The brightness and spatial distributions of terrestrial radio sources

A. R. Offringa 1 A. G. de Bruyn 2 S. Zaroubi 2 L. V. E. Koopmans 2 S. J. Wijnholds 3 F. B. Abdalla 4 B. Ciardi 5 W. N. Brouw 3 I. T. Iliev 6 G. J. A. Harker 7 G. Mellema 8 G. Bernardi 9 P. Zarka 10 A. Ghosh 2 A. Alexov 11 A. Asgekar 3 J. Anderson 12 I. M. Avruch 13 R. Beck 12 M. R. Bell 5 M. J. Bentum 3 P. Best 14 L. Bîrzan 15 F. Breitling 16 J. Broderick 17 M. Brüggen 18 H. R. Butcher 3 F. de Gasperin 18 E. de Geus 3 M. de Vos 3 S. Duscha 3 J. Eislöffel 19 W. Frieswijk 3 R. A. Fallows 3 C. Ferrari 20 M. A. Garrett 15 Jean-Mathias Grießmeier 21 T. E. Hassall 17 A. Horneffer 12 M. Iacobelli 15 E. Juette 22 J. P. Mckean 3 A. Karastergiou 23 W. Klijn 3 V. I. Kondratiev 3 M. Kuniyoshi 12 G. Kuper 3 J. van Leeuwen 24 M. Loose 3 P. Maat 3 G. Macario 20 G. Mann 16 H. Meulman 3 M. J. Norden 3 E. Orru 3 H. Paas 25 M. Pandey-Pommier 26 R. Pizzo 3 A. G. Polatidis 3 D. Rafferty 15 W. Reich 12 R. van Nieuwpoort 3 H. Röttgering 15 A. M. M. Scaife 17 J. Sluman 3 O. Smirnov 27 C. Sobey 12 Michel Tagger 21 Y. Tang 3 C. Tasse 10 S. ter Veen 28 C. Toribio 3 R. Vermeulen 3 C. Vocks 16 R. J. van Weeren 9 M. W. Wise 29 O. Wucknitz 30
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 Reionisation (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 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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A. R. Offringa, A. G. de Bruyn, S. Zaroubi, L. V. E. Koopmans, S. J. Wijnholds, et al.. The brightness and spatial distributions of terrestrial radio sources. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2013, 435, pp.584-596. ⟨10.1093/mnras/stt1337 ⟩. ⟨insu-01291191⟩

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