Research Outputs
CLASS angular power spectra and map-component analysis for 40 GHz observations through 2022
Measurement of the largest angular scale (â„“ < 30) features of the cosmic microwave background (CMB) polarization is a powerful way to constrain the optical depth to reionization and search for the signature of inflation through the detection of primordial B-modes. We present an analysis of maps covering 73.6% of the sky made from the 40 GHz channel of the Cosmology Large Angular Scale Surveyor (CLASS) from 2016 August to 2022 May. Taking advantage of the measurement stability enabled by front-end polarization modulation and excellent conditions from the Atacama Desert, we show this channel achieves higher sensitivity than the analogous frequencies from satellite measurements in the range 10 < â„“ < 100. Simulations show the CLASS linear (circular) polarization maps have a white noise level of 125(130) uK arcmin. We measure the Galaxy-masked EE and BB spectra of diffuse synchrotron radiation and compare to space-based measurements at similar frequencies. In combination with external data, we expand measurements of the spatial variations of the synchrotron spectral energy density (SED) to include new sky regions and measure the diffuse SED in the harmonic domain. We place a new upper limit on a background of circular polarization in the range 5 < â„“ < 125 with the first bin showing Dâ„“ < 0.023 uK2CMB at 95% confidence. These results establish a new standard for recovery of the largest-scale CMB polarization from the ground and signal exciting possibilities when the higher sensitivity and higher-frequency CLASS channels are included in the analysis.
Venus observations at 40 and 90 GHz with CLASS
Using the Cosmology Large Angular Scale Surveyor, we measure the disk-averaged absolute Venus brightness temperature to be 432.3 ± 2.8 K and 355.6 ± 1.3 K in the Q and W frequency bands centered at 38.8 and 93.7 GHz, respectively. At both frequency bands, these are the most precise measurements to date. Furthermore, we observe no phase dependence of the measured temperature in either band. Our measurements are consistent with a CO2-dominant atmospheric model that includes trace amounts of additional absorbers like SO2 and H2SO4.
Two Year Cosmology Large Angular Scale Surveyor (CLASS) Observations: Long timescale stability achieved with a front-end variable-delay polarization modulator at 40 GHz
The Cosmology Large Angular Scale Surveyor (CLASS) is a four-telescope array observing the largest angular scales (2 < â„“ < 200) of the cosmic microwave background (CMB) polarization. These scales encode information about reionization and inflation during the early universe. The instrument stability necessary to observe these angular scales from the ground is achieved through the use of a variable-delay polarization modulator as the first optical element in each of the CLASS telescopes. Here, we develop a demodulation scheme used to extract the polarization timestreams from the CLASS data and apply this method to selected data from the first 2 yr of observations by the 40 GHz CLASS telescope. These timestreams are used to measure the 1/f noise and temperature-to-polarization (T → P) leakage present in the CLASS data. We find a median knee frequency for the pair-differenced demodulated linear polarization of 15.12 mHz and a T → P leakage of <3.8 Ă— 10−4 (95% confidence) across the focal plane. We examine the sources of 1/f noise present in the data and find the component of 1/f due to atmospheric precipitable water vapor (PWV) has an amplitude of 203 12 K s  m RJ for 1 mm of PWV when evaluated at 10 mHz; accounting for ∼17% of the 1/f noise in the central pixels of the focal plane. The low levels of T → P leakage and 1/f noise achieved through the use of a front-end polarization modulator are requirements for observing of the largest angular scales of the CMB polarization by the CLASS telescopes.
On-sky performance of the CLASS Q-band telescope
The Cosmology Large Angular Scale Surveyor (CLASS) is mapping the polarization of the cosmic microwave background (CMB) at large angular scales (2 < ℓ lesssim 200) in search of a primordial gravitational wave B-mode signal down to a tensor-to-scalar ratio of r ≈ 0.01. The same data set will provide a near sample-variance-limited measurement of the optical depth to reionization. Between 2016 June and 2018 March, CLASS completed the largest ground-based Q-band CMB survey to date, covering over 31,000 square-degrees (75% of the sky), with an instantaneous array noise-equivalent temperature sensitivity of $32\,\mu {{\rm{K}}}_{\mathrm{cmb}}\sqrt{{\rm{s}}}$. We demonstrate that the detector optical loading (1.6 pW) and noise-equivalent power (19 $\mathrm{aW}\sqrt{{\rm{s}}}$) match the expected noise model dominated by photon bunching noise. We derive a 13.1 ± 0.3 K pW−1 calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of 0.48 ± 0.02 and a 27 K system noise temperature. Finally, we report a Tau A flux density of 308 ± 11 Jy at 38.4 ± 0.2 GHz, consistent with the Wilkinson Microwave Anisotropy Probe Tau A time-dependent spectral flux density model.
Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz telescope pointing, beam profile, window function, and polarization performance
The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over 75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale (1°  θ  90°) CMB polarization to constrain the tensor-to-scalar ratio at the r ∼ 0.01 level and the optical depth to last scattering to the sample variance limit. This paper presents the optical characterization of the 40 GHz telescope during its first observation era, from 2016 September to 2018 February. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is <0°. 023 (<1.6% of the beam’s full width at half maximum (FWHM)). We estimate beam parameters per detector and in aggregate, as in the CMB survey maps. The aggregate beam has an FWHM of 1°. 579 ± 0°.001 and a solid angle of 838 ± 6 μsr, consistent with physical optics simulations. The corresponding beam window function has a sub-percent error per multipole at ℓ < 200. An extended 90° beam map reveals no significant far sidelobes. The observed Moon polarization shows that the instrument polarization angles are consistent with the optical model and that the temperature-to-polarization leakage fraction is <10−4 (95% C.L.). We find that the Moon-based results are consistent with measurements of M42, RCW 38, and Tau A from CLASS’s CMB survey data. In particular, Tau A measurements establish degree level precision for instrument polarization angles.
Cosmology Large Angular Scale Surveyor (CLASS): 90 GHz telescope pointing, beam profile, window function, and polarization performance
The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over ∼75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale CMB polarization to constrain the tensor-to-scalar ratio and the optical depth to last scattering. This paper presents the optical characterization of the 90 GHz telescope. Observations of the Moon establish the pointing while dedicated observations of Jupiter are used for beam calibration. The standard deviations of the pointing error in azimuth, elevation, and boresight angle are 1 3, 2 1, and 2 0, respectively, over the first 3 yr of observations. This corresponds to a pointing uncertainty ∼7% of the beam’s full width at half-maximum (FWHM). The effective azimuthally symmetrized instrument 1D beam estimated at 90 GHz has an FWHM of 0°. 620 ± 0°.003 and a solid angle of 138.7 ± 0.6(stats.) ± 1.1(sys.) μsr integrated to a radius of 4°. The corresponding beam window function drops to bâ„“ = 0.93, 0.71, 0.14 2 at â„“ = 30, 100, 300, respectively. Far-sidelobes are studied using detector-centered intensity maps of the Moon and measured to be at a level of 10−3 or below relative to the peak. The polarization angle of Tau A estimated from preliminary survey maps is 149°.6 ± 0°.2(stats.) in equatorial coordinates. The instrumental temperature-to-polarization (T → P) leakage fraction, inferred from per-detector demodulated Jupiter scan data, has a monopole component at the level of 1.7 Ă— 10−3, a dipole component with an amplitude of 4.3 Ă— 10−3, with no evidence of quadrupolar leakage.
Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A measurement of circular polarization at 40 GHz
We report measurements of circular polarization from the first two years of observation with the 40 GHz polarimeter of the Cosmology Large Angular Scale Surveyor (CLASS). CLASS is conducting a multi-frequency survey covering 75% of the sky from the Atacama Desert designed to measure the cosmic microwave background (CMB) linear E and B polarization on angular scales 1°  θ 90°, corresponding to a multipole range of 2 ℓ  200. The modulation technology enabling measurements of linear polarization at the largest angular scales from the ground, the Variable-delay Polarization Modulator, is uniquely designed to provide explicit sensitivity to circular polarization (Stokes V ). We present a first detection of circularly polarized atmospheric emission at 40 GHz that is well described by a dipole with an amplitude of 124 4 K  m when observed at an elevation of 45°, and discuss its potential impact on the recovery of linear polarization by CLASS. Filtering the atmospheric component, CLASS places a 95% confidence upper limit of 0.4 Km 2 to 13.5 Km 2 on ℓℓ C ( ) () + 1 2 ℓ p VV for 1 120  ℓ , representing an improvement by two orders of magnitude over previous CMB limits.
Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q band
The Earth’s magnetic field induces Zeeman splitting of the magnetic dipole transitions of molecular oxygen in the atmosphere, which produces polarized emission in the millimeter-wave regime. This polarized emission is primarily circularly polarized and manifests as a foreground with a dipole-shaped sky pattern for polarizationsensitive ground-based cosmic microwave background experiments, such as the Cosmology Large Angular Scale Surveyor (CLASS), which is capable of measuring large angular scale circular polarization. Using atmospheric emission theory and radiative transfer formalisms, we model the expected amplitude and spatial distribution of this signal and evaluate the model for the CLASS observing site in the Atacama Desert of northern Chile. Then, using two years of observations at 32°. 3 to 43.7 GHz from the CLASS Q-band telescope, we present a detection of this signal and compare the observed signal to that predicted by the model. We recover an angle between magnetic north and true north of −5°. 5 ± 0°. 6, which is consistent with the expectation of −5°.9 for the CLASS observing site. When comparing dipole sky patterns fit to both simulated and data-derived sky maps, the dipole directions match to within a degree, and the measured amplitudes match to within ∼20%.
Four-year Cosmology Large Angular Scale Surveyor (CLASS) observations: On-sky receiver performance at 40, 90, 150, and 220 GHz frequency bands
The Cosmology Large Angular Scale Surveyor (CLASS) observes the polarized cosmic microwave background (CMB) over the angular scales of 1° ≲ θ ≤ 90° with the aim of characterizing primordial gravitational waves and cosmic reionization. We report on the on-sky performance of the CLASS Q-band (40 GHz), W-band (90 GHz), and dichroic G-band (150/220 GHz) receivers that have been operational at the CLASS site in the Atacama desert since 2016 June, 2018 May, and 2019 September, respectively. We show that the noise-equivalent power measured by the detectors matches the expected noise model based on on-sky optical loading and lab-measured detector parameters. Using Moon, Venus, and Jupiter observations, we obtain power to antenna temperature calibrations and optical efficiencies for the telescopes. From the CMB survey data, we compute instantaneous array noise-equivalent-temperature sensitivities of 22, 19, 23, and 71 $\mu {{\rm{K}}}_{\mathrm{cmb}}\sqrt{{\rm{s}}}$ for the 40, 90, 150, and 220 GHz frequency bands, respectively. These noise temperatures refer to white noise amplitudes, which contribute to sky maps at all angular scales. Future papers will assess additional noise sources impacting larger angular scales.