AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy

Creators: Kool, E. C.; Reynolds, T. M.; Mattila, S.; Kankare, E.; Pérez-Torres, M. A.; Efstathiou, A.; Ryder, S.; Romero-Cañizales, C.; Lu, W.; Heikkilä, T.; Anderson, G. E.; Berton, M.; Bright, J.; Cannizzaro, G.; Eappachen, D.; Fraser, M.; Gromadzki, M.; Jonker, P. G.; Kuncarayakti, H.; Lundqvist, P.; Maeda, K.; McDermid, R. M.; Medling, A. M.; Moran, S.; Reguitti, A.; Shahbandeh, M.; Tsygankov, S.; U, V.; Wevers, T.

Abstract

We present the discovery with Keck of the extremely infrared (IR) luminous transient AT 2017gbl, coincident with the Northern nucleus of the luminous infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multiwavelength follow-up spans ∼900 d, including photometry and spectroscopy in the optical and IR, and (very long baseline interferometry) radio and X-ray observations. Radiative transfer modelling of the host galaxy spectral energy distribution and long-term pre-outburst variability in the mid-IR indicate the presence of a hitherto undetected dust obscured active galactic nucleus (AGN). The optical and near-IR spectra show broad ∼2000 km s⁻¹ hydrogen, He I, and O I emission features that decrease in flux over time. Radio imaging shows a fast evolving compact source of synchrotron emission spatially coincident with AT 2017gbl. We infer a lower limit for the radiated energy of 7.3 × 10⁵⁰ erg from the IR photometry. An extremely energetic supernova would satisfy this budget, but is ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is related to an accretion event by the central supermassive black hole, where the spectral signatures originate in the AGN broad line region and the IR photometry is consistent with re-radiation by polar dust. Given the fast evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a more plausible scenario than a dramatic change in the AGN accretion rate. This makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to the so far considered TDE population discovered at optical wavelengths and hosted preferably by post-starburst galaxies.

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 August 3. Received 2020 August 3; in original form 2020 March 5. Published: 19 August 2020. We thank the anonymous referee for insightful comments and suggestions. We would like to thank Petri Väisänen and Seppo Laine for useful discussions on LIRG properties and WISE data analysis, respectively. We thank David Williams for scheduling the final AMI-LA epoch and Chi-Ho Chan for comments on the submitted version of the manuscript. ECK acknowledges support from the Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT) research environment funded by Vetenskapsrädet under project no. 2016-06012, financial support from the visitor and mobility program of the Finnish Centre for Astronomy with ESO (FINCA), funded by the Academy of Finland grant no. 306531, and support from The Wenner-Gren Foundations under project no. UPD2019-0070. TMR acknowledges the financial support of the Jenny and Antti Wihuri foundation and the Vilho, Yrjö and Kalle Väisälä Foundation of the Finnish Academy of Science and Letters. MPT acknowledges financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and through grant PGC2018-098915-B-C21 (MCI/AEI/FEDER). CRC acknowledges support by the Chinese Academy of Sciences (CAS), through grant CAS16013 of the CAS South America Center for Astronomy (CASSACA) and Programa de Astronomía CONICYT, Chile. PGJ and GC acknowledge support from European Research Council Consolidator Grant 647208. MG is supported by the Polish NCN MAESTRO grant 2014/14/A/ST9/00121. GEA is the recipient of an Australian Research Council Discovery Early Career Researcher Award (project DE180100346) and acknowledges partial support through the Australian Research Council's Discovery Projects funding scheme (project DP200102471). ST acknowledges financial support from the Russian Foundation for Basic Research project 17-52-80139 BRICS-a. RMcD is the recipient of an Australian Research Council Future Fellowship Award (project number FT150100333). WL is supported by the David and Ellen Lee Fellowship at Caltech. The NOT Unbiased Transient Survey 2 (NUTS2) is funded in part by the Instrument Center for Danish Astronomy. This work is based in part 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. Some of the data (PI: S. Ryder; program IDs Z229N2L, Z271N2L) 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. We thank the Mullard Radio Astronomy Observatory for carrying out the AMI-LA observations. This work is based in part on observations (PIs: E. Kool, K. Maeda; program IDs GN-2017B-DD-2, GN-2018B-FT-109) obtained at 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), National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). The scientific results reported in this article are based in part on observations (PI: T. Heikkilä; program ID 18208589) made by the Chandra X-ray Observatory, and this research has made use of the CIAO software package provided by the Chandra X-ray Center (CXC). This article includes results based on observations made by the Neil Gehrels Swift observatory. The authors thank Chandra and Swift teams for the approval and rapid scheduling of our observations. This work is based in part on observations made with the William Herschel Telescope (WHT). The WHT 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. The ISIS and ACAM data were obtained as part of (17A)N4/N6, (18A)N4, and (18B)N5. This work is based in part on observations (PI: S. Mattila; program IDs 13226, 14054) made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication also makes use of data products from NEOWISE, which is a project of the Jet Propulsion Laboratory/California Institute of Technology, funded by the Planetary Science Division of the National Aeronautics and Space Administration. This work is based in part on observations from programs RP028B and BP225 (PI: M. Pérez-Torres) obtained with the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA), respectively. The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. 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 Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington. Data Availability: The photometric data underlying this article are available in the article. The spectroscopic data will be made available on WISeREP12 under the designation AT 2017gbl.

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