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  • 1
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    Apertif: Phased array feeds for the Westerbork Synthesis Radio Telescope : System overview and performance characteristics
    EDP Sciences ; 2022
    In:  Astronomy & Astrophysics Vol. 658 ( 2022-02), p. A146-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 658 ( 2022-02), p. A146-
    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.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202141739
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2022
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 2
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    First release of Apertif imaging survey data
    EDP Sciences ; 2022
    In:  Astronomy & Astrophysics Vol. 667 ( 2022-11), p. A38-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 667 ( 2022-11), p. A38-
    Abstract: Context. Apertif is a phased-array feed system for the Westerbork Synthesis Radio Telescope, providing forty instantaneous beams over 300 MHz of bandwidth. A dedicated survey program utilizing this upgrade started on 1 July 2019, with the last observations taken on 28 February 2022. The imaging survey component provides radio continuum, polarization, and spectral line data. Aims. Public release of data is critical for maximizing the legacy of a survey. Toward that end, we describe the release of data products from the first year of survey operations, through 30 June 2020. In particular, we focus on defining quality control metrics for the processed data products. Methods. The Apertif imaging pipeline, Apercal, automatically produces non-primary beam corrected continuum images, polarization images and cubes, and uncleaned spectral line and dirty beam cubes for each beam of an Apertif imaging observation. For this release, processed data products are considered on a beam-by-beam basis within an observation. We validate the continuum images by using metrics that identify deviations from Gaussian noise in the residual images. If the continuum image passes validation, we release all processed data products for a given beam. We apply further validation to the polarization and line data products and provide flags indicating the quality of those data products. Results. We release all raw observational data from the first year of survey observations, for a total of 221 observations of 160 independent target fields, covering approximately one thousand square degrees of sky. Images and cubes are released on a per beam basis, and 3374 beams (of 7640 considered) are released. The median noise in the continuum images is 41.4 uJy beam −1 , with a slightly lower median noise of 36.9 uJy beam −1 in the Stokes V polarization image. The median angular resolution is 11.6″/sin δ . The median noise for all line cubes, with a spectral resolution of 36.6 kHz, is 1.6 mJy beam −1 , corresponding to a 3- σ H i column density sensitivity of 1.8 × 10 20 atoms cm −2 over 20 km s −1 (for a median angular resolution of 24″ × 15″). Line cubes at lower frequency have slightly higher noise values, consistent with the global RFI environment and overall Apertif system performance. We also provide primary beam images for each individual Apertif compound beam. The data are made accessible using a Virtual Observatory interface and can be queried using a variety of standard tools.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202244007
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2022
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
    Location Call Number Limitation Availability
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  • 3
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    Repeating fast radio bursts with WSRT/Apertif
    EDP Sciences ; 2020
    In:  Astronomy & Astrophysics Vol. 635 ( 2020-03), p. A61-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 635 ( 2020-03), p. A61-
    Abstract: Context. Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments as well as to decipher the underlying emission mechanism. Detailed studies of repeating FRBs might also hold clues as to the origin of FRBs as a population. Aims. We aim to detect bursts from the first two repeating FRBs, FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and to characterise their repeat statistics. We also want to significantly improve the sky localisation of R2 and identify its host galaxy. Methods. We used the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows one to cover the entire sky position uncertainty of R2 with fine spatial resolution in a single pointing. The data were searched for bursts around the known dispersion measures of the two sources. We characterise the energy distribution and the clustering of detected R1 bursts. Results. We detected 30 bursts from R1. The non-Poissonian nature is clearly evident from the burst arrival times, which is consistent with earlier claims. Our measurements indicate a dispersion measure (DM) of 563.5(2) pc cm −3 , suggesting a significant increase in DM over the past few years. Assuming a constant position angle across the burst, we place an upper limit of 8% on the linear polarisation fraction for the brightest burst in our sample. We did not detect any bursts from R2. Conclusions. A single power-law might not fit the R1 burst energy distribution across the full energy range or widely separated detections. Our observations provide improved constraints on the clustering of R1 bursts. Our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium at 1400 MHz, which is not observed at higher frequencies. The non-detection of any bursts from R2, despite nearly 300 h of observations, implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of the two assuming that the source is still active. Another possibility is that R2 has turned off completely, either permanently or for an extended period of time.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/201937422
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2020
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 4
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    Online Resource
    Characterising the Apertif primary beam response
    EDP Sciences ; 2022
    In:  Astronomy & Astrophysics Vol. 667 ( 2022-11), p. A40-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 667 ( 2022-11), p. A40-
    Abstract: Context. Phased array feeds (PAFs) are multi-element receivers in the focal plane of a telescope that make it possible to simultaneously form multiple beams on the sky by combining the complex gains of the individual antenna elements. Recently, the Westerbork Synthesis Radio Telescope (WSRT) was upgraded with PAF receivers to carry out several observing programs, including two imaging surveys and a time-domain survey. The Apertif imaging surveys use a configuration of 40 partially overlapping compound beams (CBs) simultaneously formed on the sky and arranged in an approximately rectangular shape. Aims. This work is aimed at characterising the response of the 40 Apertif CBs to create frequency-resolved I, XX, and YY polarization empirical beam shapes. The measured CB maps can be used for the image deconvolution, primary beam correction, and mosaicking processes of Apertif imaging data. Methods. We used drift scan measurements to measure the response of each of the 40 Apertif CBs. We derived beam maps for all individual beams in I, XX, and YY polarisation in 10 or 18 frequency bins over the same bandwidth as the Apertif imaging surveys. We sampled the main lobe of the beams and the side lobes up to a radius of 0.6 degrees from the beam centres. In addition, we derived beam maps for each individual WSRT dish. Results. We present the frequency and time dependence of the beam shapes and sizes. We compared the compound beam shapes derived with the drift scan method to beam shapes derived with an independent method using a Gaussian Process Regression comparison between the Apertif continuum images and the NRAO VLA Sky Survey (NVSS) catalogue. We find a good agreement between the beam shapes derived with the two independent methods.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202244045
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2022
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 5
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    Continuum source catalog for the first APERTIF data release
    EDP Sciences ; 2022
    In:  Astronomy & Astrophysics Vol. 667 ( 2022-11), p. A39-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 667 ( 2022-11), p. A39-
    Abstract: The first data release from Apertif survey contains 3074 radio continuum images, covering a thousand square degrees of the sky. The observations were performed between August 2019 and July 2020. The continuum images were produced at a central frequency 1355 MHz, with a bandwidth of ~150 MHz and angular resolution of up to 10″. In this work, we introduce and apply a new method to obtain a primary beam model based on a machine-learning approach, namely, Gaussian process regression. The primary beam models obtained with this method have been published, along with the data products for the first Apertif data release. We applied the method to the continuum images, carried out a mosaicking process on their basis, and extracted the source catalog. The catalog contains 249672 radio sources, many of which have been detected for the first time at these frequencies. We cross-matched the coordinates with the NVSS, LOFAR/DR1/value-added, and LOFAR/DR2 catalogs – resulting in 44523, 22825, and 152824 common sources, respectively. The first sample provides a unique opportunity for detecting long-term transient sources, which have significantly changed their flux density over the past 25 yr. A combination of the second and the third samples provides valuable information on the spectral properties of the sources in addition to redshift estimates.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202244008
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2022
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
    Location Call Number Limitation Availability
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  • 6
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    Apercal—The Apertif calibration pipeline
    Elsevier BV ; 2022
    In:  Astronomy and Computing Vol. 38 ( 2022-01), p. 100514-
    Details
    In: Astronomy and Computing, Elsevier BV, Vol. 38 ( 2022-01), p. 100514-
    Type of Medium: Online Resource
    ISSN: 2213-1337
    URL: Article
    DOI: 10.1016/j.ascom.2021.100514
    Language: English
    Publisher: Elsevier BV
    Publication Date: 2022
    detail.hit.zdb_id: 2695868-5
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  • 7
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    A search for radio emission from double-neutron star merger GW190425 using Apertif
    EDP Sciences ; 2021
    In:  Astronomy & Astrophysics Vol. 650 ( 2021-06), p. A131-
    Details
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 650 ( 2021-06), p. A131-
    Abstract: Context. Detection of the electromagnetic emission from coalescing binary neutron stars (BNS) is important for understanding the merger and afterglow. Aims. We present a search for a radio counterpart to the gravitational-wave (GW) source GW190425, a BNS merger, using Apertif on the Westerbork Synthesis Radio Telescope (WSRT). Methods. We observed a field of high probability in the associated localisation region for three epochs at Δ T  = 68, 90, 109 d post merger. We identified all sources that exhibit flux variations consistent with the expected afterglow emission of GW190425. We also looked for possible transients. These are sources that are only present in one epoch. In addition, we quantified our ability to search for radio afterglows in the fourth and future observing runs of the GW detector network using Monte Carlo simulations. Results. We found 25 afterglow candidates based on their variability. None of these could be associated with a possible host galaxy at the luminosity distance of GW190425. We also found 55 transient afterglow candidates that were only detected in one epoch. All of these candidates turned out to be image artefacts. In the fourth observing run, we predict that up to three afterglows will be detectable by Apertif. Conclusions. While we did not find a source related to the afterglow emission of GW190425, the search validates our methods for future searches of radio afterglows.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    URL: Article
    DOI: 10.1051/0004-6361/202140578
    RVK:
    UA 1000
    RVK:
    US 1090
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2021
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 8
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    A bright, high rotation-measure FRB that skewers the M33 halo
    Oxford University Press (OUP) ; 2020
    In:  Monthly Notices of the Royal Astronomical Society Vol. 499, No. 4 ( 2020-11-04), p. 4716-4724
    Details
    In: Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP), Vol. 499, No. 4 ( 2020-11-04), p. 4716-4724
    Abstract: We report the detection of a bright fast radio burst, FRB 191108, with Apertif on the Westerbork Synthesis Radio Telescope. The interferometer allows us to localize the FRB to a narrow 5 arcsec × 7 arcmin ellipse by employing both multibeam information within the Apertif phased-array feed beam pattern, and across different tied-array beams. The resulting sightline passes close to Local Group galaxy M33, with an impact parameter of only 18 kpc with respect to the core. It also traverses the much larger circumgalactic medium (CGM) of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute ∼10 per cent of its dispersion measure of 588 pc cm−3. FRB 191108 has a Faraday rotation measure (RM) of +474 $\pm \, 3$ rad m−2, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetized plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with Δν ≈ 40 MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.
    Type of Medium: Online Resource
    ISSN: 0035-8711 , 1365-2966
    URL: Article
    DOI: 10.1093/mnras/staa3009
    Language: English
    Publisher: Oxford University Press (OUP)
    Publication Date: 2020
    detail.hit.zdb_id: 2016084-7
    SSG: 16,12
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  • 9
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    Online Resource
    Genetic and lifestyle risk factors for MRI-defined brain infarcts in a population-based setting
    Ovid Technologies (Wolters Kluwer Health) ; 2019
    In:  Neurology Vol. 92, No. 5 ( 2019-01-29)
    Details
    In: Neurology, Ovid Technologies (Wolters Kluwer Health), Vol. 92, No. 5 ( 2019-01-29)
    Type of Medium: Online Resource
    ISSN: 0028-3878 , 1526-632X
    URL: Article
    DOI: 10.1212/WNL.0000000000006851
    RVK:
    XA 10000
    Language: English
    Publisher: Ovid Technologies (Wolters Kluwer Health)
    Publication Date: 2019
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  • 10
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    Genetic contributions to variation in general cognitive function: a meta-analysis of genome-wide association studies in the CHARGE consortium (N=53 949)
    Springer Science and Business Media LLC ; 2015
    In:  Molecular Psychiatry Vol. 20, No. 2 ( 2015-02), p. 183-192
    Details
    In: Molecular Psychiatry, Springer Science and Business Media LLC, Vol. 20, No. 2 ( 2015-02), p. 183-192
    Abstract: General cognitive function is substantially heritable across the human life course from adolescence to old age. We investigated the genetic contribution to variation in this important, health- and well-being-related trait in middle-aged and older adults. We conducted a meta-analysis of genome-wide association studies of 31 cohorts ( N =53 949) in which the participants had undertaken multiple, diverse cognitive tests. A general cognitive function phenotype was tested for, and created in each cohort by principal component analysis. We report 13 genome-wide significant single-nucleotide polymorphism (SNP) associations in three genomic regions, 6q16.1, 14q12 and 19q13.32 (best SNP and closest gene, respectively: rs10457441, P =3.93 × 10 −9 , MIR2113 ; rs17522122, P =2.55 × 10 −8 , AKAP6 ; rs10119, P =5.67 × 10 −9 , APOE/TOMM40 ). We report one gene-based significant association with the HMGN1 gene located on chromosome 21 ( P =1 × 10 −6 ). These genes have previously been associated with neuropsychiatric phenotypes. Meta-analysis results are consistent with a polygenic model of inheritance. To estimate SNP-based heritability, the genome-wide complex trait analysis procedure was applied to two large cohorts, the Atherosclerosis Risk in Communities Study ( N =6617) and the Health and Retirement Study ( N =5976). The proportion of phenotypic variation accounted for by all genotyped common SNPs was 29% (s.e.=5%) and 28% (s.e.=7%), respectively. Using polygenic prediction analysis, ~1.2% of the variance in general cognitive function was predicted in the Generation Scotland cohort ( N =5487; P =1.5 × 10 −17 ). In hypothesis-driven tests, there was significant association between general cognitive function and four genes previously associated with Alzheimer’s disease: TOMM40 , APOE , ABCG1 and MEF2C .
    Type of Medium: Online Resource
    ISSN: 1359-4184 , 1476-5578
    URL: Article
    DOI: 10.1038/mp.2014.188
    Language: English
    Publisher: Springer Science and Business Media LLC
    Publication Date: 2015
    detail.hit.zdb_id: 1502531-7
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