| 1. |
- Moldón, J., et al.
(författare)
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The LOFAR long baseline snapshot calibrator survey
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
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Tidskriftsartikel (refereegranskat)abstract
- Aims. An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz.Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators.Results. More than 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the sky density of calibrators that are sufficiently bright to calibrate dispersive and non-dispersive delays for the international LOFAR using existing methods is 1.0 per square degree.Conclusions. The observed density of satisfactory delay calibrator sources means that observations with international LOFAR should be possible at virtually any point in the sky provided that a fast and efficient search, using the methodology described here, is conducted prior to the observation to identify the best calibrator.
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| 2. |
- Andre, M., Behlke, R., Wahlund, J.E., Vaivads, A., Eriksson, A., Tjulin, A., Carozzi, T. D., Cully, C., Gustafsson, G., Sundkvist, D., Khotyaintsev, Y., Cornilleau-Wehrlin, N., Rezeau, L., Maksimovic, M., Lucek, E., Balogh, A., Dunlop, M., Lindqvist, P.A.
(författare)
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Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause.
- 2001
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Ingår i: Annales Geophysicae. ; 19:6, s. 1471-1481
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Tidskriftsartikel (refereegranskat)abstract
- Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have a
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| 3. |
- Andre, M., et al.
(författare)
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Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause
- 2001
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 19:12-okt, s. 1471-1481
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Tidskriftsartikel (refereegranskat)abstract
- Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.
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| 4. |
- Neld, A., et al.
(författare)
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Reliable detection and characterization of low-frequency polarized sources in the LOFAR M51 field
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 617
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Tidskriftsartikel (refereegranskat)abstract
- The new generation of broad-band radio continuum surveys will provide large data sets with polarization information. New algorithms need to be developed to extract reliable catalogs of linearly polarized sources that can be used to characterize those sources and produce a dense rotation measure (RM) grid to probe magneto-ionized structures along the line of sight via Faraday rotation. Aims. The aim of the paper is to develop a computationally efficient and rigorously defined source-finding algorithm for linearly polarized sources. Methods. We used a calibrated data set from the LOw Frequency ARray (LOFAR) at 150 MHz centered on the nearby galaxy M 51 to search for polarized background sources. With a new imaging software, we re-imaged the field at a resolution of 18″ × 15″ and cataloged a total of about 3000 continuum sources within 2.5° of the center of M 51. We made small Stokes Q and U images centered on each source brighter than 100 mJy in total intensity (201 sources) and used RM synthesis to create corresponding Faraday cubes that were analyzed individually. For each source, the noise distribution function was determined from a subset of the measurements at high Faraday depths where no polarization is expected; the peaks in polarized intensity in the Faraday spectrum were identified and the p-value of each source was calculated. Finally, the false discovery rate method was applied to the list of p-values to produce a list of polarized sources and quantify the reliability of the detections. We also analyzed sources fainter than 100 mJy but that were reported as polarized in the literature at at least another radio frequency. Results. Of the 201 sources that were searched for polarization, six polarized sources were detected confidently (with a false discovery rate of 5%). This corresponds to a number density of one polarized source per 3.3 square degrees, or 0.3 source per square degree. Increasing the false discovery rate to 50% yields 19 sources. A majority of the sources have a morphology that is indicative of them being double-lobed radio galaxies, and the ones with literature redshift measurements have 0.5 < z < 1.0. Conclusions. We find that this method is effective in identifying polarized sources, and is well suited for LOFAR observations. In the future, we intend to develop it further and apply it to larger data sets such as the LOFAR Two-meter Survey of the whole northern sky, LOTSS, and the ongoing deep LOFAR observations of the GOODS-North field.
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| 7. |
- Ibragimov, Nail H., et al.
(författare)
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Utilization of photon orbital angular momentum in the low-frequency radio domain
- 2007
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 99:8
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Tidskriftsartikel (refereegranskat)abstract
- We show numerically that vector antenna arrays can generate radio beams that exhibit spin and orbital angular momentum characteristics similar to those of helical Laguerre-Gauss laser beams in paraxial optics. For low frequencies (1 GHz), digital techniques can be used to coherently measure the instantaneous, local field vectors and to manipulate them in software. This enables new types of experiments that go beyond what is possible in optics. It allows information-rich radio astronomy and paves the way for novel wireless communication concepts.
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| 8. |
- Johnson, Owen A., et al.
(författare)
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A Simultaneous Dual-site Technosignature Search Using International LOFAR Stations
- 2023
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Ingår i: Astronomical Journal. - 1538-3881 .- 0004-6256. ; 166:5
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Tidskriftsartikel (refereegranskat)abstract
- The Search for Extraterrestrial Intelligence aims to find evidence of technosignatures, which can point toward the possible existence of technologically advanced extraterrestrial life. Radio signals similar to those engineered on Earth may be transmitted by other civilizations, motivating technosignature searches across the entire radio spectrum. In this endeavor, the low-frequency radio band has remained largely unexplored; with prior radio searches primarily above 1 GHz. In this survey at 110-190 MHz, observations of 1,631,198 targets from TESS and Gaia are reported. Observations took place simultaneously with two international stations (noninterferometric) of the Low Frequency Array in Ireland and Sweden. We can reject the presence of any Doppler drifting narrowband transmissions in the barycentric frame of reference, with equivalent isotropic radiated power of 1017 W, for 0.4 million (or 1.3 million) stellar systems at 110 (or 190) MHz. This work demonstrates the effectiveness of using multisite simultaneous observations for rejecting anthropogenic signals in the search for technosignatures.
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