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.

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LOFAR: The LOw-Frequency ARray

MILLISECOND radio pulsars, which are old (∼109yr), rapidly rotating neutron stars believed to be spun up by accretion of matter from their stellar companions, are usually found in binary systems with other degenerate stars1. Using the 305-m Arecibo radiotelescope to make precise timing measurements of pulses from the recently discovered 6.2-ms pulsar PSR1257 +12 (ref. 2), we demonstrate that, rather than being associated with a stellar object, the pulsar is orbited by two or more planet-sized bodies. The planets detected so far have masses of at least 2.8 M⊕ and 3.4 M⊕ where M⊕ is the mass of the Earth. Their respective distances from the pulsar are 0.47 AU and 0.36 AU, and they move in almost circular orbits with periods of 98.2 and 66.6 days. Observations indicate that at least one more planet may be present in this system. The detection of a planetary system around a nearby (∼500 pc), old neutron star, together with the recent report on a planetary companion to the pulsar PSR1829–10 (ref. 3) raises the tantalizing possibility that a non-negligible fraction of neutron stars observable as radio pulsars may be orbited by planet-like bodies. The utility of the clock-like pulses emitted by pulsars [see Nature 217, 709-713 (1968)] did not end with the insights they have yielded into the properties of neutron stars, and hence of high-density matter. In 1992, Wolszczan and Frail reported precise pulsar timing measurements which exhibited periodic variations. Unlike similar observations reported in the previous year (cited as ref. 3 in the paper), these variations were not an artefact, but revealed the presence of two or more planet-sized objects orbiting the neutron star — the first such objects to be detected outside the Solar System.

A planetary system around the millisecond pulsar PSR1257 + 12

Formation and evolution of binary and millisecond radio pulsars

This article reviews the current knowledge and understanding of the interstellar medium of our galaxy. The author first presents each of the three basic constituents---ordinary matter, cosmic rays, and magnetic fields---of the interstellar medium, with emphasis on their physical and chemical properties as inferred from a broad range of observations. The interaction of these interstellar constituents, both with each other and with stars, is then discussed in the framework of the general galactic ecosystem.

The interstellar environment of our galaxy

Laser-cooling of atoms and atom-trapping are finding increasing application in many areas of science1 One important use of laser-cooled atoms is in atom interferometers2 In these devices, an atom is placed into a superposition of two or more spatially separated atomic states; these states are each described by a quantum-mechanical phase term, which will interfere with one another if they are brought back together at a later time Atom interferometers have been shown to be very precise inertial sensors for acceleration3,4, rotation5 and for the measurement of the fine structure constant6 Here we use an atom interferometer based on a fountain of laser-cooled atoms to measure g, the acceleration of gravity Through detailed investigation and elimination of systematic effects that may affect the accuracy ofthe measurement, we achieve an absolute uncertainty of Δg/g ≈ 3 × 10−9, representing a million-fold increase in absoluteaccuracy compared with previous atom-interferometer experiments7 We also compare our measurement with the value of g obtained at the same laboratory site using a Michelson interferometer gravimeter (a modern equivalent of Galileo's ‘leaning tower’ experiment in Pisa) We show that the macroscopic glass object used in this instrument falls with the same acceleration, to within 7 parts in 109, as a quantum-mechanical caesium atom

Measurement of gravitational acceleration by dropping atoms

This paper presents an empirical analysis of the geometry of the conal emission region for a total population of some 150 pulsars for which an adequate body of observations now exists. It continues the analysis begun in a previous paper that explored the geometry of the core emission region and provided a means of estimating the angle α between the rotation and magnetic axes of pulsars with core components. The various pulsars are thus divided into groups according to their morphological classification, and those species that have core components are treated first. Special consideration is given to the five-component (M) class and to those other, entirely conal species which are closely related to it, the conal triple (cT) and the conal quadruple (cQ)

Toward an Empirical Theory of Pulsar Emission. VI. The Geometry of the Conal Emission Region

The core-component widths of pulsars with interpulses are studied in an effort to define the geometric properties of the core emission region. The results are then applied to a large population of core-single, triple, and five-component pulsars which all have core components. Core-component widths are intimately related to the polar-cap geometry at the stellar surface. A simple mathematical expression, established through the study of two-pole interpulsars, indicates that the core-component widths depend only upon the pulsar period and alpha, the angle between the rotation and magnetic axes of the star. The relationship can then be used to estimate alpha in any pulsar with a core component. Values of alpha are estimated for about 110 core single (St), triple (T), and five-component (M) pulsars. These results have important implications for the nature of the core radiation process. Core emission appears to come essentially from the stellar surface, filling the entire polar-cap 'gap' region where particle acceleration is thought to take place. 70 refs.

Toward an empirical theory of pulsar emission. IV. Geometry of the core emission region

The ‘drifting’ sub-pulses exhibited by some radio pulsars have fascinated both observers and theorists for 30 years, and have been widely regarded as one of the most critical and potentially insightful aspects of their emission. Moreover, Ruderman & Sutherland, in their classic model, suggested that such regular modulation was produced by a system of sub-beams, rotating around the magnetic axis under the action of E×B drift. Such ‘drift’ sequences have thus been thoroughly studied in a number of pulsars, but it has proven difficult to verify the rotating sub-beam hypothesis, and thus to establish an illuminating connection between the phenomenon and the actual physics of the emission.



Here, we report on detailed studies of pulsar B0943+10, whose nearly coherent sequences of ‘drifting’ sub-pulses have permitted us to identify their origin as a system of sub-beams that appear to circulate around the magnetic axis of the star. We introduce several new techniques of analysis, and we find that both the primary and secondary features in the fluctuation spectra of the star are aliases of their actual values. We have also developed a method of tracing the underlying pattern responsible for the observed sequences, using a ‘cartographic’ transform and its inverse, permitting us to study the characteristics of the polar emission ‘map’ and to confirm that such a ‘map’ in turn represents the observed sequence. We apply these techniques to the study of three different Arecibo observations: a 1992 430-MHz sequence which includes a transition from the highly organized ‘B’ profile mode of the star to its disorganized ‘Q’ mode; a 1972 430-MHz ‘B’-mode sequence; and a 1990 111-MHz ‘B’-mode sequence.



The ‘B’-mode sequences are consistent in revealing that the emission pattern consists of 20 sub-beams, which rotate around the magnetic axis in about 37 periods or 41 s. Even in the ‘Q’-mode sequence, we find evidence of a compatible circulation time. The similarity of the sub-beam patterns at different radio frequencies strongly suggests that the radiation is produced within a set of columns, which extend from close to the stellar surface up through the emission region and reflect some manner of ‘seeding’ phenomenon at their base. The sub-beam emission is then tied neither to the stellar surface nor to the field. While the origin of the ‘memory’ responsible for the stability of the pattern over several circulation times is unknown, the hollow conical form of the average pattern is almost certainly the origin of the conal beam forms observed in most pulsars.

The topology and polarization of sub‐beams associated with the ‘drifting’ sub‐pulse emission of pulsar B0943+10 – I. Analysis of Arecibo 430‐ and 111‐MHz observations

The beautiful sequences of drifting subpulses observed in some radio pulsars have been regarded as among the most salient and potentially instructive characteristics of their emission, not least because they have appeared to represent a system of subbeams in motion within the emission zone of the star. Numerous studies of these drift sequences have been published, and a model of their generation and motion was articulated long ago by Ruderman & Sutherland; but thus far, efforts have failed to establish an illuminating connection between the drift phenomenon and the actual sites of radio emission. Through a detailed analysis of a nearly coherent sequence of drifting pulses from the pulsar B0943+10, we have in fact identified a system of subbeams circulating around the magnetic axis of the star. A mapping technique, involving a cartographic transform and its inverse, permits us to study the character of the polar cap emission map and then to confirm that it, in turn, represents the observed pulse sequence. On this basis, we have been able to trace the physical origin of the drifting subpulse emission to a stably rotating and remarkably organized configuration of emission columns, in turn traceable possibly to the magnetic polar cap gap region envisioned by some theories.

http://iopscience.iop.org/article/10.1086/307862/pdf

Pulsar magnetospheric emission mapping: images and implications of polar-cap weather

The ``drifting'' subpulses exhibited by some radio pulsars have fascinated both observers and theorists for 30 years, and have been widely regarded as one of the most critical and potentially insightful aspects of their emission. Here, we report on detailed studies of pulsar B0943+10, whose nearly coherent sequences of ``drifting'' subpulses have permitted us to identify their origin as a system of subbeams that appear to circulate around the star's magnetic axis. We introduce several new techniques of analysis, and we find that both the primary and secondary features in the star's fluctuation spectra are aliases of their actual values. We have also developed a method of tracing the underlying pattern responsible for the observed sequences, using a ``cartographic'' transform and its inverse, permitting us to study the characteristics of the polar-cap emission ``map'' and to confirm that such a ``map'' in turn represents the observed sequence. We apply these techniques to the study of three different Arecibo observations. The ``B''-mode sequences are consistent in revealing that the emission pattern consists of 20 subbeams, which rotate around the magnetic axis in about 37 periods or 41 seconds. Even in the ``Q'' mode sequence, we find evidence of a compatible circulation time. The similarity of the subbeam patterns at different radio frequencies strongly suggests that the radiation is produced within a set of columns, which extend from close to the stellar surface up though the emission region and reflect some manner of a ``seeding''phenomenon at their base. The subbeam emission is then tied neither to the stellar surface nor to the field.

http://export.arxiv.org/pdf/astro-ph/0010048

Topology and Polarisation of Subbeams Associated With Pulsar 0943+10's ``Drifting''-Subpulse Emission: I. Analysis of Arecibo 430- and 111-MHz Observations

Joanna M. Rankin

Papers

Toward an Empirical Theory of Pulsar Emission. VI. The Geometry of the Conal Emission Region

Toward an empirical theory of pulsar emission. IV. Geometry of the core emission region

The topology and polarization of sub‐beams associated with the ‘drifting’ sub‐pulse emission of pulsar B0943+10 – I. Analysis of Arecibo 430‐ and 111‐MHz observations

Pulsar magnetospheric emission mapping: images and implications of polar-cap weather

Topology and Polarisation of Subbeams Associated With Pulsar 0943+10's ``Drifting''-Subpulse Emission: I. Analysis of Arecibo 430- and 111-MHz Observations