The redshifted 21 cm line of neutral hydrogen is a promising probe of the epoch of reionization (EoR). However, its detection requires a thorough understanding and control of the systematic errors. We study two systematic biases observed in the Low-Frequency Array-EoR residual data after calibration and subtraction of bright discrete foreground sources. The first effect is a suppression in the diffuse foregrounds, which could potentially mean a suppression of the 21 cm signal. The second effect is an excess of noise beyond the thermal noise. The excess noise shows fluctuations on small frequency scales, and hence it cannot be easily removed by foreground removal or avoidance methods. Our analysis suggests that sidelobes of residual sources due to the chromatic point spread function (PSF) and ionospheric scintillation cannot be the dominant causes of the excess noise. Rather, both the suppression of diffuse foregrounds and the excess noise can occur due to calibration with an incomplete sky model containing predominantly bright discrete sources. The levels of the suppression and excess noise depend on the relative flux of sources which are not included in the model with respect to the flux of modelled sources. We predict that the excess noise will reduce with more observation time in the same way as the thermal noise does. We also discuss possible solutions such as using only long baselines to calibrate the interferometric gain solutions as well as simultaneous multifrequency calibration along with their benefits and shortcomings.

We present the first limits on the Epoch of Reionization 21 cm H I 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 I 56 13 mK D2 =  2 ( ) (1-σ) excess variance and a best 2-σ upper limit of 21 79.6 mK D2 < 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.