The project is composed by a technical and a scientific part.
Technical goals: with the ULU project, we want to enable the exploration of the ultra-low frequencies (<100 MHz) emitted by cosmic sources. These electromagnetic wavelengths are the longest observable from the Earth. Any attempt to explore longer wavelength needs to be done from space. Observing the lowest frequencies is very challenging, mostly because of the distorting effects of the Earth’s ionosphere, which severely blur the radio images. Techniques developed in the framework of this project enabled the systematic calibration and imaging of the ultra-low frequency radio sky. This resulted in a set of codes (or "pipeline") to be used on raw data coming from the LOFAR interferometer. LOFAR is the world’s most powerful low-frequency radio telescope.
LOFAR is a hierarchical collection of thousands of dipole antennas grouped into stations (aperture arrays capable of multi-beam forming) each about the size of a football field. As a pan-European project, LOFAR has 52 stations spread across nine countries, which provide sub-arcsec resolution (LOFAR-VLBI), a capability that at these frequencies will remain unique in the next decades. LOFAR uses two antenna types: the High Band Antenna (HBA, band: 110− 240 MHz) and the Low Band Antenna (LBA, band: 10− 90 MHz). The latter touches the bottom edge of the radio window, below which the ionosphere becomes opaque. The LOFAR LBA frequency band is not covered by any other existing or planned large scale interferometer, and therefore has the potential for unique discoveries.
By combining our pipeline with more than 2500 hrs of LOFAR LBA telescope time, we are producing a large survey in the frequency range 42-66 MHz of the northern sky above declination 24 deg. This is a massive production of data that will require a total of 15 millions CPU hours to be run through the pipeline and be ready for scientific exploitation. The survey will provide the scientific community with images 100 times the sensitivity (400 for selected deep fields) and 5 times the resolution compared to the state-of-the-art. Currently, about 1/3 of the survey has been processed. Beside the wide-area survey, the technique developed in this project are being used to attempt: 1. the first exploration of the high-resolution radio sky at 15 MHz. At the moment, a few fields were successfully imaged. 2. the first images at 1 arcsec resolution of the ultra-low frequency sky. At the moment, a few bright sources were successfully imaged with this technique.
Scientific goals: The main scientific goal of the project is to understand the cosmic ray life cycle in the large scale structure of the Universe. This requires to tackle the objective from several sides. Here is a list of ongoing works:
Local universe:
1. Detailed study of a nearby galaxy cluster to study the acceleration mechanisms in shocked and turbulent environments. For this project, we are using low-frequency data from the MeerKAT radio interferometer. A complete data reduction strategy (including polarimetry) has been developed, and the result shows unexpected features that will be analysed in the coming months.
2. First comprehensive study of the closest massive galaxy clusters to our galaxy: the Virgo cluster. We acquired and reduced low-frequency data, and we are producing now a mid-frequency large survey (more than 300 pointings) using MeerKAT data.
Statistical analysis:
1. Discovery and classification of the elusive "Megahalos", faint radio sources covering the whole galaxy cluster, whose origin is still unknown. At the moment, 4 new candidates are being analysed.
2. Analysis of a large sample of galaxy clusters, selecting them at high redshift. A large reprocessing of dozens of LOFAR datasets are being carried out.
3. Analysis of radio groups (less massive siblings of galaxy clusters). In this case, we identified a good selection of candidates and started reducing existing data or acquiring the required telescope time.
