tokens: We thank the referee for his/her helpful comments/suggestions that have improved this Letter. We thank K. Maguire, X-F. Wang for useful discussions. Y-Z.C is supported by the China Scholarship Council (No. 201606040170). KM and SJP are supported by H2020 ERC grant no. 758638. MF is supported by a Royal Society - Science Foundation Ireland University Research Fellowship. SB and AF are partially supported by PRIN-INAF 2017 of Toward the SKA and CTA era: discovery, localization, and physics of transient sources.(PI: M. Giroletti). T.R. acknowledges the financial support of the Jenny and Antti Wihuri and the Vilho, Yrjö and Kalle Väisälä foundations. AR acknowledges financial support by the “Millennium Institute of Astrophysics (MAS)” of the Iniciativa Cientìfica Milenio. CG acknowledges support by the VILLUM FONDEN research grant (project number 00025501) and Investigator grant (project number 16599). MG is supported by the Polish NCN MAESTRO grant 2014/14/A/ST9/00121. FO acknowledges the support of the H2020 Hemera program, grant agreement No 730970. The Nordic Optical Telescope (NOT), operated by the NOT Scientific Association at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Observations from the NOT were obtained through the NUTS1 and NUTS2 collaboration which are supported in part by the Instrument Centre for Danish Astrophysics (IDA). The data presented here were obtained (in part) with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOTSA. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. Based on observations made with the GTC telescope in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofssica de Canarias, under Director’s Discretionary Time (GTC2019-127; PI: A. Morales-Garoffolo). This research makes use of Lasair data ( https://lasair.roe.ac.uk/ ), which is supported by the UKRI Science and Technology Facilities Council and is a collaboration between the University of Edinburgh (grant ST/N002512/1) and Queen’s University Belfast (grant ST/N002520/1) within the LSST:UK Science Consortium. IRAF is written and supported by the National Optical Astronomy Observatories (NOAO) in Tucson, Arizona. NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation. 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. SNOoPy is a package developed by E. Cappellaro which performs photometry of point-like sources in complex environments using PSF-fitting and template subtraction methods. A package description can be found at http://sngroup.oapd.inaf.it/snoopy.html .
ner_tags: O O O O O O O O O O O O O O O B-Person I-Person B-Person I-Person O O O B-Person O O O O B-Organization I-Organization I-Organization O B-Grant B-Person O B-Person O O O B-Grant B-Organization O O B-Grant B-Person O O O O B-Fellowship I-Fellowship I-Fellowship I-Fellowship I-Fellowship I-Fellowship I-Fellowship I-Fellowship I-Fellowship B-Person O B-Person O O O O B-Grant I-Grant O B-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal I-Proposal B-Person I-Person B-Person O O O O O O B-Organization I-Organization I-Organization I-Organization O O B-Organization I-Organization I-Organization I-Organization I-Organization I-Organization B-Person O O O O O B-Organization I-Organization I-Organization I-Organization B-Organization O O B-Organization I-Organization I-Organization B-Person O O O O B-Grant I-Grant I-Grant I-Grant O O B-Grant O B-Grant I-Grant O O B-Grant B-Person O O O O B-Organization I-Organization B-Grant I-Grant I-Grant B-Person O O O O O B-Grant I-Grant I-Grant I-Grant I-Grant I-Grant I-Grant O B-Telescope I-Telescope I-Telescope B-Telescope O O O B-Organization I-Organization I-Organization O O B-Location B-Observatory I-Observatory I-Observatory I-Observatory I-Observatory I-Observatory O O B-Organization I-Organization I-Organization I-Organization I-Organization O O O B-Telescope O O O O B-Collaboration O B-Collaboration O O O O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization B-Organization O O O O O O O O O B-Instrument O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization B-Organization O O O O O O B-Organization I-Organization I-Organization O B-Organization O B-Telescope I-Telescope O O O O B-Location I-Location I-Location I-Location O B-Organization I-Organization I-Organization I-Organization O O B-Location B-Observatory I-Observatory I-Observatory I-Observatory I-Observatory I-Observatory O O B-Organization I-Organization I-Organization I-Organization I-Organization O O O O O B-Location B-Organization I-Organization I-Organization I-Organization I-Organization O O O O O O B-Telescope O O O B-Location B-Observatory I-Observatory I-Observatory I-Observatory I-Observatory I-Observatory O O B-Organization I-Organization I-Organization I-Organization I-Organization O B-Grant I-Grant I-Grant B-Grant O B-Person I-Person O O O O O B-Instrument O O B-URL O O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization I-Organization O O O O O O B-Organization I-Organization I-Organization O B-Grant O B-Organization I-Organization I-Organization O B-Grant O O B-Collaboration I-Collaboration I-Collaboration B-Software O O O O O O B-Observatory I-Observatory I-Observatory I-Observatory B-Observatory O B-Location I-Location B-Observatory O O O O B-Organization I-Organization I-Organization I-Organization I-Organization I-Organization I-Organization I-Organization I-Organization O O O O O B-Organization I-Organization I-Organization O O O O O O O B-Database I-Database I-Database B-Database O O O O O B-Organization I-Organization I-Organization B-Organization I-Organization I-Organization I-Organization O O O O B-Organization I-Organization I-Organization I-Organization I-Organization B-Software O O O O O B-Person I-Person O O O O O O O O O O O O O O O O O O O O O O B-URL O