A. H. Patil et al., "Upper Limits on the 21 cm Epoch of Reionization Power Spectrum from One Night with LOFAR," The Astrophysical Journal, vol. 838, no. 1, March 2017.

abstract:
We present the first limits on the Epoch of Reionization 21 cm HI power spectra, in the redshift range z = 7.9–10.6, using the Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total, 13.0 hr of data were used from observations centered on the North Celestial Pole. After subtraction of the sky model and the noise bias, we detect a non-zero Delta_I² = (56 +/- 13 mK)² (1-σ) excess variance and a best 2-σ upper limit of Delta₂₁² < (79.6 mK)² at k = 0.053 h cMpc⁻¹ in the range z = 9.6–10.6. The excess variance decreases when optimizing the smoothness of the direction- and frequency-dependent gain calibration, and with increasing the completeness of the sky model. It is likely caused by (i) residual side-lobe noise on calibration baselines, (ii) leverage due to nonlinear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications.

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