A.R. Offringa et al., "The brightness and spatial distribution
of terrestrial radio sources", Monthly Notices of the Royal
Astronomical Society, V435, no. 1, pp584-596, October 2013
abstract: Faint undetected sources of radio-frequency
interference (RFI) might become visible in long radio observations
when they are consistently present over time. Thereby, they might
obstruct the detection of the weak astronomical signals of
interest. This issue is especially important for Epoch of Reionization
(EoR) projects that try to detect the faint redshifted HI signals
from the time of the earliest structures in the Universe.We explore
the RFI situation at 30-163 MHz by studying brightness histograms of
visibility data observed with Low-Frequency Array (LOFAR), similar to
radio-source-count analyses that are used in cosmology. An empirical
RFI distribution model is derived that allows the simulation of RFI in
radio observations. The brightness histograms show an RFI distribution
that follows a power-law distribution with an estimated exponent
around -1.5. With several assumptions, this can be explained with a
uniform distribution of terrestrial radio sources whose radiation
follows existing propagation models. Extrapolation of the power law
implies that the current LOFAR EoR observations should be severely RFI
limited if the strength of RFI sources remains strong after time
integration. This is in contrast with actual observations, which
almost reach the thermal noise and are thought not to be limited by
RFI. Therefore, we conclude that it is unlikely that there are
undetected RFI sources that will become visible in long
observations. Consequently, there is no indication that RFI will
prevent an EoR detection with LOFAR.
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