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-function of dwarf satellite galaxies that are observed around Local Group galaxies substantially differs from simulations 1-5 based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard 6, 7 . A massive dark satellite in an early-type lens galaxy at z = 0.222 was recently found 8 using a new method based on gravitational lensing 9, 10 , suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a 1.9 ± 0.1 × 10 8 M ⊙ dark satellite in the Einstein-ring system JVAS B1938+666 (ref. 11) at z = 0.881, where M ⊙ denotes solar mass. This satellite galaxy has a mass similar to the Sagittarius 12 galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be α = 1.1 +0.6 −0.4 , with an average mass-fraction of f = 3.3 +3.6 −1.8 %, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations 13-15 at the 95 per cent confidence level,and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.The gravitational lens system JVAS B1938+666 (ref. 11) has a bright infrared background galaxy at redshift 2.059 (ref. 16), which is gravitationally lensed into an almost complete Einstein ring of diameter ∼ 0.9 arcseconds by a massive elliptical galaxy at redshift 0. 881 (ref. 17). The bright, highly-magnified Einstein ring made this system an excellent candidate in which to to search for surface brightness anomalies caused by very low mass (dark matter) substructure in the halo around the high redshift elliptical lens galaxy. The presence of a low-mass substructure (e.g. a luminous or dark satellite galaxy; also denoted as substructure hereafter) in the lens galaxy 1

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120–168 MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00′00″ to 57°00′00″) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325 694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ∼10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S144 MHz = 71 μJy beam−1 and the point-source completeness is 90% at an integrated flux density of 0.45 mJy. The resolution of the images is 6″ and the positional accuracy is within 0.2″. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ∼20 articles that highlight the scientific potential of LoTSS.

The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168 MHz imaging survey that will eventually cover the entire northern sky. Each of the 3170 pointings will be observed for 8 h, which, at most declinations, is sufficient to produce ∼5 resolution images with a sensitivity of ∼100 µJy/beam and accomplish the main scientific aims of the survey, which are to explore the formation and evolution of massive black holes, galaxies, clusters of galaxies and large-scale structure. Owing to the compact core and long baselines of LOFAR, the images provide excellent sensitivity to both highly extended and compact emission. For legacy value, the data are archived at high spectral and time resolution to facilitate subarcsecond imaging and spectral line studies. In this paper we provide an overview of the LoTSS. We outline the survey strategy, the observational status, the current calibration techniques, a preliminary data release, and the anticipated scientific impact. The preliminary images that we have released were created using a fully automated but direction-independent calibration strategy and are significantly more sensitive than those produced by any existing large-area low-frequency survey. In excess of 44 000 sources are detected in the images that have a resolution of 25 , typical noise levels of less than 0.5 mJy/beam, and cover an area of over 350 square degrees in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45• 00 00 to 57• 00 00 ).

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.

We present the measurement of the Hubble Constant, H 0 , with three strong gravitational lens systems. We describe a blind analysis of both PG 1115+080 and HE 0435−1223 as well as an extension of our previous analysis of RXJ 1131−1231. For each lens, we combine new adaptive optics (AO) imaging from the Keck Telescope, obtained as part of the SHARP AO effort, with Hubble Space Telescope (HST ) imaging, velocity dispersion measurements, and a description of the line-of-sight mass distribution to build an accurate and precise lens mass model. This mass model is then combined with the COSMOGRAIL measured time delays in these systems to determine H 0 . We do both an AO-only and an AO+HST analysis of the systems and find that AO and HST results are consistent. After unblinding, the AO-only analysis gives H 0 = 82.8 +9.4 −8.3 km s −1 Mpc −1 for PG 1115+080, H 0 = 70.1 +5.3 −4.5 km s −1 Mpc −1 for HE 0435−1223, and H 0 = 77.0 +4.0 −4.6 km s −1 Mpc −1 for RXJ 1131−1231. The joint AOonly result for the three lenses is H 0 = 75.6 +3.2 −3.3 km s −1 Mpc −1 . The joint result of the AO+HST analysis for the three lenses is H 0 = 76.8 +2.6−2.6 km s −1 Mpc −1 . All of the above results assume a flat Λ cold dark matter cosmology with a uniform prior on Ω m in [0.05, 0.5] and H 0 in [0, 150] km s −1 Mpc −1 . This work is a collaboration of the SHARP and H0LiCOW teams, and shows that AO data can be used as the high-resolution imaging component in lens-based measurements of H 0 . The full time-delay cosmography results from a total of six strongly lensed systems are presented in a companion paper.

Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric "radio window": 10-240 MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR.

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...