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.

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.

Pulsars emit low-frequency radio waves through to high-energy gamma-rays that are generated anywhere from the surface out to the edges of the magnetosphere. Detecting correlated mode changes in the multi-wavelength emission is therefore key to understanding the physical relationship between these emission sites. Through simultaneous observations, we have detected synchronous switching in the radio and X-ray emission properties of PSR B0943+10. When the pulsar is in a sustained radio 'bright' mode, the X-rays show only an un-pulsed, non-thermal component. Conversely, when the pulsar is in a radio 'quiet' mode, the X-ray luminosity more than doubles and a 100%-pulsed thermal component is observed along with the non-thermal component. This indicates rapid, global changes to the conditions in the magnetosphere, which challenge all proposed pulsar emission theories. Main Text:Radio pulsars are powered by the energy released as the highly magnetized neutron star spins down. The radio pulses are generated in the pulsar magnetosphere, most probably close to the neutron star surface (1,2). Shortly after the discovery of pulsars, it was observed that the radio pulse behavior can discretely change on timescales as short as a rotation period. These changes in emission mode can manifest as switches between ordered and disordered states or variations in intensity and pulse shape, including the complete cessation of observable radio emission (3,4).Because the emitted radio luminosity is a negligible fraction of the available spin-down energy, usually substantially less than 10 -5 , this phenomenology was presumed to be related solely to microphysics of the radio emission mechanism itself. This perception has recently been challenged by the identification of a relationship between the spin properties of neutron stars and their radio emission modes. PSR B1931+24 was observed to cease emitting for tens of days, during which it spins down ~50% less rapidly (5). PSR J1841-0500 (6) and PSR J1832+0029 (7) exhibit similar behaviors. A number of other pulsars display smaller changes in spin-down rate, which correlate with variations in their average radio pulse shapes (8). The implication of these results is that mode changing is due to an inherent, perhaps universal pulsar process which causes a sudden change in the rate of angular momentum loss that is communicated along the open field lines of the magnetosphere. Whereas changes in spindown rate can only be detected on time-scales of a few days or longer, the recently identified link with the rapid switching observed in radio emission modes suggests a transformation of the global magnetospheric state in less than a rotation period. Despite the recent flurry of pulsar detections at high energies (9), the only causal relation between the radio pulses and emission at other wavelengths, likely emanating from different locations in the magnetosphere, has been made for optical emission and giant radio pulses from the Crab pulsar (10) PSR B0943+10 is a paragon of mode-changing pul...