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The Nascent Milliquasar VT J154843.06+220812.6: Tidal Disruption Event or Extreme Accretion State Change?

Somalwar, Jean J. and Ravi, Vikram and Dong, Dillon and Graham, Matthew and Hallinan, Gregg and Law, Casey and Lu, Wenbin and Myers, Steven T. (2022) The Nascent Milliquasar VT J154843.06+220812.6: Tidal Disruption Event or Extreme Accretion State Change? Astrophysical Journal, 929 (2). Art. No. 184. ISSN 0004-637X. doi:10.3847/1538-4357/ac5e29.

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We present a detailed multiwavelength follow-up of the nuclear radio flare VT J154843.06+220812.6, hereafter VT J1548. VT J1548 was selected as a ∼1 mJy radio flare in 3 GHz observations from the Very Large Array Sky Survey. It is located in the nucleus of a low-mass (log M_(BH)/M⊙ ∼ 6) host galaxy with weak or no past active galactic nuclei (AGN) activity. VT J1548 is associated with a slow rising (multiple year), bright mid-IR flare in the Wide-field Infrared Survey Explorer survey, peaking at ∼10%L_(edd). No associated optical transient is detected, although we cannot rule out a short, early optical flare given the limited data available. Constant late-time (∼3 yr post-flare) X-ray emission is detected at ∼10⁴² erg s⁻¹. The radio spectral energy distribution is consistent with synchrotron emission from an outflow incident on an asymmetric medium. A follow-up, optical spectrum shows transient, bright, high-ionization coronal line emission ([Fe XI] λ6375, [Fe XI] λ7894, [S XII] λ7612). Transient broad Hα is also detected but without corresponding broad Hβ emission, suggesting high nuclear extinction. We interpret this event as either a tidal disruption event or an extreme flare of an AGN, in both cases obscured by a dusty torus. Although these individual properties have been observed in previous transients, the combination is unprecedented. This event highlights the importance of searches across all wave bands for assembling a sample of nuclear flares that spans the range of observable properties and possible triggers.

Item Type:Article
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URLURL TypeDescription Paper
Somalwar, Jean J.0000-0001-8426-5732
Ravi, Vikram0000-0002-7252-5485
Dong, Dillon0000-0001-9584-2531
Graham, Matthew0000-0002-3168-0139
Hallinan, Gregg0000-0002-7083-4049
Law, Casey0000-0002-4119-9963
Lu, Wenbin0000-0002-1568-7461
Additional Information:© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 August 27; revised 2022 February 28; accepted 2022 March 14; published 2022 April 27. We would like to thank Sterl Phinney for useful discussions. We would like to thank Phil Hopkins for insight into the formation and evolution of AGN tori. Finally, we would like to thank the entire ZTF TDE/AGN group for useful discussions, and in particular, Suvi Gezari and Sjoert van Velzen. C.J.L. acknowledges support from the National Science Foundation under grant No. 2022546. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. CIRADA is funded by a grant from the Canada Foundation for Innovation 2017 Innovation Fund (Project 35999), as well as by the Provinces of Ontario, British Columbia, Alberta, Manitoba and Quebec. Some of the 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. 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 website is 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. Based on observations obtained with the Samuel Oschin 48 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. Facility: Swift(XRT), Keck I(LRIS), Keck II(ESI), VLA, WISE, XMM-Newton(EPIC). Software: Astropy (Robitaille et al. 2013; Astropy Collaboration et al. 2018), Matplotlib (Hunter 2007), NumPy (Harris et al. 2020; van der Walt et al. 2011), Pandas (pandas development team 2020; McKinney 2010), SciPy (Virtanen et al. 2020), dynesty (Speagle 2020; Skilling 2004; Higson et al. 2019), emcee (Foreman-Mackey et al. 2013), pybdsf (Mohan & Rafferty 2015).
Group:Astronomy Department, Zwicky Transient Facility
Funding AgencyGrant Number
Canada Foundation for Innovation35999
W. M. Keck FoundationUNSPECIFIED
Alfred P. Sloan FoundationUNSPECIFIED
Participating InstitutionsUNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Japanese MonbukagakushoUNSPECIFIED
Max Planck SocietyUNSPECIFIED
Higher Education Funding Council for EnglandUNSPECIFIED
ZTF partner institutionsUNSPECIFIED
Subject Keywords:Tidal disruption; Active galactic nuclei; Radio transient sources; X-ray transient sources; Transient sources
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Tidal disruption (1696); Active galactic nuclei (16); Radio transient sources (2008); X-ray transient sources (1852); Transient sources (1851)
Record Number:CaltechAUTHORS:20220125-215134238
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Official Citation:Jean J. Somalwar et al 2022 ApJ 929 184
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:113104
Deposited By: George Porter
Deposited On:26 Jan 2022 15:14
Last Modified:03 May 2022 19:20

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