# Upper limits on the 21-cm Epoch of Reionization power spectrum from one night with LOFAR

## TL;DR

This paper reports the first upper limits on the 21-cm power spectrum during the Epoch of Reionization using LOFAR, achieving a 2-sigma upper limit of (79.6 mK)^2 at redshifts 9.6-10.6, and discusses residual noise sources.

## Contribution

First to set observational upper limits on the EoR 21-cm power spectrum with LOFAR, improving understanding of early universe signals.

## Key findings

- Detected a non-zero excess variance of (56 ± 13 mK)^2.
- Established a 2-sigma upper limit of (79.6 mK)^2 on the power spectrum.
- Identified residual noise and calibration effects as primary sources of excess variance.

## Abstract

We present the first limits on the Epoch of Reionization (EoR) 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\,h of data were used from observations centred on the North Celestial Pole (NCP). After subtraction of the sky model and the noise bias, we detect a non-zero $\Delta^2_{\rm I} = (56 \pm 13 {\rm mK})^2$ (1-$\sigma$) excess variance and a best 2-$\sigma$ upper limit of $\Delta^2_{\rm 21} < (79.6 {\rm mK})^2$ 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 non-linear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications.