The Primordial Inflation Explorer (PIXIE) is an Explorer-class mission concept to measure the spectral energy distribution and linear polarization of the cosmic microwave background (CMB) and astrophysical foregrounds. A single cryogenic Fourier transform spectrometer compares the sky to an external blackbody calibration target, measuring the Stokes I, Q, U parameters to levels ~200 Jy/sr in each 2.65 degree diameter beam over the full sky, in each of 300 frequency channels from 28 GHz to 6 THz.

The PIXIE instrument (Figure 1) is a polarizing Fourier transform spectrometer (FTS) with two input ports and two output ports. The input ports are fed by two off-axis telescopes that produce twin beams co-pointed along the spacecraft spin axis. PIXIE carries a full-aperture blackbody calibrator to provide an absolute reference source with sub-percent accuracy. The calibrator can be deployed to fully cover either beam, or be stowed so both beams view the sky. When both beams view the sky, the instrument nulls unpolarized emission, so the fringe pattern encodes only the frequency spectrum of polarized emission. When the calibrator covers either beam, the fringe pattern encodes information for both polarization and the absolute intensity of sky emission.

PIXIE observes from a thermally-stable Sun-Earth Lagrange Point 2 (L2) halo orbit (Figure 2). The spacecraft spins about the instrument optical boresight at 1.25 RPM while simultaneously scanning the boresight through a 5.12 hour period great circle perpendicular to the Earth-Sun line. The annual orbital motion precesses the great-circle orientation to achieve full-sky coverage every six months. The 2-year baseline mission produces 4 redundant full-sky maps for jackknife comparison.

The baseline mission assumes 2 years of observations with 45% of the time spent in spectral distortion mode (calibrator deployed to cover one beam) and 45% of the time spent in polarization mode (calibrator stowed so both beams view the sky). Figure 3 compares selected cosmological signals to the baseline mission sensitivity integrated over the cleanest 70% of the sky.

Figure 1. PIXIE instrument and observatory. Twin telescopes with 55 cm diameter primary mirrors feed a polarizing Fourier Transform Spectrometer. The 2.65 deg diameter tophat beams are oriented along the spacecraft spin axis.

Figure 2. Schematic view of PIXIE operations in its L2 halo orbit. The spacecraft spins about the beam boresight with a 48-second period while simultaneously scanning the beams in a great circle perpendicular to the sun line with a 5.12 hour period. The annual motion of the sun line from the L2 orbit precesses the direction of the great-circle scan to provide full-sky coverage every six months.

Figure 3. Sensitivity and spectral coverage for the PIXIE baseline 2-year baseline mission in spectral distortions (left) and polarization (right). Predicted cosmological signals are shown for the electron pressure (y) and temperature (kT_e) from structure formation, the dissipation of primordial density perturbations (mu), E-mode polarization from reionization, and B-mode polarization from inflation with tensor-to-scalar ratio r=0.01 and r=0.001. Dashed lines indicate signals with negative amplitude.

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Goddard Space Flight Center, National Aeronautics and Space Administration
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