Paper: F.G. Mertens, M. Mevius, L.V.E. Koopmans, A.R. Offringa, G. Mellema, S. Zaroubi, M.A. Brentjens, H. Gan, B.K. Gehlot, V. N. Pandey, A. M. Sardarabadi, H. K. Vedantham, S. Yatawatta, K. M. B. Asad, B. Ciardi, E. Chapman, S. Gazagnes, R. Ghara, A. Ghosh, S. K. Giri, I. T. Iliev, V. Jelic ́, R. Kooistra, R. Mondal, J. Schaye, and M. B. Silva. Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR. MNRAS, 493[2]:1662–1685, Apr. 2020

3- Upper limits from The LOFAR EoR KSP

Figure 3: This Figure shows the LOFAR EoR recent results. Shown are final 10 nights Stokes I spherically averaged power spectra after residual foreground removal and noise bias removal (orange). The green and blue dashed lines represent, respectively, the estimated frequency-uncorrelated noise (∆2N) and thermal noise (∆2th) power of the 10 nights dataset. The black dashed line represents the 2σ upper limit theoretically achievable if the residual of the 10 nights dataset were thermal noise dominated. From Mertens et al. 2020.

The redshifted 21 cm, especially, its EoR era, has been the target of a number of ground-based radio observato- ries, especially, radio interferometers. Unfortunately, these telescopes have not hitherto been able to measure the EoR signal power spectrum, simply because the data quality have not reached the required level of refinement in order to do so. In the last year, however, significant progress has been made and the reported upper limits have started to reach low enough values that allowed us to start excluding certain theoretical models. The best upper limits on the δTb power spectrum so far have been published in the last couple of years from three telescopes: LOFAR has reported a 2σ upper limit of (79.6 mK)2 at k ≈ 0.053 hcMpc−1; WMA has reported a 2σ upper limit of (43 mK)2 at k ≈ 0.14 hcMpc−1; and HERA, which has very recently published the best 2σ upper limits yet of (30.76 mK)2 at k ≈ 0.192 hcMpc−1. These results have been analyzed further to constraint theoretical models for LOFAR, MWA and HERA.

The most recent measurement of the 21 cm power spectrum measured at z ≈ 9.1 by the LOFAR EoR-KSP, is shown in Fig. 3 (Mertens et al. 2020).