2013/06/10 by H. K. Vedantham, L. V. E. Koopmans, A. G. de Bruyn +4 · 4 citations
Engineering · Physics and Astronomy · #Antenna (radio) #Astronomy #Astrophysics #Astrophysics and Cosmic Phenomena #Brightness #COSMIC cancer database #Galaxy #Ionosphere #LOFAR #Optics #Physics #Radio Astronomy Observations and Technology #Radio Wave Propagation Studies #Radio telescope #Redshift #Sky #Sky brightness #Telecommunications #astro-ph.CO #astro-ph.IM
paper · pdf · doi:10.1093/mnras/stt1878
16 pages, 12 figures
arxiv created 2013/06/10 · openalex publication_date 2013/11/21 · arxiv updated 2015/06/16 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
The redshifted 21 cm brightness distribution from neutral hydrogen is a promising probe into the cosmic dark ages, cosmic dawn and re-ionization. Low Frequency Array's (LOFAR) Low Band Antennas (LBA) may be used in the frequency range 45 to 85 MHz (30 > z > 16) to measure the sky-averaged redshifted 21 cm brightness temperature as a function of frequency, or equivalently, cosmic redshift. These low frequencies are affected by strong Galactic foreground emission that is observed through frequency-dependent ionospheric and antenna beam distortions which lead to chromatic mixing of spatial structure into spectral structure. Using simple models, we show that (i) the additional antenna temperature due to ionospheric refraction and absorption are at an ∼1 per cent level – two-to-three orders of magnitude higher than the expected 21 cm signal, and have an approximate ν−2 dependence, (ii) ionospheric refraction leads to a knee-like modulation on the sky spectrum at ν ≈ 4 times plasma frequency. Using more realistic simulations, we show that in the measured sky spectrum, more than 50 per cent of the 21 cm signal variance can be lost to confusion from foregrounds and chromatic effects. To mitigate this confusion, we recommend modelling of chromatic effects using additional priors and interferometric visibilities rather than subtracting them as generic functions of frequency as previously proposed.