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Apeldoorn, Netherlands

Van Leeuwen J.,Stichting ASTRON
Advances in Space Research | Year: 2011

LOFAR, the low-frequency array, is an innovative new radio telescope currently under construction in The Netherlands. With its continuous monitoring of the radio sky we expect LOFAR will detect many new transient events, including GRB afterglows and pulsating/single-burst neutron stars. We here describe all-sky surveys ranging from a time resolution of microseconds to a cadence span of years. © 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. Source


Portegies Zwart S.,Leiden University | Van Den Heuvel E.P.J.,Astronomical Institute Anton Pannekoek | Van Leeuwen J.,Stichting ASTRON | Nelemans G.,Radboud University Nijmegen
Astrophysical Journal | Year: 2011

The millisecond pulsar (MSP) J1903+0327 is accompanied by an ordinary G dwarf star in an unusually wide (P orb ≃ 95.2days) and eccentric (e ≃ 0.44) orbit. The standard model for producing MSPs fails to explain the orbital characteristics of this extraordinary binary, and alternative binary models are unable to explain the observables. We present a triple-star model for producing MSPs in relatively wide eccentric binaries with a normal (main-sequence) stellar companion. We start from a stable triple system consisting of a low-mass X-ray binary (LMXB) with an orbital period of at least 1 day, accompanied by a G dwarf in a wide and possibly eccentric orbit. Variations in the initial conditions naturally provide a satisfactory explanation for the unexplained triple component in the eclipsing soft X-ray transient 4U 2129+47 or the cataclysmic variable EC 19314-5915. The best explanation for J1903+0327, however, results from the expansion of the orbit of the LMXB, driven by the mass transfer from the evolving donor star to its neutron star companion, which causes the triple eventually to become dynamically unstable. Using numerical computations we show that, depending on the precise system configuration at the moment the triple becomes dynamically unstable, the ejection of each of the three components is possible. If the donor star of the LMXB is ejected, a system resembling J1903+0327 will result. If the neutron star is ejected, a single MSP results. This model therefore also provides a straightforward mechanism for forming a single MSP in the Galactic disk. We conclude that the Galaxy contains some 30-300 binaries with characteristics similar to J1903+0327 and about an order of magnitude fewer single MSPs produced with the proposed triple scenario. © 2011. The American Astronomical Society. All rights reserved. Source


Van Leeuwen J.,Stichting ASTRON | Stappers B.W.,University of Manchester
Astronomy and Astrophysics | Year: 2010

We investigate the number and type of pulsars that will be discovered with the low-frequency radio telescope LOFAR. We consider different search strategies for the Galaxy, for globular clusters and for other galaxies. We show that a 25-day all-sky Galactic survey can find approximately 900 new pulsars, probing the local pulsar population to a deep luminosity limit. For targets of smaller angular size such as globular clusters and galaxies many LOFAR stations can be combined coherently, to make use of the full sensitivity. Searches of nearby northern-sky globular clusters can find new low luminosity millisecond pulsars. Giant pulses from Crab-like extragalactic pulsars can be detected out to over a Mpc. © 2010 ESO. Source


Holwerda B.W.,European Space Agency | Allen R.J.,US Space Telescope Science Institute | de Blok W.J.G.,Stichting ASTRON | Bouchard A.,McGill University | And 3 more authors.
Astronomische Nachrichten | Year: 2013

Our aim is to explore the relation between gas, atomic and molecular, and dust in spiral galaxies. Gas surface densities are from atomic hydrogen and CO line emission maps. To estimate the dust content, we use the disk opacity as inferred from the number of distant galaxies identified in twelve HST/WFPC2 fields of ten nearby spiral galaxies. The observed number of distant galaxies is calibrated for source confusion and crowding with artificial galaxy counts and here we verify our results with sub-mm surface brightnesses from archival Herschel -SPIRE data. We find that the opacity of the spiral disk does not correlate well with the surface density of atomic (H I) or molecular hydrogen (H2) alone implying that dust is not only associated with the molecular clouds but also the diffuse atomic disk in these galaxies. Our result is a typical dust-to-gas ratio of 0.04, with some evidence that this ratio declines with galactocentric radius, consistent with recent Herschel results. We discuss the possible causes of this high dust-to-gas ratio; an over-estimate of the dust surface-density, an under-estimate of the molecular hydrogen density from CO maps or a combination of both. We note that while our value of the mean dust-to-gas ratio is high, it is consistent with the metallicity at the measured radii if one assumes the Pilyugin & Thuan (2005) calibration of gas metallicity. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source


Stappers B.,University of Manchester | Hessels J.,Netherlands Institute for Radio Astronomy | Hessels J.,University of Amsterdam | Alexov A.,University of Amsterdam | And 14 more authors.
AIP Conference Proceedings | Year: 2011

The LOw Frequency ARray is the first of the next generation of radio telescopes to be completed. It uses large numbers of small receptors and vast computing and data transport capabilities to achieve a high degree of sensitivity over large fields of view. It uses two different types of receptor to enable it to observe over the frequency range 10-260 MHz. Here we report on some of the capabilities of this telescope for pulsar and fast transient research. We also present some results of the commissioning work that we have been carrying out which highlight the exciting potential of this telescope. These include simultaneous imaging and pulsar observations, simultaneous observations spanning 30-8000 MHz, a large number of known pulsars detected in the high band and the detection of PSR B0809+74 down to a frequency of 16 MHz. © 2011 American Institute of Physics. Source

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