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 binary neutron-star merger GW170817 was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of about 41 megaparsecs from Earth. The radio and X-ray afterglows of GW170817 exhibited delayed onset, a gradual increase in the emission with time (proportional to t) to a peak about 150 days after the merger event, followed by a relatively rapid decline. So far, various models have been proposed to explain the afterglow emission, including a choked-jet cocoon and a successful-jet cocoon (also called a structured jet). However, the observational data have remained inconclusive as to whether GW170817 launched a successful relativistic jet. Here we report radio observations using very long-baseline interferometry. We find that the compact radio source associated with GW170817 exhibits superluminal apparent motion between 75 days and 230 days after the merger event. This measurement breaks the degeneracy between the choked- and successful-jet cocoon models and indicates that, although the early-time radio emission was powered by a wide-angle outflow (a cocoon), the late-time emission was most probably dominated by an energetic and narrowly collimated jet (with an opening angle of less than five degrees) and observed from a viewing angle of about 20 degrees. The imaging of a collimated relativistic outflow emerging from GW170817 adds substantial weight to the evidence linking binary neutron-star mergers and short γ-ray bursts.

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International audienceEarth's nearest candidate supermassive black hole lies at the centre of the Milky Way. Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its environment, which is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which the black hole can be fed. The magnetization of the gas, however, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of accretion, remove angular momentum from the infalling gas, expel matter through relativistic jets and lead to synchrotron emission such as that previously observed. Here we report multi-frequency radio measurements of a newly discovered pulsar close to the Galactic Centre and show that the pulsar's unusually large Faraday rotation (the rotation of the plane of polarization of the emission in the presence of an external magnetic field) indicates that there is a dynamically important magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission-from radio to X-ray wavelengths-from the black hole

GW170817 was the first gravitational-wave detection of a binary neutron-star merger. It was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet being launched during the merger (and successfully breaking out of the surrounding material), directed away from our line of sight (off-axis). The presence of such a jet is predicted from models that posit neutron-star mergers as the drivers of short hard-γ-ray bursts. Here we report that the radio light curve of GW170817 has no direct signature of the afterglow of an off-axis jet. Although we cannot completely rule out the existence of a jet directed away from the line of sight, the observed γ-ray emission could not have originated from such a jet. Instead, the radio data require the existence of a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material that was ejected dynamically during the merger, or a cocoon of material that breaks out when a jet launched during the merger transfers its energy to the dynamical ejecta. Because the cocoon model explains the radio light curve of GW170817, as well as the γ-ray and X-ray emission (and possibly also the ultraviolet and optical emission), it is the model that is most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a hitherto unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.

Fast Radio Bursts (FRBs) are brief radio emissions from distant astronomical sources. Some are known to repeat, but most are single bursts. Non-repeating FRB observations have had insufficient positional accuracy to localize them to an individual host galaxy. We report the interferometric localization of the single pulse FRB 180924 to a position 4 kpc from the center of a luminous galaxy at redshift 0.3214. The burst has not been observed to repeat. The properties of the burst and its host are markedly different from the only other accurately localized FRB source. The integrated electron column density along the line of sight closely matches models of the intergalactic medium, indicating that some FRBs are clean probes of the baryonic component of the cosmic web.Cosmological observations have shown that baryons comprise 4% of the energy density of the Universe, of which only about 10% is in cold gas and stars (1), with the remainder residing in a diffuse plasma surrounding and in between galaxies and galaxy clusters. The location and density of this material has been challenging to characterize, and up to 50% of it remains unaccounted (2).Fast radio bursts (FRBs; ref.(3)) are bright bursts of radio waves with millisecond duration. They can potentially be used to detect, study, and map this medium, as bursts of emission are dispersed and scattered by their 1 arXiv:1906.11476v1 [astro-ph.HE] 27 Jun 2019 dual-polarization beams on the sky using digital beamforming, producing a total field-of-view of ∼ 30 deg 2 . For burst detection, the beamformers produces channelized autocorrelation spectra for both linear polarizations of all beams, with an integration time of 864 µs and channel bandwidth of 1 MHz in these observations. We used 336 channels centered at 1320 MHz. A real-time detection pipeline incoherently adds the spectra from all available antennas (24 antennas in these observations) and polarization channels, then searches (16) the result for dispersed pulses (17).Burst localization is completed with a second data product that utilizes both the amplitude and phase information of the burst radiation. The beamformers store samples of the complex electric field for all beams and both polarizations in a ring buffer of 3.1 s duration, with the oldest data being continuously overwritten by new data. The data are saved for offline interferometric analysis only when the pipeline identifies a candidate. For the searches reported here the triggering required pulses with widths less than 9 ms and S/N > 10.Previous searches with ASKAP used antennas pointed in different directions to maximize sky coverage (10,16). In contrast, our observations used antennas all pointed in the same direction, enabling the array to act as an interferometer capable of sub-arcsecond localization with a 30 deg 2 field of view. We targeted high Galactic latitude fields (Galactic latitude |b| ∼ 50 • ), that had been observed previously (10, 16), and Southern circumpolar fields. The high-latitude fields were observed regularly through 2017 and earl...

Software correlation, where a correlation algorithm written in a high-level language such as C++ is run on commodity computer hardware, has become increasingly attractive for small to medium sized and/or bandwidth constrained radio interferometers. In particular, many long baseline arrays (which typically have fewer than 20 elements and are restricted in observing bandwidth by costly recording hardware and media) have utilized software correlators for rapid, costeffective correlator upgrades to allow compatibility with new, wider bandwidth recording systems and improve correlator flexibility. The DiFX correlator, made publicly available in 2007, has been a popular choice in such upgrades and is now used for production correlation by a number of observatories and research groups worldwide. Here we describe the evolution in the capabilities of the DiFX correlator over the past three years, including a number of new capabilities, substantial performance improvements, and a large amount of supporting infrastructure to ease use of the code. New capabilities include the ability to correlate a large number of phase centers in a single correlation pass, the extraction of phase calibration tones, correlation of disparate but overlapping sub-bands, the production of rapidly sampled filterbank and kurtosis data at minimal cost, and many more.The latest version of the code is at least 15% faster than the original, and in certain situations many times this value. Finally, we also present detailed test results validating the correctness of the new code.

The Hubble constant (H 0 ) measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decades to measure H 0 1-10 . Notably, Planck cosmic microwave background (CMB) and the local Cepheid-supernovae distance ladder measurements determine H 0 with a precision of ∼ 1% and ∼ 2% respectively 3, 4, 11 . A 3-σ level of discrepancy exists between the two measurements 4, 12 , for reasons that have yet to be understood. Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer a completely independent standard siren (the GW analogue of an astronomical standard candle) measurement of H 0 13-15 , as demonstrated following the discovery of the neutron star merger, GW170817 16-18 . This measurement does not assume a cosmological model and is independent of a cosmic distance ladder. The first joint analysis of the GW signal from GW170817 and its EM localization led to a measurement of H 0 = 74 +16 −8 km/s/Mpc (median and symmetric 68% credible interval) 15 . In this analysis, the degeneracy in the GW signal between the source distance and the weakly constrained viewing angle dominated the H 0 measurement uncertainty. Recently, Mooley et al. (2018) 19