SDSS1335+0728: The awakening of a ∼10⁶ M_⊙ black hole^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: psanchez@eso.org
² Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
³ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña No. 1111, Playa Ancha, Valparaíso, Chile
⁴ Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), 4030000 Concepción, Chile
⁵ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁶ Centro de Astroingeniería, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁸ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
⁹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹⁰ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
¹¹ California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹² Gemini Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile
¹³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁴ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36D, Santiago, Chile
¹⁶ Astronomy Department, Universidad de Concepción, Barrio Universitario s/n, Concepción 4030000, Chile
¹⁷ Center for Mathematical Modeling, University of Chile, AFB170001, Chile
¹⁸ INAF – Institute for Space Astrophysics and Planetology, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁹ Data and Artificial Intelligence Initiative (ID&IA), University of Chile, Santiago, Chile
²⁰ Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA

Received: 13 September 2023
Accepted: 15 May 2024

Abstract

Context. The early-type galaxy SDSS J133519.91+072807.4 (hereafter SDSS1335+0728), which had exhibited no prior optical variations during the preceding two decades, began showing significant nuclear variability in the Zwicky Transient Facility (ZTF) alert stream from December 2019 (as ZTF19acnskyy). This variability behaviour, coupled with the host-galaxy properties, suggests that SDSS1335+0728 hosts a ∼10⁶ M_⊙ black hole (BH) that is currently in the process of “turning on”.

Aims. We present a multi-wavelength photometric analysis and spectroscopic follow-up performed with the aim of better understanding the origin of the nuclear variations detected in SDSS1335+0728.

Methods. We used archival photometry (from WISE, 2MASS, SDSS, GALEX, eROSITA) and spectroscopic data (from SDSS and LAMOST) to study the state of SDSS1335+0728 prior to December 2019, and new observations from Swift, SOAR/Goodman, VLT/X-shooter, and Keck/LRIS taken after its turn-on to characterise its current state. We analysed the variability of SDSS1335+0728 in the X-ray/UV/optical/mid-infrared range, modelled its spectral energy distribution prior to and after December 2019, and studied the evolution of its UV/optical spectra.

Results. From our multi-wavelength photometric analysis, we find that: (a) since 2021, the UV flux (from Swift/UVOT observations) is four times brighter than the flux reported by GALEX in 2004; (b) since June 2022, the mid-infrared flux has risen more than two times, and the W1 − W2 WISE colour has become redder; and (c) since February 2024, the source has begun showing X-ray emission. From our spectroscopic follow-up, we see that (i) the narrow emission line ratios are now consistent with a more energetic ionising continuum; (ii) broad emission lines are not detected; and (iii) the [OIII] line increased its flux ∼3.6 years after the first ZTF alert, which implies a relatively compact narrow-line-emitting region.

Conclusions. We conclude that the variations observed in SDSS1335+0728 could be either explained by a ∼10⁶ M_⊙ AGN that is just turning on or by an exotic tidal disruption event (TDE). If the former is true, SDSS1335+0728 is one of the strongest cases of an AGN observed in the process of activating. If the latter were found to be the case, it would correspond to the longest and faintest TDE ever observed (or another class of still unknown nuclear transient). Future observations of SDSS1335+0728 are crucial to further understand its behaviour.