More Like this

1. Effects of model incompleteness on the drift-scan calibration of radio telescopes

   https://doi.org/10.1093/mnras/stab2072  

   Gehlot, Bharat K ; Jacobs, Daniel C ; Bowman, Judd D ; Mahesh, Nivedita ; Murray, Steven G ; Kolopanis, Matthew ; Beardsley, Adam P ; Abdurashidova, Zara ; Aguirre, James E ; Alexander, Paul ; et al ( August 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Precision calibration poses challenges to experiments probing the redshifted 21-cm signal of neutral hydrogen from the Cosmic Dawn and Epoch of Reionization (z ∼ 30–6). In both interferometric and global signal experiments, systematic calibration is the leading source of error. Though many aspects of calibration have been studied, the overlap between the two types of instruments has received less attention. We investigate the sky based calibration of total power measurements with a HERA dish and an EDGES-style antenna to understand the role of autocorrelations in the calibration of an interferometer and the role of sky in calibrating a total power instrument. Using simulations we study various scenarios such as time variable gain, incomplete sky calibration model, and primary beam model. We find that temporal gain drifts, sky model incompleteness, and beam inaccuracies cause biases in the receiver gain amplitude and the receiver temperature estimates. In some cases, these biases mix spectral structure between beam and sky resulting in spectrally variable gain errors. Applying the calibration method to the HERA and EDGES data, we find good agreement with calibration via the more standard methods. Although instrumental gains are consistent with beam and sky errors similar in scale to those simulated, the receiver temperatures show significant deviations from expected values. While we show that it is possible to partially mitigate biases due to model inaccuracies by incorporating a time-dependent gain model in calibration, the resulting errors on calibration products are larger and more correlated. Completely addressing these biases will require more accurate sky and primary beam models. 

   more » « less
2. Improved polarization calibration of the BICEP3 CMB polarimeter at the South Pole

   https://doi.org/10.1117/12.2620212  

   Cornelison, James ; Verges, Clara ; Ade, P A. ; Ahmed, Zeeshan ; Amiri, Mandana ; Barkats, Denis ; Basu Thakur, Ritoban ; Beck, Dominic ; Bischoff, Colin A. ; Bock, James J. ; et al ( August 2022 , Proceedings of the SPIE)

   Zmuidzinas, Jonas ; Gao, Jian-Rong (Ed.)

   The BICEP3 Polarimeter is a small aperture, refracting telescope, dedicated to the observation of the Cosmic Microwave Background (CMB) at 95GHz. It is designed to target degree angular scale polarization patterns, in particular the very-much-sought-after primordial B-mode signal, which is a unique signature of cosmic inflation. The polarized signal from the sky is reconstructed by differencing co-localized, orthogonally polarized superconducting Transition Edge Sensor (TES) bolometers. In this work, we present absolute measurements of the polarization response of the detectors for more than approximately 800 functioning detector pairs of the BICEP3 experiment, out of a total of approximately 1000. We use a specifically designed Rotating Polarized Source (RPS) to measure the polarization response at multiple source and telescope boresight rotation angles, to fully map the response over 360 degrees. We present here polarization properties extracted from on-site calibration data taken in January 2022. A similar calibration campaign was performed in 2018, but we found that our constraint was dominated by systematics on the level of approximately 0.5° . After a number of improvements to the calibration set-up, we are now able to report a significantly lower level of systematic contamination. In the future, such precise measurements will be used to constrain physics beyond the standard cosmological model, namely cosmic birefringence. 

   more » « less
3. Low-frequency Radio Recombination Lines Away from the Inner Galactic Plane

   https://doi.org/10.3847/1538-3881/ad08ba  

   Vydula, Akshatha K. ; Bowman, Judd D. ; Lewis, David ; Crawford, Kelsie ; Kolopanis, Matthew ; Rogers, Alan E. E. ; Murray, Steven G. ; Mahesh, Nivedita ; Monsalve, Raul A. ; Sims, Peter ; et al ( December 2023 , The Astronomical Journal)

   Diffuse radio recombination lines (RRLs) in the Galaxy are possible foregrounds for redshifted 21 cm experiments. We use EDGES drift scans centered at −26.°7 decl. to characterize diffuse RRLs across the southern sky. We find that RRLs averaged over the large antenna beam (72° × 110°) reach minimum amplitudes of R.A. = 2–6 hr. In this region, the Cαabsorption amplitude is 33 ± 11 mK (1σ) averaged over 50–87 MHz (27 ≳z≳ 15 for the 21 cm line) and increases strongly as frequency decreases. Cβand Hαlines are consistent with no detection with amplitudes of 13 ± 14 and 12 ± 10 mK (1σ), respectively. At 108–124.5 MHz (z≈ 11) in the same region, we find no evidence for carbon or hydrogen lines at the noise level of 3.4 mK (1σ). Conservatively assuming that observed lines come broadly from the diffuse interstellar medium, as opposed to a few compact regions, these amplitudes provide upper limits on the intrinsic diffuse lines. The observations support expectations that Galactic RRLs can be neglected as significant foregrounds for a large region of sky until redshifted 21 cm experiments, particularly those targeting cosmic dawn, move beyond the detection phase. We fit models of the spectral dependence of the lines averaged over the large beam of EDGES, which may contain multiple line sources with possible line blending, and find that including degrees of freedom for expected smooth, frequency-dependent deviations from local thermodynamic equilibrium (LTE) is preferred over simple LTE assumptions for Cαand Hαlines. For Cαwe estimate departure coefficients 0.79 <b_nβ_n< 4.5 along the inner Galactic plane and 0 <b_nβ_n< 2.3 away from the inner Galactic plane.

    

   more » « less
4. A Bayesian approach to high-fidelity interferometric calibration – I. Mathematical formalism

   https://doi.org/10.1093/mnras/stac1861  

   Sims, Peter H. ; Pober, Jonathan C. ; Sievers, Jonathan L. ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   High-fidelity radio interferometric data calibration that minimizes spurious spectral structure in the calibrated data is essential in astrophysical applications, such as 21 cm cosmology, which rely on knowledge of the relative spectral smoothness of distinct astrophysical emission components to extract the signal of interest. Existing approaches to radio interferometric calibration have been shown to impart spurious spectral structure to the calibrated data if the sky model used to calibrate the data is incomplete. In this paper, we introduce BayesCal: a novel solution to the sky-model incompleteness problem in interferometric calibration, designed to enable high-fidelity data calibration. The BayesCal data model supplements the a priori known component of the forward model of the sky with a statistical model for the missing and uncertain flux contribution to the data, constrained by a prior on the power in the model. We demonstrate how the parameters of this model can be marginalized out analytically, reducing the dimensionality of the parameter space to be sampled from and allowing one to sample directly from the posterior probability distribution of the calibration parameters. Additionally, we show how physically motivated priors derived from theoretical and measurement-based constraints on the spectral smoothness of the instrumental gains can be used to constrain the calibration solutions. In a companion paper, we apply this algorithm to simulated observations with a HERA-like array and demonstrate that it enables up to four orders of magnitude suppression of power in spurious spectral fluctuations relative to standard calibration approaches.

    

   more » « less
5. JWST Constraints on the UV Luminosity Density at Cosmic Dawn: Implications for 21 cm Cosmology

   https://doi.org/10.3847/2041-8213/ad0239  

   Hassan, Sultan ; Lovell, Christopher C. ; Madau, Piero ; Huertas-Company, Marc ; Somerville, Rachel S. ; Burkhart, Blakesley ; Dixon, Keri L. ; Feldmann, Robert ; Starkenburg, Tjitske K. ; Wu, John F. ; et al ( November 2023 , The Astrophysical Journal Letters)

   An unprecedented array of new observational capabilities are starting to yield key constraints on models of the epoch of first light in the Universe. In this Letter we discuss the implications of the UV radiation background at cosmic dawn inferred by recent JWST observations for radio experiments aimed at detecting the redshifted 21 cm hyperfine transition of diffuse neutral hydrogen. Under the basic assumption that the 21 cm signal is activated by the Lyαphoton field produced by metal-poor stellar systems, we show that a detection at the low frequencies of the EDGES and SARAS3 experiments may be expected from a simple extrapolation of the declining UV luminosity density inferred atz≲ 14 from JWST early galaxy data. Accounting for an early radiation excess above the cosmic microwave background suggests a shallower or flat evolution to simultaneously reproduce low- and high-zcurrent UV luminosity density constraints, which cannot be entirely ruled out, given the large uncertainties from cosmic variance and the faint-end slope of the galaxy luminosity function at cosmic dawn. Our findings raise the intriguing possibility that a high star formation efficiency at early times may trigger the onset of intense Lyαemission at redshiftz≲ 20 and produce a cosmic 21 cm absorption signal 200 Myr after the Big Bang.

    

   more » « less