Fast giant flares in discs around supermassive black holes

Abstract

We studied the thermal stability of non-self-gravitating turbulent $\alpha$-discs around supermassive black holes (SMBHs) to test a new type of high-amplitude galactic nucleus flares. 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 $\xi$. 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 $\sim 10^6-10^8\, M_\odot$ if the prior geometrically thin and optically thick disc surrounded a central radiatively inefficient accretion flow. An increase in the viscosity parameter $\alpha$ 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 estimates, the mass of about $\sim 4-3000\, M_\odot$ 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 $10^7\, M_\odot$. The accretion rate on the SMBH peaks about 10 times faster at the potentially super-Eddington level. An optically thick outflow leads to anisotropy of the emission. At the beginning of the giant flare, the region near the truncation radius is heated to $\sim 10^5\,$K, and its UV/optical luminosity is at least $\sim 0.3-4 \,L_\mathrm{Edd}$ depending on the SMBH mass. 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.