AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system

¹ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
² Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS) Chile
³ Instituto de Física y Astronomía, Facultad de Ciencias,Universidad de Valparaíso, Gran Bretana 1111, Playa Ancha, Valparaíso, Chile
⁴ Center for Mathematical Modeling, Universidad de Chile, Beauchef 851, Santiago 8370456, Chile
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36D, Santiago, Chile
⁶ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, Roma 00133, Italy
⁷ Departamento de Ciencias, Facultad de Artes Liberales, Universidad Adolfo Ibáñez, Av. Padre Hurtado 750, Viña del Mar, Chile
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna, 4860 Macul Santiago, Chile
¹⁰ Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago, Chile
¹¹ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
¹² Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
¹⁴ Data Observatory Foundation, Santiago, Chile
¹⁵ Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 001, India
¹⁶ Department of Physics, Deen Dayal Upadhyaya Gorakhpur University, Gorakhpur-273009, India
¹⁷ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Tonantzintla, Puebla 72840, México
¹⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁹ Instituto de Astrofísica, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
²⁰ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
²¹ Data and Artificial Intelligence Initiative (IDIA), Faculty of Physical and Mathematical Sciences, Universidad de Chile, Chile
²² Department of Astronomy, The Oskar Klein Center, Stockholm University, AlbaNova 106 91, Stockholm, Sweden
²³ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, I-10125 Torino, Italy
²⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
²⁵ Indian Institute of Astrophysics, Koramangala 2nd Block, Bangalore 560034, India

^⋆ Corresponding author; lorena.hernandez@uv.cl

Received: 28 June 2024
Accepted: 10 October 2024

Abstract

With a growing number of facilities able to monitor the entire sky and produce light curves with a cadence of days, in recent years there has been an increased rate of detection of sources whose variability deviates from standard behavior, revealing a variety of exotic nuclear transients. The aim of the present study is to disentangle the nature of the transient AT 2021hdr, whose optical light curve used to be consistent with a classic Seyfert 1 nucleus, which was also confirmed by its optical spectrum and high-energy properties. From late 2021, AT 2021hdr started to present sudden brightening episodes in the form of oscillating peaks in the Zwicky Transient Facility (ZTF) alert stream, and the same shape is observed in X-rays and UV from Swift data. The oscillations occur every ∼60–90 days with amplitudes of ∼0.2 mag in the g and r bands. Very Long Baseline Array (VLBA) observations show no radio emission at milliarcseconds scale. It is argued that these findings are inconsistent with a standard tidal disruption event (TDE), a binary supermassive black hole (BSMBH), or a changing-look active galactic nucleus (AGN); neither does this object resemble previous observed AGN flares, and disk or jet instabilities are an unlikely scenario. Here, we propose that the behavior of AT 2021hdr might be due to the tidal disruption of a gas cloud by a BSMBH. In this scenario, we estimate that the putative binary has a separation of ∼0.83 mpc and would merge in ∼7 × 10⁴ years. This galaxy is located at 9 kpc from a companion galaxy, and in this work we report this merger for the first time. The oscillations are not related to the companion galaxy.