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 lightning flashes used in this work were mapped using data from the LO-FAR (LOw Frequency ARray) radio telescope. Due to its effective lightning protection system, LOFAR is able to continue to operate during thunderstorm activity[1]. The Dutch LOFAR stations consist of 38 (24 core + 14 remote) stations spread over 3200 km 2 in the northern Netherlands. The largest baseline between core stations is about 3 km, the largest baseline between remote stations is about 100 km. From each station we use 6 dual-polarized low band dipole antennas (LBA), sampled at 200 MHz, to observe the 30-80 MHz band. The raw time series data were saved to the transient buffer boards, which continuously buffer the last 5 s of data from a maximum of 48 dual-polarized antennas per station. The resulting relative timing accuracy is better than 1 ns. See [2] for more details on LOFAR. When a lightning flash occurs within the area enclosed by the Dutch LOFAR stations, as observed by www.lightningmaps.org, the transient buffer boards are stopped and the data is read to disk. The method we used to map each lightning flash has three major steps. In the first step we fitted plane-waves to the time of pulses received by individual LOFAR stations. Note that the LOFAR stations are less than 100 m in diameter and the lighting is many kilometers from the closest LOFAR station, so that a plane-wave approximation is very good for individual LOFAR stations. These plane-waves were used to identify non-functional antennas, and the intersection of their arrival directions gave a rough first estimate of the flash location, accurate to a few kilometers. Since each station has its own clock and cable delays, in the second step we found the clock offsets between the different LOFAR stations by simultaneously fitting the locations of multiple events and station clock offsets to the measured times of radio pulses, with a Levenberg-Marquardt minimizer. In order to achieve the highest precision, we chose to fit locations of 5 events that created pulses that were strong but not saturating, had a simple structure, and did not change shape significantly across different stations. After fitting, the root-mean-square difference between the modeled and measured arrival times of the radio pulses was around 1 ns. The resulting station clock offsets are consistent with LOFAR station clock calibrations, which are known

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...

We present the first wide area (19 deg 2 ), deep (≈ 120-150 µJy beam −1 ), high resolution (5.6× 7.4 arcsec) LOFAR High Band Antenna image of the Boötes field made at 130-169 MHz. This image is at least an order of magnitude deeper and 3 − 5 times higher in angular resolution than previously achieved for this field at low frequencies. The observations and data reduction, which includes full direction-dependent calibration, are described here. We present a radio source catalogue containing 6 276 sources detected over an area of 19 deg 2 , with a peak flux density threshold of 5σ . As the first thorough test of the facet calibration strategy, introduced by van Weeren et al., we investigate the flux and positional accuracy of the catalogue. We present differential source counts that reach an order of magnitude deeper in flux density than previously achieved at these low frequencies, and show flattening at 150 MHz flux densities below 10 mJy associated with the rise of the low flux density star-forming galaxies and radioquiet AGN.