Fast giant flares in discs around supermassive black holes

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³ McWilliams Center for Cosmology & Astrophysics, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
⁴ Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 W. Green St., IL 61801, USA

^⋆ Corresponding author: gvlipunova@gmail.com

Received: 11 April 2024
Accepted: 3 June 2025

Abstract

Aims. We studied the thermal stability of non-self-gravitating turbulent α-discs around supermassive black holes (SMBHs) to test a new type of high-amplitude galactic nucleus flares.

Methods. By calculating the disc structures, we computed the critical points of equilibrium curves for discs around SMBHs, which cover a wide range of accretion rates and resemble the shape of a ξ curve.

Results. We find that a transition of a disc ring from a recombined cold state to a hot, fully ionised, advection dominated, geometrically thick state is possible. Such a transition can trigger a giant flare for SMBHs with masses ∼10⁶ − 10⁸ M_⊙ if the prior geometrically thin and optically thick disc with convective energy transport surrounded a central radiatively inefficient accretion flow. An increase in the viscosity parameter α is a necessary condition for this scenario. This increase may be related to the fact that the magnetic Prandtl number increases and exceeds 1 during ionisation. When self-gravity effects in the disc are negligible, the duration and power of the flare exhibit a positive correlation with the prior truncation radius of the geometrically thin disc. According to our rough estimates, the mass of about ∼4 − 3000 M_⊙ can be involved in the giant flare lasting 1 to 400 years if the flare is triggered somewhere between 60 and 600 gravitational radii from the SMBH of 10⁷ M_⊙. The accretion rate on the SMBH peaks about ten times faster at the potentially super-Eddington level. An optically thick outflow with the comparable mass loss rate leads to anisotropy of the emission. At the beginning of the giant flare, the region near the truncation radius is heated to ∼10⁵ K, and its UV/optical luminosity is at least ∼0.3 − 4 L_Edd depending on the SMBH mass.

Conclusions. The sudden heating of a cold disc around a SMBH can trigger a massive outburst, similar in appearance to what is proposed to occur after a tidal disruption event.