Abstract
The largely unexplored decametre radio band (10–30 MHz) provides a unique window for studying a range of astronomical topics, such as auroral emission from exoplanets, inefficient cosmic ray acceleration mechanisms and fossil radio plasma. The scarcity of low-frequency studies is mainly due to severe ionospheric corruption. Here we present a calibration strategy to correct for the ionosphere in the decametre band. We apply this to an observation from the Low-Frequency Array (LOFAR) between 16 and 30 MHz. The resulting image covers 330 square degrees of sky at a resolution of 45″, reaching a sensitivity of 12 mJy per beam, which is an improvement by an order of magnitude in terms of sensitivity and resolution compared to previous decametre observations. Residual ionospheric effects cause additional blurring between 60″ and 100″. We have identified four fossil plasma sources in the surveyed region. These sources probably harbour rejuvenated radio plasma from past active galactic nuclei outbursts. Three are near the centre of low-mass galaxy clusters. Notably, two of these sources display the steepest radio spectral index among all the sources detected at 23 MHz. This indicates that fossil plasma sources constitute the primary population of steep-spectrum sources at these frequencies, emphasizing the large discovery potential of ground-based decametre observations.
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Data availability
The catalogue produced in this work is available in the Supplementary Data. The calibrated mosaic image is available upon reasonable request to the authors.
Code availability
The following codes were used in this work and are readily available: https://github.com/saopicc/DDFacet (DDFacet); https://git.astron.nl/RD/DP3 (DP3); https://gitlab.com/aroffringa/wsclean (WSClean); https://github.com/rvweeren/lofar_facet_selfcal (facetselfcal); and https://github.com/lofar-astron/PyBDSF (PyBDSF). Other code used in this work is available upon request to the authors.
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Acknowledgements
C.G. and R.J.v.W. acknowledge support from the European Research Council (Starting Grant ClusterWeb No. 804208). M.B. is funded by the German Research Foundation under Germany’s Excellence Strategy (Project EXC 2121, Quantum Universe, 390833306). E.O. acknowledges support from the VIDI research programme (Project No. 639.042.729), which is financed by the Netherlands Organisation for Scientific Research. A.B. acknowledges financial support from the Next Generation programme of the European Union. LOFAR was designed and constructed by ASTRON. It has observing, data processing and data storage facilities in several countries, which are owned by various parties (each with their own funding sources) and are collectively operated by the ILT Foundation under a joint scientific policy. The ILT resources have benefitted from the following recent major funding sources: the National Institute for Earth Sciences and Astronomy of the National Centre for Scientific Research, the Paris Observatory and the University of Orléans, France; the Federal Ministry of Education and Research, the Ministry of Innovation, Science and Research of North Rhine-Westphalia and the Max Planck Society, Germany; Science Foundation Ireland, Department of Business, Enterprise and Innovation, Ireland; the Netherlands Organisation for Scientific Research, the Netherlands; the Science and Technology Facilities Council, UK; Ministry of Science and Higher Education, Poland; and the Italian National Institute for Astrophysics, Italy. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max Planck Society and its participating institutes (the Max Planck Institute for Astronomy, Heidelberg, and the Max Planck Institute for Extraterrestrial Physics, Garching), Johns Hopkins University, Durham University, the University of Edinburgh, Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, NASA (Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate), the US National Science Foundation (NSF; Grant No. AST-1238877), the University of Maryland, Eotvos Lorand University, Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. When using data from the Legacy Surveys in papers, please use the following acknowledgment: The Legacy Surveys consist of three individual and complementary projects: the Dark Energy Camera Legacy Survey (DECaLS; Proposal ID 2014B-0404; PIs D. Schlegel and A. Dey), the Beijing–Arizona Sky Survey (BASS; National Optical Astronomy Observatory, Prop. ID 2015A-0801; PIs Z. Xu and X. Fan) and the Mayall z-band Legacy Survey (Prop. ID 2016A-0453; PI A. Dey). DECaLS, BASS and the Mayall z-band Legacy Survey together include data obtained, respectively, at the Blanco telescope, Cerro Tololo Inter-American Observatory, NSF’s NOIRLab; the Bok telescope, Steward Observatory, University of Arizona; and the Mayall telescope, Kitt Peak National Observatory, NOIRLab. Pipeline processing and analyses of the data were supported by NOIRLab and Lawrence Berkeley National Laboratory (LBNL). The Legacy Surveys project is honoured to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation. NOIRLab is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF. LBNL is managed by the Regents of the University of California under contract to the US Department of Energy (DOE). This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. Funding for the DES projects has been provided by the DOE, the NSF, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council, UK, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, the Funding Authority for Studies and Projects, Brazil, the Carlos Chagas Filho Foundation, Amparo, the Carlos Chagas Filho Foundation for Research Support in the State of Rio de Janeiro, the National Council for Scientific and Technological Development, Brazil, the Ministry of Science, Technology and Innovation, Brazil, the German Research Foundation, and the collaborating institutions in DES. The collaborating institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, the Centre for Energy, Environmental and Technological Research, Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenossische Technische Hochschule (ETH) Zurich, the Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institute of Space Sciences of the Spanish National Research Council, the Institute for High Energy Physics, Spain, LBNL, the Ludwig Maximilian University of Munich and the associated Excellence Cluster Universe, the University of Michigan, NSF’s NOIRLab, the University of Nottingham, Ohio State University, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University. BASS is a key project of the Telescope Access Program, which has been funded by the National Astronomical Observatories of China, the Chinese Academy of Sciences (the Strategic Priority Research Program the Emergence of Cosmological Structures, Grant No. XDB09000000) and the Special Fund for Astronomy from the Ministry of Finance. BASS is also supported by the External Cooperation Program of the Chinese Academy of Sciences (Grant No. 114A11KYSB20160057) and the Chinese National Natural Science Foundation (Grant Nos. 12120101003 and 11433005). The Legacy Survey team makes use of data products from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which is a project of the Jet Propulsion Laboratory/California Institute of Technology. NEOWISE is funded by NASA. The Legacy Surveys imaging of the Dark Energy Spectroscopic Instrument footprint is supported by the Director, Office of Science, Office of High Energy Physics of the DOE (Contract No. DE-AC02-05CH1123), by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility under the same contract, and by the NSF, Division of Astronomical Sciences (Contract No. AST-0950945 to the National Optical Astronomy Observatory). This work made use of EveryStamp (https://tikk3r.github.io/EveryStamp/). This work made use of Astropy (http://www.astropy.org), a community-developed core Python package and an ecosystem of tools and resources for astronomy61,62,63.
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C.G. coordinated and wrote the paper, reduced the data and produced the LOFAR images. R.J.v.W. developed the self-calibration strategy and led the proposal that provided the data that this work is based on. E.O. worked on the initial calibration strategy for the decametre observations. F.d.G. developed some of the procedures used for this work. J.R.C provided scientific background information on the physics of radio emissions from stellar systems. W.L.W., J.R.C., H.J.A.R., M.B., G.B, G.K.M. and R.J.v.W. helped with writing the paper and provided feedback on the manuscript. A.B. performed the LoTSS target extraction. T.S. performed the LoTSS data reduction and survey management. F.S. had a critical role in developing the self-calibration software used in this work. J.M.G.H.J.d.J. developed the software required for managing the direction-dependent correction files. L.F.J. produced the cross-matched catalogues between this work and LoTSS. W.L.W., J.R.C and L.F.J. helped with the verification of the data products.
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Groeneveld, C., van Weeren, R.J., Osinga, E. et al. Characterization of the decametre sky at subarcminute resolution. Nat Astron 8, 786–795 (2024). https://doi.org/10.1038/s41550-024-02266-z
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DOI: https://doi.org/10.1038/s41550-024-02266-z
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