A low-frequency radio imager for NenuFAR – NRI

Before the NRI project, NenuFAR consisted of a dense set of 96 arrays of 19 antennas (called «Mini-Arrays« ; 56 were funded and under construction), distributed in a 400 m diameter disk, and connected to a «master« receiver ensuring the digitization of signals, their splitting into spectral bands, and their summation to form sensitive beams (of ~1° aperture). To produce images, it was necessary to add a correlator to perform the products of all pairs of antenna signals. This correlator had to be fed from the «master« receiver and included in the telescope control interface. A hardware and software solution had to be developed to efficiently process the very large data stream produced by the imager. Finally, the imaging sensitivity of a 400 m diameter instrument at 10-85 MHz is severely limited by the so-called «confusion« spatial noise, linked to the limited angular resolution (~1°). It was therefore planned to add several Mini-Arrays of antennas up to 3 km distance from the core, improving the angular resolution by a factor of ~7 and thus reducing the confusion noise by a factor of ~50 (~72). These additional arrays were to be digitised locally, and their signals synchronised with the core signals.

Main results of the project :
The project was completed according to plan. After a comparative study, the new LOFAR COBALT2 correlator was selected and adapted to NenuFAR. The infrastructure of 4 remote Mini-Arrays has been prepared and 3 have been built and connected (the 4th one will be built mid-2021). The construction of the last 2 Remote Mini-Arrays requires more extensive civil works than planned, for which additional funding is being sought. The processing machines and pipelines have been put in place, allowing efficient data reduction. The «1st light« images (core only) have been completed and published. The 2nd light images, including the remote Mini-Arrays, have just been obtained (see Figure 1). The NenuFAR-Radio-Imager is fully operational. In parallel, the core of NenuFAR has grown to 80 Mini-Arrays, the imaging science programmes have started, and France has decided on its involvement in SKA.

The 4th distant Mini-Array is under construction. The realisation of the last 2 distant Mini-Arrays requires more important civil engineering works than expected, for which additional funding has been obtained (ERC Advanced Grant of P. Zarka). The 2nd light images will be published in the SKA journal «Contact«. The scientific imaging programmes are taking off.

Scientific production since the beginning of the project :
The «1st light« images have been published in the SKA-France Newsletter and on the ASTRON website (LOFAR). The 2nd light results will be published in June 2021 in the international magazine SKA-Contact #8 and on the Paris Observatory website. The article describing NenuFAR and the radio imager is in preparation. The «Cosmic Dawn« project is preparing its first publications. The NenuFAR-Radio-Imager mode has already been used for several student training courses (Master).

LOFAR (the European LOw Frequency ARray) is the first of the new-generation radiotelescopes of the 21st century, that will culminate with SKA (Square Kilometer Array) after 2020. NenuFAR is a giant extension of LOFAR, that is also a powerful standalone low-frequency (LF) radiotelescope in the range 10-85 MHz. Its construction has started in 2014 in Nançay, supported by CNRS/INSU, Observatoire de Paris, Université d’Orléans/OSUC, and the FLOW consortium. NenuFAR is a compact antenna array (400 m in diameter) connected to the LOFAR receivers and to a local digitizer and beamformer (synthesizing narrow steerable 200 kHz beams in the sky up to a total instantaneous bandwidth of 150 MHz).
We propose here to give NenuFAR the additional capability of a powerful LF imager with ~7’ resolution, by adding a realtime correlator, offline computing power, massive storage, and 6 small arrays (of 19 antennas each) within 3 km of the compact core. These extensions will considerably reduce the confusion noise limiting the imaging sensitivity of the instrument, down to about the thermal noise. The scientific topics that will greatly benefit from the use of NenuFAR in Standalone as an imager include:
(a) in multi-frequency, long integration imaging: the detection of the cosmological HI signal from the “dark ages” to the reionization era through the "cosmic dawn", the systematic and sensitive search for yet unconfirmed LF radio signatures of exoplanets and star-planet plasma interactions (SPI), and the radio afterglows of high energy collapses generating gravitational waves and Gamma Ray Bursts ;
(b) in snapshot imaging mode: the systematic sensitive search for fast radio transients in a broad field of view, including the prompt emission associated with gravitational waves, pulsar giant pulses, Fast Radio Bursts, and exoplanetary / stellar / SPI bursts ;
Detection and study of ~10^5 galactic and extragalactic sources, with their diffuse LF emission, will also be made possible.
For topics (a) & (b) the NenuFAR Radio Imager (NRI) will be in its frequency range the most powerful existing instrument before SKA (more powerful than LOFAR and the LWA), combining a high instantaneous sensitivity, very large total bandwidth, good angular resolution, and full polarization measurement capability. The proposed imaging capability, adding to its beamformer capability and its enhancement of LOFAR as a “Super Station” (LSS) will make NenuFAR a truly unique LF radio facility of the 21st century. In the SKA era, it will remain complementary to SKA in terms of spectral range and hemisphere. France has a strong expertise in development of imaging, calibration, deconvolution, and RFI mitigation algorithms for LF radioastronomy, that will be applied to and optimized for the NRI, preparing the exploitation of SKA. NenuFAR has received the official status of SKA pathfinder from the SKA office, for the experience it will bring in terms of hardware, algorithms, multimode operations (super-station / standalone beamformer / imager), and science preparation. The NRI will contribute to the organization and development of the French LF radio community – which started with the collective writing of a broad science case by 80 French and International scientists –, reinforcing its role and influence in Europe. The CNRS prospective in 2014 and the Haut Conseil TGIR in 2016 gave a high priority to the completion of NenuFAR (labelled “Infrastructure de Recherche” by the MESR), and the “Conseil Stratégique de Direction” of USN identified the NRI as the development of highest importance. The NRI project includes a Public Outreach part in the form of an Art Project integrated with the instrument and that will enhance its interest for the general public.