The fast transient AT 2023clx in the nearby LINER galaxy NGC 3799 as a tidal disruption of a very low-mass star

¹ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁴ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark
⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
⁶ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁷ Institute for Gravitational Wave Astronomy, University of Birmingham, Birmingham B15 2TT, UK
⁸ School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
⁹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
¹⁰ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, København N DK-2200, Denmark
¹¹ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile
¹² School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
¹³ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, P.R. China
¹⁴ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, P.R. China
¹⁵ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, P.R. China
¹⁶ Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan
¹⁷ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis Bd Arago 75014 Paris, France
¹⁸ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹⁹ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
²⁰ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, PL-00-478 Warszawa, Poland
²¹ Isaac Newton Group of Telescopes, ING, 38700 La Palma, S.C. Tenerife, Spain
²² Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Finland
²³ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
²⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
²⁵ Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
²⁶ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
²⁷ INAF-Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, I-64100 Teramo, Italy
²⁸ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
²⁹ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, China
³⁰ Physics & Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK

Received: 20 January 2024
Accepted: 28 June 2024

Abstract

We present an extensive analysis of the optical and ultraviolet (UV) properties of AT 2023clx, the closest optical/UV tidal disruption event (TDE) to date (z = 0.01107), which occurred in the nucleus of the interacting low-ionization nuclear emission-line region (LINER) galaxy, NGC 3799. After correcting for the host reddening (E(B − V)_h = 0.179 mag), we find its peak absolute g-band magnitude to be −18.03 ± 0.07 mag, and its peak bolometric luminosity to be L_pk = (1.57 ± 0.19)×10⁴³ erg s⁻¹. AT 2023clx displays several distinctive features: first, it rose to peak within 10.4 ± 2.5 days, making it the fastest rising TDE to date. Our SMBH mass estimate of M̄_BH ≈ 10^6.0 M_⊙ –estimated using several standard methods– rules out the possibility of an intermediate-mass BH as the reason for the fast rise. Dense spectral follow-up reveals a blue continuum that cools slowly and broad Balmer and He II lines as well as weak He Iλλ5876,6678 emission features that are typically seen in TDEs. The early, broad (width ∼15 000 km s⁻¹) profile of Hα matches theoretical expectations from an optically thick outflow. A flat Balmer decrement (L_Hα/L_Hβ ∼ 1.58) suggests that the lines are collisionally excited rather than being produced via photoionisation, in contrast to typical active galactic nuclei. A second distinctive feature, seen for the first time in TDE spectra, is a sharp, narrow emission peak at a rest wavelength of ∼6353 Å. This feature is clearly visible up to 10 d post-peak; we attribute it to clumpy material preceding the bulk outflow, which manifests as a high-velocity component of Hα (−9584 km s⁻¹). Its third distinctive feature is the rapid cooling during the first ∼20 days after peak, reflected as a break in the temperature evolution. Combining these findings, we propose a scenario for AT 2023clx involving the disruption of a very low-mass star (≲0.1 M_⊙) with an outflow launched in our line of sight and with disruption properties that led to efficient circularisation and prompt accretion disc formation, observed through a low-density photosphere.