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- Shimwell, T. W., et al.
(author)
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The LOFAR Two-metre Sky Survey: V. Second data release
- 2022
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 659
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Journal article (peer-reviewed)abstract
- In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey we present 120a 168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44 30a and 1h00m +28 00a and spanning 4178 and 1457 square degrees respectively. The images were derived from 3451 h (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4 396 228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6a resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 μJy beama 1; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2a; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy beama 1. By creating three 16 MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of > a ±a 0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20a resolution 120a168 MHz continuum images have a median rms sensitivity of 95 μJy beama 1, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480a A a 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mJy beama 1 at 4a and 2.2 mJy beama 1 at 20a; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.
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- Shimwell, T. W., et al.
(author)
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The LOFAR Two-metre Sky Survey: II. First data release
- 2019
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 622
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Research review (peer-reviewed)abstract
- The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168 MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00′00″ to 57°00′00″) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325 694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ∼10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S144 MHz = 71 μJy beam -1 and the point-source completeness is 90% at an integrated flux density of 0.45 mJy. The resolution of the images is 6″ and the positional accuracy is within 0.2″. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ∼20 articles that highlight the scientific potential of LoTSS.
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- Smirnov, O. M., et al.
(author)
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The RATT PARROT : serendipitous discovery of a peculiarly scintillating pulsar in MeerKAT imaging observations of the Great Saturn – Jupiter Conjunction of 2020. I. Dynamic imaging and data analysis
- 2024
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 528:4, s. 6517-6537
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Journal article (peer-reviewed)abstract
- We report on a radiopolarimetric observation of the Saturn–Jupiter Great Conjunction of 2020 using the MeerKAT L-band system, initially carried out for science verification purposes, which yielded a serendipitous discovery of a pulsar. The radiation belts of Jupiter are very bright and time variable: coupled with the sensitivity of MeerKAT, this necessitated development of dynamic imaging techniques, reported on in this work. We present a deep radio ‘movie’ revealing Jupiter’s rotating magnetosphere, a radio detection of Callisto, and numerous background radio galaxies. We also detect a bright radio transient in close vicinity to Saturn, lasting approximately 45 min. Follow-up deep imaging observations confirmed this as a faint compact variable radio source, and yielded detections of pulsed emission by the commensal MeerTRAP search engine, establishing the object’s nature as a radio emitting neutron star, designated PSR J2009−2026. A further observation combining deep imaging with the PTUSE pulsar backend measured detailed dynamic spectra for the object. While qualitatively consistent with scintillation, the magnitude of the magnification events and the characteristic time–scales are odd. We are tentatively designating this object a pulsar with anomalous refraction recurring on odd time-scales (PARROT). As part of this investigation, we present a pipeline for detection of variable sources in imaging data, with dynamic spectra and light curves as the products, and compare dynamic spectra obtained from visibility data with those yielded by PTUSE. We discuss MeerKAT’s capabilities and prospects for detecting more of such transients and variables.
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- Ekblad, M., et al.
(author)
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Influence of operational conditions and wastewater properties on the removal of organic micropollutants through ozonation
- 2021
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In: Journal of Environmental Management. - : Elsevier BV. - 0301-4797. ; 286
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Journal article (peer-reviewed)abstract
- The objective of this study was to evaluate the influence of operational conditions and wastewater properties on the removal of pharmaceuticals, contrast media and antibiotics through ozonation, in order to facilitate the optimization of treatment and its implementation on a full scale. Pilot-scale ozone oxidation trials were performed on treated wastewater, before and after post-precipitation, over a seven-month period, including summer and winter months. Hydraulic retention times as short as 7 min were found to be sufficient for organic micropollutant removal. A short hydraulic retention time reduces both investment costs and land use. Neither the choice of ozone dispersion method, a static mixer or a Venturi injector, nor the wastewater temperature had any significant effect on the removal efficiency of organic micropollutants, however, higher removal was achieved after on-site post-precipitation with aluminum chloride.
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- Ekblad, M., et al.
(author)
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Is dissolved COD a suitable design parameter for ozone oxidation of organic micropollutants in wastewater?
- 2019
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In: Science of the Total Environment. - : Elsevier BV. - 0048-9697. ; 658, s. 449-456
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Journal article (peer-reviewed)abstract
- Ozone oxidation of organic micropollutants in biologically treated wastewater was investigated in pilot-scale after a high- and a low loaded activated sludge process. Higher ozone doses were required to remove organic micropollutants in the effluent wastewater from the high loaded activated sludge process. Further comparison of the micropollutant removal was based on normalized ozone doses, expressed as g O3/g DOC and g O3/g soluble COD (sCOD). A clear difference was noted for the two effluents when the micropollutant removal was normalized by DOC. This difference disappeared almost completely when the removal was linked to ozone doses normalized by sCOD. The dose-response curves for the organic micropollutants were practically linear in the removal range up to 95%. A linear prediction model was developed and compared with literature values to test the transferability of the obtained results. Results from this comparison indicated that the slope of the dose-response functions could be used to predict the removal efficiency of organic micropollutants at a third plant with an average uncertainty of 10%. The modeled ozone requirements were then set in relation to the COD concentrations in the discharged water from approximately 90 Swedish activated sludge treatment plants with and without nitrogen removal. This comparison highlighted the need for a well-functioning biological treatment for an effective ozone oxidation of organic micropollutants. The results in this study suggest that soluble COD should be further explored for design and modeling of ozone oxidation of organic micropollutants in biologically treated wastewater.
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- El-taliawy, H., et al.
(author)
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Ozonation efficiency in removing organic micro pollutants from wastewater with respect to hydraulic loading rates and different wastewaters
- 2017
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In: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 325, s. 310-321
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Journal article (peer-reviewed)abstract
- Organic micro pollutants can be removed from water by ozonation. In this article we studied the performance of ozonation under real life conditions and compared results of the same ozonation pilot plant installed at different wastewater treatment plants (WWTPs) thus operating with different waters. The comparability of the removal and reaction rate constants from one waste water treatment plant were low in respect to reaction rate, removal as well as to response to the specific ozone dose. Neither pH-value nor residual nitrite concentrations were the driving force considering these differences. Further tests with different loadings were conducted at the same WWTP under different weather conditions. For the different hydraulic loading of the biological plant, the ozonation was running with rather similar removal rates concerning the same specific (TOC normalized) ozone dose. The compounds that were removed quantitatively under dry weather were still removed well with three times dry weather flow. Using a dataset from one WWTP to optimize operation in another one is thus questionable.
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