Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR

Abstract

A new upper limit on the 21-cm signal power spectrum at a redshift of z ≈ 9.1 is presented, based on 141 hours of data obtained with the Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally-smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally-weighted power spectrum inference. Previously seen 'excess power' due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25 − 0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-σ upper limit of ∆ 2 21 < (73) 2 mK 2 at k = 0.075 h cMpc −1 is found, an improvement by a factor ≈ 8 in power compared to the previously reported upper limit. The remaining excess power could be due to residual foreground emission from sources or diffuse emission far away from the phase centre, polarization leakage, chromatic calibration errors, ionosphere, or low-level radio-frequency interference. We discuss future improvements to the signal processing chain that can further reduce or even eliminate these causes of excess power.

Constraining the intergalactic medium at z ≈ 9.1 using LOFAR Epoch of Reionization observations

“…The smallest k-scale which can be probed with this resolution is ≈ 1.9 h Mpc −1 (corresponding to scale ≈ 3.3 h −1 Mpc). The smallest scale probed in Mertens et al (2020), which is ≈ 0.4 h Mpc −1 (corresponding to scale ≈ 15 h −1 Mpc), remains within the Nyquist limit of our simulation and free from the aliasing effect (Mao et al 2012).…”

“…Gehlot et al (2019) placed upper limits on the power spectrum in the redshift range z = 19.8 − 25.2 using observations with the LOFAR-Low Band Antenna array and Eastwood et al (2019) placed upper limits at z ≈ 18.4 using observations with the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) 6 . Mertens et al (2020) have provided the second LOFAR upper limit on the 21-cm power spectrum at redshift ≈ 9.1 using 10 nights of observations. At k = 0.1 h Mpc −1 the 2 − σ upper limit is (106.65) 2 mK 2 , a factor of ≈ 8 improvement at the same k− scale over the value obtained from 1 night of observations (Patil et al 2017) and the best upper limit so far on the large-scale power spectrum at redshift 9.…”

“…Right panel: the curves show the power spectra of the 21-cm brightness temperature as a function of scale for 8556 different combinations of ζ and M min . We assume 1 − T γ /T S = −12, which corresponds to a uniform T S = 2.1 K. The red points with error-bars (2-sigma) show the upper limits from the 10-night observations with LOFAR(Mertens et al 2020). The dashed blue curve refers to the spherically average power spectrum of the brightness temperature cube from which the slice in the left panel has been extracted.…”

Redundant-baseline calibration of the hydrogen epoch of reionization array

“…At lower redshifts, 21 cm tomography can enable intensity mapping of self-shielded gas within galaxies, allowing for precise measurements of baryon acoustic oscillations across cosmic time (Chang et al 2008;Loeb & Wyithe 2008;Cosmic Visions 21 cm Collaboration et al 2018;Kovetz et al 2019;Slosar et al 2019). At higher redshifts, 21 cm cosmology promises a new window on the "Cosmic Dawn", spanning from the first stars through to the Epoch of Reionization (EoR) by probing the intergalactic medium's (IGM) temperature, density, and ionization state (Furlanetto et al 2019b;Mirocha et al 2019;Chang et al 2019;Furlanetto et al 2019a;Alvarez et al 2019). Both the sky-averaged 21 cm brightness temperature and its fluctuations encode key information about these processes.…”

“…This effect can be mitigated by calibrating with only the shortest, least spectrallycomplex baselines (Ewall-Wice et al 2017), though this may make the calibration less accurate as it relies more heavily on the poorlymodeled diffuse Galactic emission. Alternatively, one can impose a priori constraints on calibration solutions, either by the consensus optimization technique (Yatawatta 2015;Yatawatta 2016) used in Patil et al (2017) and Mertens et al (2020), via low-order polynomials (Barry et al 2019a; Barry et al 2019b), or by directly filtering gains (Kern et al 2019b). This general approach relies on the inherent spectral smoothness smoothness of the instrument response, including its complex, per-antenna gains.…”