The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole?

Creators: Perley, Daniel A.; Yan, Lin; Blagorodnova, Nadia; Fremling, Christoffer; Coughlin, Michael W.; De, Kishalay; Drake, Andrew J.; Duev, Dmitry A.; Heida, Marianne; Ho, Anna Y. Q.; Jencson, Jacob E.; Kasliwal, Mansi M.; Kulkarni, S. R.; Mahabal, Ashish; Neill, James D.; Tinyanont, Samaporn

Abstract

Wide-field optical surveys have begun to uncover large samples of fast (t_(rise) ≲ 5 d), luminous (M_(peak) < −18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered detailed investigation of their properties. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT 2018cow (ATLAS 18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between 3 and 8 days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R < 10^(14) cm after 1 month). This behaviour does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT 2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins.

Additional Information

© 2018 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 2018 December 10. Received 2018 December 10; in original form 2018 August 3. Published: 22 December 2018. DAP acknowledges useful discussions with Matt Darnley and Helen Jermak. We thank the referee for helpful comments which improved the quality of this paper. This work was supported by the GROWTH project funded by the National Science Foundation under Grant No 1545949. GROWTH is a collaborative project between California Institute of Technology, Pomona College, San Diego State University, Los Alamos National Laboratory, University of Maryland College Park, University of Wisconsin Milwaukee (USA), Tokyo Institute of Technology (Japan), National Central University (Taiwan), Indian Institute of Astrophysics (India), Weizmann Institute of Science (Israel), The Oskar Klein Centre at Stockholm University (Sweden), Humboldt University (Germany), and Liverpool John Moores University (UK). This paper used the GROWTH marshal to filter alerts and co-ordinate follow-up. FT and JS gratefully acknowledge support from the Knut and Alice Wallenberg Foundation. JS acknowledges the support of Vetenskapsrådet through VR grants 2012-2265 and 2017-03699. AYQH was supported by a National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144469. MC was supported by the David and Ellen Lee Postdoctoral Fellowship at the California Institute of Technology. JSB was supported by a Data-Driven Discovery grant from the Moore Foundation. C-C Ngeow and P-C Yu thank the Ministry of Science and Technology (MoST, Taiwan) for funding under grant 104-2923-M-008-004-MY5 and 106-2112-M-008-007. This publication has made use of data collected at Lulin Observatory, partly supported by MoST grant 105-2112-M-008-024-MY3. RI is supported by JSPS and NSF under the JSPS-NSF Partnerships for International Research and Education (PIRE), YT and NK supported by JSPS KAKENHI Grant Numbers JP16J05742 and JP17H06362. YT is also financially supported by Academy for Global Leadership (AGL) of Tokyo Institute of Technology. MITSuME Akeno 50cm telescope is also supported by the joint research program of the Institute for Cosmic Ray Research (ICRR), the University of Tokyo, and Optical and Near-Infrared Astronomy Inter-University Cooperation Program in Japan (KLM). GCA and VB acknowledge the support of the Science and Engineering Research Board, Department of Science and Technology, India and the Indo-US Science and Technology Forum for the GROWTH-India project. BK acknowledges the Science and Engineering Research Board under the Department of Science & Technology, Govt. of India, for financial assistance in the form of National Post-Doctoral Fellowship (PDF/2016/001563) and BRICS grant DST/IMRCD/BRICS/PilotCall1/MuMeSTU/2017(G). 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. We acknowledge helpful support from the entire LT staff, including Robert Smith, Jon Marchant, and Iain Steele, and to the LT review panel for approving our requests for Reactive time (JQ18A01). We acknowledge the use of public data from the Swift data archive, and thank the Swift team for executing such a thorough observing campaign of this transient. This work is partly based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. 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 William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. These results made use of the Discovery Channel Telescope at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the DCT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. The upgrade of the DeVeny optical spectrograph has been funded by a generous grant from John and Ginger Giovale. We thank Debra Fischer for graciously assisting with our Target of Opportunity request at the DCT. This work is partly based on observations made with the Kitt Peak EMCCD Demonstrator (KPED) camera on the Kitt Peak 84 inch telescope. The KPED team thanks the National Science Foundation, discretionary funds of SRK, and donors to SRK for support in the building and operation of KPED. In addition, they thank the Chimera project for use of the EMCCD. Some of the data used in this paper were acquired with the COATLI telescope and interim instrument, and with the RATIR instrument on the 1.5-meter Harold L. Johnson telescope; both at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, México. COATLI is funded by CONACyT (LN 232649, 260369, and 271117) and the Universidad Nacional Autónoma de México (CIC and DGAPA/PAPIIT IT102715, IG100414, and IN109408). RATIR is funded by the University of California and NASA Goddard Space Flight Center. COATLI and the Johnson 1.5-m telescope are operated and maintained by the Observatorio Astronómico Nacional and the Instituto de Astronomía of the Universidad Nacional Autónoma de México. We acknowledge the contribution of Leonid Georgiev and Neil Gehrels to the development of RATIR. Based partially on data from the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). Data were taken under program GN-2018A-Q-902 and acquired through the Gemini Observatory Archive. Some data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The 2m Himalayan Chandra Telescope at the Indian Astronomical Observatory is operated by the Indian Institute of Astrophysics. Support from IAO staff and HCT observation assistants is acknowledged. We also thank all HCT observers who provided part of their time for the observations. The HCT data of 22, 23, 25 and 29 June were obtained under the ToO proposals of both D. K. Sahu (PI) and F. Sutaria (PI). AstroSat is a dedicated multi-wavelength space observatory funded and facilitated by the Indian Space Research Organisation (ISRO). We thank the AstroSat ToO Time Allocation Committee for granting ToO time, and the operations team for carrying out these observations. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. 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, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. Finally, we thank the developers and maintainers of the Open Supernova Catalog and the Weizmann Spectral Repository, whose databases greatly facilitated the comparisons of this event to other transients.

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