LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

The mass composition of cosmic rays contains important clues about their origin. Accurate measurements are needed to resolve longstanding issues such as the transition from Galactic to extraGalactic origin and the nature of the cutoff observed at the highest energies. Composition can be studied by measuring the atmospheric depth of the shower maximum X max of air showers generated by high-energy cosmic rays hitting the Earth's atmosphere. We present a new method to reconstruct X max based on radio measurements. The radio emission mechanism of air showers is a complex process that creates an asymmetric intensity pattern on the ground. The shape of this pattern strongly depends on the longitudinal development of the shower. We reconstruct X max by fitting two-dimensional intensity profiles, simulated with CoREAS, to data from the Low Frequency Array (LOFAR) radio telescope. In the dense LOFAR core, air showers are detected by hundreds of antennas simultaneously. The simulations fit the data very well, indicating that the radiation mechanism is now well understood. The typical uncertainty on the reconstruction of X max for LOFAR showers is 17 g=cm 2 .

The low frequency array (LOFAR), is the first radio telescope designed with the capability to measure radio emission from cosmic-ray induced air showers in parallel with interferometric observations. In the first ∼2 years of observing, 405 cosmic-ray events in the energy range of 10 16 −10 18 eV have been detected in the band from 30−80 MHz. Each of these air showers is registered with up to ∼1000 independent antennas resulting in measurements of the radio emission with unprecedented detail. This article describes the dataset, as well as the analysis pipeline, and serves as a reference for future papers based on these data. All steps necessary to achieve a full reconstruction of the electric field at every antenna position are explained, including removal of radio frequency interference, correcting for the antenna response and identification of the pulsed signal.

4Cosmic rays are the highest energy particles found in nature. Measurements of the mass composition of cosmic rays between 10 17 eV and 10 18 eV are essential to understand whether this energy range is dominated by Galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal 1 comes from accelerators capable of producing cosmic rays of these energies 2 . Cosmic rays initiate cascades of secondary particles (air showers) in the atmosphere and their masses are inferred from measurements of the atmospheric depth of the shower maximum, X max 3 , or the composition of shower particles reaching the ground 4 .Current measurements 5 suffer from either low precision, and/or a low duty cycle. Radio detection of cosmic rays 6-8 is a rapidly developing technique 9 , suitable for determination of X max 10, 11 with a duty cycle of in principle nearly 100%. The radiation is generated by the separation of relativistic charged particles in the geomagnetic field and a negative charge excess in the shower front 6, 12 . Here we report radio measurements of X max with a mean precision of 16 g/cm 2 between 10 17 − 10 17.5 eV. Because of the high resolution in X max we can determine the mass spectrum and find a mixed composition, containing a light mass fraction of ∼ 80%. Unless the extragalactic component becomes significant already below 10 17.5 eV, our measurements indicate an additional Galactic component dominating at this energy range.Observations were made with the Low Frequency Array (LOFAR 13 ), a radio telescope consisting of thousands of crossed dipoles, with built-in air shower detection capability 14 . LOFAR records the radio signals from air showers continuously while running astronomical observations simultaneously. It comprises a scintillator array (LORA), that triggers the readout of buffers, stor-5 ing the full waveforms received by all antennas.We have selected air showers from the period June 2011 -January 2015 with radio pulses in at least 192 antennas. The total uptime was ∼150 days, limited by construction and commissioning of the telescope. Showers that occurred within an hour from lightning activity, or have a polarisation pattern that is indicative of influences from atmospheric electric fields are excluded from the sample 15 .Radio intensity patterns from air showers are asymmetric due to the interference between geomagnetic and charge excess radiation. They can be reproduced from first principles by summing the radio contributions of all electrons and positrons in the shower. We use the radio simulation code CoREAS 16 , a plug-in of CORSIKA 17 , which follows this approach.It has been shown that X max can be accurately reconstructed from densely sampled radio measurements 18 . We use a hybrid approach, simultaneously fitting the radio and particle data. The radio component is very sensitive to X max , while the particle component is used for the energy measurement.The fit contains four free parameters: the shower core position (x, y), and scaling factors for the partic...

Motivated by the recent high-precision measurements of cosmic rays by several new-generation experiments, we have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to about 10 18 eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of different elements, optimised to explain the observed elemental spectra below ∼ 10 14 eV and the 'knee' in the all-particle spectrum, cannot explain the observed all-particle spectrum above ∼ 2 × 10 16 eV. We discuss two approaches for a second component of Galactic cosmic rays -re-acceleration at a Galactic wind termination shock, and supernova explosions of Wolf-Rayet stars, and show that the latter scenario can explain almost all observed features in the all-particle spectrum and the composition up to ∼ 10 18 eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. In this two-component Galactic model, the knee at ∼ 3 × 10 15 eV and the 'second knee' at ∼ 10 17 eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. We also discuss several variations of the extra-galactic component, from a minimal contribution to scenarios with a significant component below the 'ankle' (at ∼ 4 × 10 18 eV), and find that extra-galactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. We also provide arguments that an extra-galactic contribution is unlikely to dominate at or below the second knee. Our main result is that the second Galactic component predicts a composition of Galactic cosmic rays at and above the second knee that largely consists of helium or a mixture of helium and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast to most common assumptions. This prediction is in agreement with new measurements from LOFAR and the Pierre Auger Observatory which indicate a strong light component and a rather low iron fraction between ∼ 10 17 and 10 18 eV.