A Family Tree of Optical Transients from Narrow-line Seyfert 1 Galaxies

Creators: Frederick, Sara; Gezari, Suvi; Graham, Matthew J.; Sollerman, Jesper; van Velzen, Sjoert; Perley, Daniel A.; Stern, Daniel; Ward, Charlotte; Hammerstein, Erica; Hung, Tiara; Yan, Lin; Andreoni, Igor; Bellm, Eric C.; Duev, Dmitry A.; Kowalski, Marek; Mahabal, Ashish A.; Masci, Frank J.; Medford, Michael; Rusholme, Ben; Smith, Roger; Walters, Richard

Abstract

The Zwicky Transient Facility (ZTF) has discovered five events (0.01 < z < 0.4) belonging to an emerging class of active galactic nuclei (AGNs) undergoing smooth, large-amplitude, and rapidly rising flares. This sample consists of several transients initially classified as supernovae with narrow spectral lines. However, upon closer inspection, all of the host galaxies display Balmer lines with FWHM(Hβ) ∼ 900–1400 km s⁻¹, characteristic of a narrow-line Seyfert 1 (NLSy1) galaxy. The transient events are long lived, over 400 days on average in the observed frame. We report UV and X-ray follow-up of the flares and observe persistent UV emission, with two of the five transients detected with luminous X-ray emission, ruling out a supernova interpretation. We compare the properties of this sample to previously reported flaring NLSy1 galaxies and find that they fall into three spectroscopic categories: 1) Balmer line profiles and Fe II complexes typical of NLSy1s, 2) strong He II profiles, and 3) He II profiles including Bowen fluorescence features. The latter are members of the growing class of AGN flares attributed to enhanced accretion reported by Trakhtenbrot et al. We consider physical interpretations in the context of related transients from the literature. For example, two of the sources show high-amplitude rebrightening in the optical, ruling out a simple tidal disruption event scenario for those transients. We conclude that three of the sample belong to the Trakhtenbrot et al. class and two are tidal disruption events in NLSy1s. We also hypothesize as to why NLSy1s are preferentially the sites of such rapid enhanced flaring activity.

Additional Information

© 2021. The American Astronomical Society. Received 2020 October 16; revised 2021 June 30; accepted 2021 July 1; published 2021 October 12. We acknowledge and thank our reviewer for thorough and helpful feedback, which greatly enhanced the clarity of the presentation of this work. We would like to thank S. Mattila, L. Foschini, E.F. Borra, and A. Dittmann for useful comments. S.G. is supported in part by NSF CAREER grant 1454816. The work of D.S. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We thank C. Barbarino for reducing the Nordic Optical Telescope observation of AT2019fdr. We thank R. Foley for contributing to the Lick Shane KAST observation of AT2019pev. Based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. This work was supported by the GROWTH project funded by the National Science Foundation under grant No. 1545949. The SED Machine is based upon work supported by the National Science Foundation under grant No. 1106171. The ZTF forced photometry service was funded under the Heising-Simons Foundation grant No. 12540303 (PI: Graham). These results made use of the Discovery Channel Telescope at Lowell Observatory. Lowell is a private, nonprofit 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. Based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku, and the University of Oslo, representing Denmark, Finland, and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. We acknowledge the use of public data from the Swift data archive. 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. Facilities: PO:1.2m - Palomar Observatory's 1.2 meter Samuel Oschin Telescope, PO:1.5m - , Hale - , Swift(XRT and UVOT) - , DCT - , NOT - , Shane - , Liverpool:2m. - Software: Pyraf (Science Software Branch at STScI 2012), Lmfit (Newville et al. 2016), HEAsoft (NASA High Energy Astrophysics Science Archive Research Center (Heasarc) 2014), PIMMS (Mukai 1993).

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