LOFAR 144-MHz follow-up observations of GW170817

Creators: Broderick, J. W.; Shimwell, T. W.; Gourdji, K.; Rowlinson, A.; Nissanke, S.; Hotokezaka, K.; Jonker, P. G.; Tasse, C.; Hardcastle, M. J.; Oonk, J. B. R.; Fender, R. P.; Wijers, R. A. M. J.; Shulevski, A.; Stewart, A. J.; ter Veen, S.; Moss, V. A.; van der Wiel, M. H. D.; Nichols, D. A.; Piette, A.; Bell, M. E.; Carbone, D.; Corbel, S.; Eislöffel, J.; Grießmeier, J.-M.; Keane, E. F.; Law, C. J.; Muñoz-Darias, T.; Pietka, M.; Serylak, M.; van der Horst, A. J.; van Leeuwen, J.; Wijnands, R.; Zarka, P.; Anderson, J. M.; Bentum, M. J.; Blaauw, R.; Brouw, W. N.; Brüggen, M.; Ciardi, B.; de Vos, M.; Duscha, S.; Fallows, R. A.; Franzen, T. M. O.; Garrett, M. A.; Gunst, A. W.; Hoeft, M.; Hörandel, J. R.; Iacobelli, M.; Jütte, E.; Koopmans, L. V. E.; Krankowski, A.; Maat, P.; Mann, G.; Mulder, H.; Nelles, A.; Paas, H.; Pandey-Pommier, M.; Pekal, R.; Reich, W.; Röttgering, H. J. A.; Schwarz, D. J.; Smirnov, O.; Soida, M.; Toribio, M. C.; van Haarlem, M. P.; van Weeren, R. J.; Vocks, C.; Wucknitz, O.; Zucca, P.

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∘.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 3σ upper limits for the afterglow component of 6.6 and 19.5 mJy 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 α^(610)_(144) ≳ −2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.

Additional Information

© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) Accepted 2020 March 23. Received 2020 March 10; in original form 2019 October 5. We thank the referee for a number of helpful suggestions that improved the content and presentation of this paper. We also thank the ASTRON Radio Observatory for setting up and scheduling the observations described in this paper, as well as preprocessing the data. Additionally, we are grateful to Ben Stappers for providing very useful feedback on an earlier version of this manuscript. This paper is based on data obtained with the International LOFAR Telescope (ILT) under project codes DDT9_002 and LT10_013. LOFAR (van Haarlem et al. 2013) is the Low Frequency Array designed and constructed by ASTRON. It has observing, data processing, and data storage facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefitted from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Université d'Orléans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; The Science and Technology Facilities Council, UK; Ministry of Science and Higher Education, Poland. The LOFAR direction-independent calibration pipeline (https://github.com/lofar-astron/prefactor) was deployed by the LOFAR e-infragroup on the Dutch National Grid infrastructure with support of the SURF Co-operative through grants e-infra170194, e-infra180087, and e-infra180169 (Mechev et al. 2017). The LOFAR direction-dependent calibration and imaging pipeline (http://github.com/mhardcastle/ddf-pipeline/) was run on computing clusters at Leiden Observatory and the University of Hertfordshire, which are supported by a European Research Council Advanced Grant [NEWCLUSTERS-321271] and the UK Science and Technology Funding Council [ST/P000096/1]. PGJ acknowledges funding from the European Research Council under ERC Consolidator Grant agreement no. 647208. SC acknowledges funding support from the UnivEarthS Labex program of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). TMD acknowledges support from grants AYA2017-83216-P and RYC-2015-18148. JvL acknowledges funding from Vici research programme 'ARGO' with project number 639.043.815, financed by the Netherlands Organisation for Scientific Research (NWO). MB acknowledges support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC 2121 'Quantum Universe' – 390833306. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France (DOI: 10.26093/cds/vizier). The original description of the VizieR service was published in A&AS 143, 23 (Ochsenbein, Bauer & Marcout 2000). This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of NASA's Astrophysics Data System Bibliographic Services. This project also made use of KVIS (Gooch 1995), TOPCAT (Taylor 2005), NUMPY (Oliphant 2006), and MATPLOTLIB (Hunter 2007).

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