| 1. |
- Broderick, J. W., et al.
(författare)
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LOFAR 144-MHz follow-up observations of GW170817
- 2020
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 494:4, s. 5110-5117
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Tidskriftsartikel (refereegranskat)abstract
- We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO-Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13 degrees.7 when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130-138 and 371-374 d after the merger event, we obtain 3s upper limits for the afterglow component of 6.6 and 19.5mJy beam(-1), respectively. Using our best upper limit and previously published, contemporaneous higher frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index alpha(610)(144) greater than or similar to -2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.
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| 2. |
- Morabito, L., et al.
(författare)
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Sub-arcsecond imaging with the International LOFAR Telescope: I. Foundational calibration strategy and pipeline
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Tidskriftsartikel (refereegranskat)abstract
- The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ∼2000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 h observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ∼1.5° away, while phase solution transferral works well over ∼1°. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ∼0.3″ ×0.2″ although this varies slightly over the entire 5 deg2 field of view. We find we can achieve ∼80-300 μJy bm-1 image rms noise, which is dependent on the distance from the phase centre; typical values are ∼90 μJy bm-1 for the 8 h observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ∼ 3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution makes this estimate a lower limit.
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| 3. |
- De Gasperin, F., et al.
(författare)
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Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Tidskriftsartikel (refereegranskat)abstract
- The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (< 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15″ with a high dynamic range and good image fidelity. Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source. Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further.
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| 4. |
- Fallows, Richard A., et al.
(författare)
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A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances
- 2020
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Ingår i: Journal of Space Weather and Space Climate. - : EDP Sciences. - 2115-7251. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cassiopeia A, taken overnight on 18-19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10-80 MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR "core" reveals two different velocities in the scintillation pattern: a primary velocity of similar to 20-40 ms(-1) with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of similar to 110 ms(-1) with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller-scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.
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| 5. |
- van Bentum, Jael S., et al.
(författare)
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The Suicidal Intrusions Attributes Scale (SINAS) : a new tool measuring suicidal intrusions
- 2023
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Ingår i: Frontiers in Psychiatry. - : Frontiers Media S.A.. - 1664-0640. ; 14
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Tidskriftsartikel (refereegranskat)abstract
- Introduction: Suicidal intrusions are uncontrollable, intrusive mental images (e. g., visualizing a future suicidal act). They may also be called suicidal "flash-forwards." Despite the importance of integrating the assessment of suicidal intrusions into a clinical routine assessment, quick self-report screening instruments are lacking. This study describes the development of a new instrument-Suicidal Intrusions Attributes Scale (SINAS)-to assess the severity and characteristics of suicidal intrusions and examines its psychometric properties.Method: The sample included currently suicidal outpatients with elevated levels of depression recruited across mental health institutions in the Netherlands (N = 168). Instruments administered were 10-item SINAS, the Suicidal Ideation Attributes Scale (SIDAS), the Prospective Imagery Task (PIT), four-item Suicidal Cognitions Interview (SCI), and the Beck Depression Inventory (BDI-II).Results: An exploratory factor analysis identified a one-factor structure. The resulting SINAS demonstrated good internal consistency (Cronbach's & alpha; = 0.91) and convergent validity, as expected.Discussion: Overall, this study demonstrated acceptable levels of reliability and validity of the measure in a depressed clinical population with suicidal ideation. The SINAS may be a useful screening tool for suicidal intrusions in both research and clinical settings.
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| 6. |
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| 7. |
- Van Bentum, R, et al.
(författare)
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MICROVASCULAR CHANGES OF THE RETINA IN ANKYLOSING SPONDYLITIS, AND THE ASSOCIATION WITH CARDIOVASCULAR DISEASE - THE EYE FOR A HEART STUDY.
- 2020
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Ingår i: ANNALS OF THE RHEUMATIC DISEASES. - : BMJ. - 0003-4967 .- 1468-2060. ; 79, s. 1654-1654
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Patients with ankylosing spondylitis (AS) have an increased risk at cardiovascular disease (CVD). Microvasculature changes might precede overt CVD, but have been poorly studied in AS. The small vessels of the retina are accessible for non-invasive visualization, and microvascular changes (retinal arteriolar narrowing, venular widening, loss of tortuosity) are described in association with CVD in other diseases.Objectives:The aim of this study was to compare the retinal microvasculature of AS patients with healthy controls, and to assess gender differences.Methods:A cross-sectional, case-control study comparing AS patients (fulfilling the modified New York criteria, Rheumatology outpatient clinic of Reade and Amsterdam UMC) with healthy controls (EMIF-AD PreClinAD cohort of the Dutch Twins Register(1)), men:women=1:1. Most important inclusion criteria were: age 50-75 years, diabetes mellitus was excluded. All subjects underwent Optical Coherence Tomography Angiography and fundus photography (≥1 eye), analyzed with Singapore I Vessel Assessment software (Table 2). Differences between AS and controls were evaluated with generalised estimating equations (GEE), adjusted for demographics and cardiovascular risk, and stratified for gender.Results:In total, 59 AS patients (mean disease duration 36 years) and 105 controls were included. Controls were significantly older than patients, but did not differ in cardiovascular profile (Table 1). Patients had a significantly lower retinal arteriolar tortuosity (β-0.1;p=0.02), and higher vessel density (β 0.5,p=0.02), than controls (Table 2). Also, male AS patients showed a lower arteriovenular ratio compared to male controls (β -0.03,p=0.04). There were no differences between women with and without AS. In AS, a high disease activity was associated with a wider (unfavorable) venular diameter (p=0.05), whereas biologic use showed a wider (more favorable) arteriolar diameter (p<0.01).Conclusion:This study detected several retinal microvascular changes, in AS patients compared to controls, of which some are associated with CVD based on previous studies. Some changes were only observed in male-, but not in female, patients. A new finding was an increased capillary density in AS, of which the association with CVD-risk has not yet been studied before.References:[1]Konijnenberg E et al. The EMIF-AD PreclinAD study: study design and baseline cohort overview. Alzheimers Res Ther. 2018; 10:75.Table 1.Patient characteristics AS (n=57) and controls (n=105)ASControlspGender, women (%)30 (51)52 (50)nsAge, mean yrs (SD)60 (6)68 (4)<0.01Smoking currently, yes (%)11 (19)8 (8)0.06Body mass index, mean (SD)26 (4)26 (3)nsHypertension, yes (%)23 (39)39 (37)nsDyslipidemia, yes (%)9 (15)18 (17)nsCardiovascular disease history, yes (%)9 (15)15 (14)nsNSAIDS (%)24 (41)6 (6)<0.01Biological (mostly TNF inhibitor)* (%)29 (49)0 (0)<0.01AS Disease Activity Score, mean (SD)2.1 ±0.9Table 2.Retinal vascular parameters, differences AS and Control subjectsCrudeAdjusted for:Age, gender, BMI smoking, hypertension, dyslipidemiaRetinal vascular parametersβ(95%CI)pβ(95%CI)pDiameterArteriolar1.6(-2.0, 5.2)0.37-0.2(-4.8, 4.4)0.92Venular5.4(-0.66, 11.5)0.082.5(-5.4, 10.4)0.53Arteriovenular ratio-0.01(-0.03, 0.01)0.43-0.01(-0.03, 0.02)0.65TortuosityArteriolar-0.05(-0.12, 0.03)0.19-0.1(-0.2, -0.01)0.02ComplexityFractal dimension0.01(0.00, 0.03)0.040.0(-0.02, 0.02)0.88Vessel DensityInner ring0.8(0.5, 1.1)<0.0010.5(0.1, 0.9)0.02Outer ring0.7(0.4, 1.0)<0.0010.2(-0.2, 0.6)0.42Disclosure of Interests:Rianne van Bentum: None declared, Milad Baniaamam: None declared, Buket Kinaci-Tas: None declared, Jacoba van de Kreeke: None declared, Pieter Jelle Visser: None declared, Erik Serné: None declared, Michael Nurmohamed Grant/research support from: Not related to this research, Consultant of: Not related to this research, Speakers bureau: Not related to this research, Irene van der Horst-Bruinsma Grant/research support from: AbbVie, Novartis, Eli Lilly, Bristol-Myers Squibb, MSD, Pfizer, UCB Pharma, Consultant of: AbbVie, Novartis, Eli Lilly, Bristol-Myers Squibb, MSD, Pfizer, UCB Pharma
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