemisphere, the CTP will point toward the South Celestial Pole instead, to configure the observation.