Using its first station of distributed radio antennas, the Low Frequency Array (LOFAR) radio telescope has successfully detected the pulsar PSR B0329+54. The measurement took 15 minutes on 14 June and used only six of the prototype high-band antennas recently installed in the eastern part of the Netherlands. The results demonstrate the technical performance of the antennas.
LOFAR will be the largest radio telescope ever built, using a new concept based on a vast array of simple omni-directional antennas. The idea is to digitize the signals before sending them to a central digital processor where software will combine them to create the effect of a large conventional antenna. When finished, it will consist of 15,000 small antennas, distributed to more than 77 stations in the north east of the Netherlands and nearby parts of Germany. The array will operate at the lowest frequencies that can be observed from Earth, at 10–240 MHz. Plans exist for the extension of the array beyond its initial 100 km scale, by building stations further into Germany and also in the UK, France, Sweden, Poland and Italy.
One important area of research, in addition to more conventional astronomy, will be the detection of extensive air showers originating from high-energy cosmic rays, and perhaps even neutrinos. Researchers have known since the 1960s that these showers produce radio signals that are detectable for cosmic-ray energies above 1017 eV. The radio emission comes from charged particles in the shower, mainly electrons and positrons, which are deflected in the Earth’s magnetic field and produce coherent synchrotron radiation. Electronic devices in the 1960s were not sensitive enough for reliable measurements of the radio emission. However, researchers have now developed new observational techniques and radio receiver systems – such as those that LOFAR employs. Through its observations, LOFAR should be able to study the longitudinal development of air showers and reconstruct the original directions of the incident cosmic rays.
Two other European experiments – CODALEMA in France, and LOPES in Germany – have already confirmed that radio detection techniques can be used to observe extensive air showers induced by cosmic rays. In addition, the Auger collaboration in Argentina is testing the same technique, with plans to implement a large array of antennas in conjunction with the existing air-Cherenkov detectors.
