ORCID Profile
0000-0002-1429-9010
Current Organisation
Universiteit Utrecht
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Publisher: Oxford University Press (OUP)
Date: 19-02-2016
DOI: 10.1093/MNRAS/STW395
Publisher: American Astronomical Society
Date: 18-09-2019
Abstract: The Low-Frequency Array radio telescope discovered the 707 Hz binary millisecond pulsar (MSP) J0952−0607 in a targeted radio pulsation search of an unidentified Fermi gamma-ray source. This source shows a weak energy flux of F γ = 2.6 × 10 −12 erg cm −2 s −1 in the energy range between 100 MeV and 100 GeV. Here we report the detection of pulsed gamma-ray emission from PSR J0952−0607 in a very sensitive gamma-ray pulsation search. The pulsar’s rotational, binary, and astrometric properties are measured over 7 years of Fermi -Large Area Telescope data. For this we take into account the uncertainty on the shape of the gamma-ray pulse profile. We present an updated radio-timing solution now spanning more than 2 years and show results from optical modeling of the black-widow-type companion based on new multiband photometric data taken with HiPERCAM on the Gran Telescopio Canarias on La Palma and ULTRACAM on the New Technology Telescope at ESO La Silla (based on observations collected at the European Southern Observatory, Chile programme 0101.D-0925, PI: Clark, C. J.). PSR J0952−0607 is now the fastest-spinning pulsar for which the intrinsic spin-down rate has been reliably constrained ( ). The inferred surface magnetic field strength of is among the 10 lowest of all known pulsars. This discovery is another ex le of an extremely fast spinning black-widow pulsar hiding within an unidentified Fermi gamma-ray source. In the future such systems might help to pin down the maximum spin frequency and the minimum surface magnetic field strength of MSPs.
Publisher: Springer Science and Business Media LLC
Date: 23-02-2022
DOI: 10.1038/S41586-021-04354-W
Abstract: Fast radio bursts (FRBs) are flashes of unknown physical origin
Publisher: Oxford University Press (OUP)
Date: 19-11-2015
Publisher: Oxford University Press (OUP)
Date: 30-07-2016
Publisher: American Association for the Advancement of Science (AAAS)
Date: 26-04-2013
Abstract: Because of their extremely high densities, massive neutron stars can be used to test gravity. Based on spectroscopy of its white dwarf companion, Antoniadis et al. (p. 448 ) identified a millisecond pulsar as a neutron star twice as heavy as the Sun. The observed binary's orbital decay is consistent with that predicted by general relativity, ruling out previously untested strong-field phenomena predicted by alternative theories. The binary system has a peculiar combination of properties and poses a challenge to our understanding of stellar evolution.
Publisher: Oxford University Press (OUP)
Date: 16-10-2022
Abstract: We present LOFAR imaging observations from the April/May 2020 active episode of magnetar SGR 1935 + 2154. We place the earliest radio limits on persistent emission following the low-luminosity fast radio burst FRB 200428 from the magnetar. We also perform an image-plane search for transient emission and find no radio flares during our observations. We examine post-FRB radio upper limits in the literature and find that all are consistent with the multiwavelength afterglow predicted by the synchrotron maser shock model interpretation of FRB 200428. However, early optical observations appear to rule out the simple versions of the afterglow model with constant-density circumburst media. We show that these constraints may be mitigated by adapting the model for a wind-like environment, but only for a limited parameter range. In addition, we suggest that late-time non-thermal particle acceleration occurs within the afterglow model when the shock is no longer relativistic, which may prove vital for detecting afterglows from other Galactic FRBs. We also discuss future observing strategies for verifying either magnetospheric or maser shock FRB models via rapid radio observations of Galactic magnetars and nearby FRBs.
Publisher: American Astronomical Society
Date: 02-2005
DOI: 10.1086/426683
Publisher: American Astronomical Society
Date: 17-10-2016
Publisher: American Astronomical Society
Date: 15-06-2015
Publisher: Oxford University Press (OUP)
Date: 21-05-2013
DOI: 10.1093/MNRAS/STT721
Publisher: Oxford University Press (OUP)
Date: 28-07-2014
Publisher: Oxford University Press (OUP)
Date: 02-03-2016
DOI: 10.1093/MNRAS/STW483
Publisher: Oxford University Press (OUP)
Date: 07-03-2011
Publisher: Oxford University Press (OUP)
Date: 12-05-2016
Publisher: Oxford University Press (OUP)
Date: 28-07-2016
Publisher: Springer Science and Business Media LLC
Date: 14-08-2013
DOI: 10.1038/NATURE12499
Abstract: Earth'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.
Publisher: Oxford University Press (OUP)
Date: 02-09-2013
Abstract: We report on the setup and initial discoveries of the Northern High Time Resolution Universe survey for pulsars and fast transients, the first major pulsar survey conducted with the 100-m Effelsberg radio telescope and the first in 20 years to observe the whole northern sky at high radio frequencies. Using a newly developed 7-beam receiver system combined with a state-of-the-art polyphase filterbank, we record an effective bandwidth of 240 MHz in 410 channels centred on 1.36 GHz with a time resolution of 54 μs. Such fine time and frequency resolution increases our sensitivity to millisecond pulsars and fast transients, especially deep inside the Galaxy, where previous surveys have been limited due to intrachannel dispersive smearing. To optimize observing time, the survey is split into three integration regimes dependent on Galactic latitude, with 1500, 180 and 90-s integrations for latitude ranges |b| & 3 $_{.}^{\circ}$5, |b| & 15° and |b| & 15°, respectively. The survey has so far resulted in the discovery of 15 radio pulsars, including a pulsar with a characteristic age of ∼18 kyr, PSR J2004+3429, and a highly eccentric, binary millisecond pulsar, PSR J1946+3417. All newly discovered pulsars are timed using the 76-m Lovell radio telescope at the Jodrell Bank Observatory and the Effelsberg radio telescope. We present timing solutions for all newly discovered pulsars and discuss potential supernova remnant associations for PSR J2004+3429.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 06-06-2008
Abstract: Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 milliseconds in a highly eccentric ( e = 0.44) 95-day orbit around a solar mass ( \\batchmode \\documentclass[fleqn,10pt,legalpaper]{article} \\usepackage{amssymb} \\usepackage{amsfonts} \\usepackage{amsmath} \\pagestyle{empty} \\begin{document} \\(\\mathrm{M}_{{\\odot}}\\) \\end{document} ) companion. Infrared observations identify a possible main-sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main-sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster, then ejecting it into the Galactic disk, or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74 ± 0.04 \\batchmode \\documentclass[fleqn,10pt,legalpaper]{article} \\usepackage{amssymb} \\usepackage{amsfonts} \\usepackage{amsmath} \\pagestyle{empty} \\begin{document} \\(\\mathrm{M}_{{\\odot}}\\) \\end{document} , an unusually high value.
Publisher: Oxford University Press (OUP)
Date: 18-07-2016
Publisher: Oxford University Press (OUP)
Date: 21-12-2012
DOI: 10.1093/MNRAS/STS449
Publisher: Oxford University Press (OUP)
Date: 13-06-2015
Publisher: Oxford University Press (OUP)
Date: 12-05-2014
DOI: 10.1093/MNRAS/STU708
Publisher: American Astronomical Society
Date: 04-2021
Abstract: The object FRB 20180916B is a well-studied repeating fast radio burst source. Its proximity (∼150 Mpc), along with detailed studies of the bursts, has revealed many clues about its nature, including a 16.3 day periodicity in its activity. Here we report on the detection of 18 bursts using LOFAR at 110–188 MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are seen down to the lowest observed frequency of 110 MHz, suggesting that their spectra extend even lower. These observations provide an order-of-magnitude stronger constraint on the optical depth due to free–free absorption in the source’s local environment. The absence of circular polarization and nearly flat polarization angle curves are consistent with burst properties seen at 300–1700 MHz. Compared with higher frequencies, the larger burst widths (∼40–160 ms at 150 MHz) and lower linear polarization fractions are likely due to scattering. We find ∼2–3 rad m −2 variations in the Faraday rotation measure that may be correlated with the activity cycle of the source. We compare the LOFAR burst arrival times to those of 38 previously published and 22 newly detected bursts from the uGMRT (200–450 MHz) and CHIME/FRB (400–800 MHz). Simultaneous observations show five CHIME/FRB bursts when no emission is detected by LOFAR. We find that the burst activity is systematically delayed toward lower frequencies by about 3 days from 600 to 150 MHz. We discuss these results in the context of a model in which FRB 20180916B is an interacting binary system featuring a neutron star and high-mass stellar companion.
Publisher: American Astronomical Society
Date: 10-2020
Abstract: We report the discovery of 1.97 ms period gamma-ray pulsations from the 75 minute orbital-period binary pulsar now named PSR J1653−0158. The associated Fermi Large Area Telescope gamma-ray source 4FGL J1653.6−0158 has long been expected to harbor a binary millisecond pulsar. Despite the pulsar-like gamma-ray spectrum and candidate optical/X-ray associations—whose periodic brightness modulations suggested an orbit—no radio pulsations had been found in many searches. The pulsar was discovered by directly searching the gamma-ray data using the GPU-accelerated Einstein@Home distributed volunteer computing system. The multidimensional parameter space was bounded by positional and orbital constraints obtained from the optical counterpart. More sensitive analyses of archival and new radio data using knowledge of the pulsar timing solution yield very stringent upper limits on radio emission. Any radio emission is thus either exceptionally weak, or eclipsed for a large fraction of the time. The pulsar has one of the three lowest inferred surface magnetic-field strengths of any known pulsar with B surf ≈ 4 × 10 7 G. The resulting mass function, combined with models of the companion star’s optical light curve and spectra, suggests a pulsar mass ≳2 M ⊙ . The companion is lightweight with mass ∼0.01 M ⊙ , and the orbital period is the shortest known for any rotation-powered binary pulsar. This discovery demonstrates the Fermi Large Area Telescope's potential to discover extreme pulsars that would otherwise remain undetected.
Publisher: EDP Sciences
Date: 10-2023
Publisher: Oxford University Press (OUP)
Date: 17-02-2014
DOI: 10.1093/MNRAS/STU067
Publisher: EDP Sciences
Date: 10-2023
Publisher: Springer Science and Business Media LLC
Date: 02-2016
DOI: 10.1038/NATURE17140
Abstract: In recent years, millisecond-duration radio signals originating in distant galaxies appear to have been discovered in the so-called fast radio bursts. These signals are dispersed according to a precise physical law and this dispersion is a key observable quantity, which, in tandem with a redshift measurement, can be used for fundamental physical investigations. Every fast radio burst has a dispersion measurement, but none before now have had a redshift measurement, because of the difficulty in pinpointing their celestial coordinates. Here we report the discovery of a fast radio burst and the identification of a fading radio transient lasting ~6 days after the event, which we use to identify the host galaxy we measure the galaxy's redshift to be z = 0.492 ± 0.008. The dispersion measure and redshift, in combination, provide a direct measurement of the cosmic density of ionized baryons in the intergalactic medium of ΩIGM = 4.9 ± 1.3 per cent, in agreement with the expectation from the Wilkinson Microwave Anisotropy Probe, and including all of the so-called 'missing baryons'. The ~6-day radio transient is largely consistent with the radio afterglow of a short γ-ray burst, and its existence and timescale do not support progenitor models such as giant pulses from pulsars, and supernovae. This contrasts with the interpretation of another recently discovered fast radio burst, suggesting that there are at least two classes of bursts.
Publisher: AIP
Date: 2008
DOI: 10.1063/1.2900272
Publisher: EDP Sciences
Date: 10-2023
Publisher: EDP Sciences
Date: 11-2022
DOI: 10.1051/0004-6361/202244377
Abstract: Context. The occultation of a radio source by the plasma tail of a comet can be used to probe structure and dynamics in the tail. Such occultations are rare, and the occurrence of scintillation, due to small-scale density variations in the tail, remains somewhat controversial. Aims. A detailed observation taken with the Low-Frequency Array (LOFAR) of a serendipitous occultation of the compact radio source 3C196 by the plasma tail of comet C/2020 F3 (Neowise) is presented. 3C196 tracked almost perpendicularly behind the tail, providing a unique profile cut only a short distance downstream from the cometary nucleus itself. Methods. Interplanetary scintillation (IPS) is observed as the rapid variation of the intensity received of a compact radio source due to density variations in the solar wind. IPS in the signal received from 3C196 was observed for five hours, covering the full transit behind the plasma tail of comet C/2020 F3 (Neowise) on 16 July 2020, and allowing an assessment of the solar wind in which the comet and its tail are embedded. Results. The results reveal a sudden and strong enhancement in scintillation which is unequivocally attributable to the plasma tail. The strongest scintillation is associated with the tail boundaries, weaker scintillation is seen within the tail, and previously-unreported periodic variations in scintillation are noted, possibly associated with in idual filaments of plasma. Furthermore, contributions from the solar wind and comet tail are separated to measure a sharp decrease in the velocity of material within the tail, suggesting a steep velocity shear resulting in strong turbulence along the tail boundary.
Publisher: American Astronomical Society
Date: 19-09-2014
Publisher: Oxford University Press (OUP)
Date: 06-09-2010
Publisher: American Astronomical Society
Date: 12-2021
Abstract: 3FGL J1544.6−1125 is a candidate transitional millisecond pulsar (tMSP). Similar to the well-established tMSPs—PSR J1023+0038, IGR J18245−2452, and XSS J12270−4859—3FGL J1544.6−1125 shows γ -ray emission and discrete X-ray “low” and “high” modes during its low-luminosity accretion-disk state. Coordinated radio/X-ray observations of PSR J1023+0038 in its current low-luminosity accretion-disk state showed rapidly variable radio continuum emission—possibly originating from a compact, self-absorbed jet, the “propellering” of accretion material, and/or pulsar moding. 3FGL J1544.6−1125 is currently the only other (candidate) tMSP system in this state, and can be studied to see whether tMSPs are typically radio-loud compared to other neutron star binaries. In this work, we present a quasi-simultaneous Very Large Array and Swift radio/X-ray c aign on 3FGL J1544.6−1125. We detect 10 GHz radio emission varying in flux density from 47.7 ± 6.0 μ Jy down to ≲15 μ Jy (3 σ upper limit) at four epochs spanning three weeks. At the brightest epoch, the radio luminosity is L 5 GHz = (2.17 ± 0.17) × 10 27 erg s −1 for a quasi-simultaneous X-ray luminosity L 2–10 keV = (4.32 ± 0.23) × 10 33 erg s −1 (for an assumed distance of 3.8 kpc). These luminosities are close to those of PSR J1023+0038, and the results strengthen the case that 3FGL J1544.6−1125 is a tMSP showing similar phenomenology to PSR J1023+0038.
Publisher: Elsevier BV
Date: 10-2017
Publisher: Oxford University Press (OUP)
Date: 28-04-2011
Publisher: Oxford University Press (OUP)
Date: 09-12-2014
Publisher: Oxford University Press (OUP)
Date: 31-08-2015
Publisher: American Astronomical Society
Date: 11-02-2021
Publisher: Oxford University Press (OUP)
Date: 07-03-2016
DOI: 10.1093/MNRAS/STW179
Publisher: American Astronomical Society
Date: 07-2015
Publisher: EDP Sciences
Date: 02-2022
DOI: 10.1051/0004-6361/202141739
Abstract: We describe the APERture Tile In Focus (Apertif) system, a phased array feed (PAF) upgrade of the Westerbork Synthesis Radio Telescope that transforms this telescope into a high-sensitivity, wide-field-of-view L -band imaging and transient survey instrument. Using novel PAF technology, up to 40 partially overlapping beams are formed on the sky simultaneously, significantly increasing the survey speed of the telescope. With this upgraded instrument, an imaging survey covering an area of 2300 deg 2 is being performed that will deliver both continuum and spectral line datasets, of which the first data have been publicly released. In addition, a time domain transient and pulsar survey covering 15 000 deg 2 is in progress. An overview of the Apertif science drivers, hardware, and software of the upgraded telescope is presented, along with its key performance characteristics.
Publisher: EDP Sciences
Date: 04-2023
DOI: 10.1051/0004-6361/202244107
Abstract: Fast radio bursts (FRBs) must be powered by uniquely energetic emission mechanisms. This requirement has eliminated a number of possible source types, but several remain. Identifying the physical nature of FRB emitters arguably requires good localisation of more detections, as well as broad-band studies enabled by real-time alerting. In this paper, we present the Apertif Radio Transient System (ARTS), a supercomputing radio-telescope instrument that performs real-time FRB detection and localisation on the Westerbork Synthesis Radio Telescope (WSRT) interferometer. It reaches coherent-addition sensitivity over the entire field of the view of the primary-dish beam. After commissioning results verified that the system performed as planned, we initiated the Apertif FRB survey (ALERT). Over the first 5 weeks we observed at design sensitivity in 2019, we detected five new FRBs, and interferometrically localised each of them to 0.4–10 sq. arcmin. All detections are broad band, very narrow, of the order of 1 ms in duration, and unscattered. Dispersion measures are generally high. Only through the very high time and frequency resolution of ARTS are these hard-to-find FRBs detected, producing an unbiased view of the intrinsic population properties. Most localisation regions are small enough to rule out the presence of associated persistent radio sources. Three FRBs cut through the halos of M31 and M33. We demonstrate that Apertif can localise one-off FRBs with an accuracy that maps magneto-ionic material along well-defined lines of sight. The rate of one every ~7 days ensures a considerable number of new sources are detected for such a study. The combination of the detection rate and localisation accuracy exemplified by the first five ARTS FRBs thus marks a new phase in which a growing number of bursts can be used to probe our Universe.
Publisher: Oxford University Press (OUP)
Date: 18-01-2022
Abstract: In this work, we study variations in the parabolic scintillation arcs of the binary millisecond pulsar PSR J1643−1224 over five years using the Large European Array for Pulsars (LEAP). The two-dimensional (2D) power spectrum of scintillation, called the secondary spectrum, often shows a parabolic distribution of power, where the arc curvature encodes the relative velocities and distances of the pulsar, ionized interstellar medium, and Earth. We observe a clear parabolic scintillation arc, which varies in curvature throughout the year. The distribution of power in the secondary spectra is inconsistent with a single scattering screen, which is fully 1D or entirely isotropic. We fit the observed arc curvature variations with two models: an isotropic scattering screen and a model with two independent 1D screens. We measure the distance to the scattering screen to be in the range 114–223 pc, depending on the model, consistent with the known distance of the foreground large-diameter H ii region Sh 2-27 (112 ± 17 pc), suggesting that it is the dominant source of scattering. We obtain only weak constraints on the pulsar’s orbital inclination and longitude of ascending node, since the scintillation pattern is not very sensitive to the pulsar’s motion and the screen is much closer to the Earth than the pulsar. More measurements of this kind – where scattering screens can be associated with foreground objects – will help to inform the origins and distribution of scattering screens within our galaxy.
Publisher: EDP Sciences
Date: 05-2022
DOI: 10.1051/0004-6361/202142636
Abstract: Discovering radio pulsars, particularly millisecond pulsars (MSPs), is important for a range of astrophysical applications, such as testing theories of gravity or probing the magneto-ionic interstellar medium. We aim to discover pulsars that may have been missed in previous pulsar searches by leveraging known pulsar observables (primarily polarisation) in the sensitive, low-frequency radio images from the Low-Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS), and have commenced the Targeted search, using LoTSS images, for polarised pulsars (TULIPP) survey. For this survey, we identified linearly and circularly polarised point sources with flux densities brighter than 2 mJy in LoTSS images at a centre frequency of 144 MHz with a 48 MHz bandwidth. Over 40 known pulsars, half of which are MSPs, were detected as polarised sources in the LoTSS images and excluded from the survey. We have obtained beam-formed LOFAR observations of 30 candidates, which were searched for pulsations using coherent de-dispersion. Here, we present the results of the first year of the TULIPP survey. We discovered two pulsars, PSRs J1049+5822 and J1602+3901, with rotational periods of P = 0.73 s and 3.7 ms, respectively. We also detected a further five known pulsars (two slowly-rotating pulsars and three MSPs) for which accurate sky positions were not available to allow a unique cross-match with LoTSS sources. This targeted survey presents a relatively efficient method by which pulsars, particularly MSPs, may be discovered using the flexible observing modes of sensitive radio telescopes such as the Square Kilometre Array and its pathfinders recursors, particularly since wide-area all-sky surveys using coherent de-dispersion are currently computationally infeasible.
Publisher: American Astronomical Society
Date: 10-07-2004
DOI: 10.1086/421259
Publisher: American Astronomical Society
Date: 06-2008
DOI: 10.1086/587505
Publisher: Center for Open Science
Date: 27-07-2022
Abstract: This article proposes improvements to an open framework for evaluating participatory science, including projects framed as citizen science. An original proposed framework, while valuable in its comprehensiveness, used problematic language that makes it unworkable in many international contexts. In countries like Australia where Indigenous data sovereignty matters profoundly, language about ‘target groups’ and ‘easing access’ to knowledge can harmfully perpetuate colonial discourses. The original proposed framework is sufficiently useful that it is worth constructively revising, so critique in this article is aimed towards collaborative progression of an open framework more suitable for international use. As well as replacing ‘target groups’ with partnership approaches, we argue that ‘easing access’ to knowledge for exploitation is a frame perpetuating the colonial doctrine of discovery, proposing recovery as an alternative aligned with several international movements for social justice and sustainability.
Publisher: American Society of Civil Engineers
Date: 29-03-2012
Publisher: IOP Publishing
Date: 05-2016
Publisher: Elsevier BV
Date: 12-2006
Publisher: Oxford University Press (OUP)
Date: 15-02-2016
DOI: 10.1093/MNRAS/STW347
No related grants have been discovered for Cees Bassa.