Unstable mass-outflows in geometrically thick accretion flows around black holes

Abstract

Accretion flows around black holes generally result in mass-outflows that exhibit irregular behavior quite often. Using 2D time-dependent hydrodynamical calculations, we show that the mass-outflow is unstable in the cases of thick accretion flows such as the low angular momentum accretion flow and the advection-dominated accretion flow. For the low angular momentum flow, the inward accreting matter on the equatorial plane interacts with the outflowing gas along the rotational axis and the centrifugally supported oblique shock is formed at the interface of both the flows, when the viscosity parameter α is as small as α 10 −3 . The hot and rarefied blobs, which result in the eruptive mass-outflow, are generated in the inner shocked region and grow up toward the outer boundary. The advection-dominated accretion flow attains finally in the form of a torus disc with the inner edge of the disc at 3R g r 6R g and the center at 6R g r 10R g , and a series of hot blobs is intermittently formed near the inner edge of the torus and grows up along the outer surface of the torus. As a result, the luminosity and the mass-outflow rate are modulated irregularly where the luminosity is enhanced by 10−40% and the mass-outflow rate is increaed by a factor of few up to ten. We interpret the unstable nature of the outflow to be due to the KelvinHelmholtz instability, examining the Richardson number for the Kelvin-Helmholtz criterion in the inner region of the flow. We propose that the flare phenomena of Sgr A* may be induced by the unstable mass-outflow as is found in this work.

A possible model for the long-term flares of Sgr A*

“…We notice that when the outer boundary in the 2D hydrodynamical simulation is chosen not far from the analytically obtained shock location, the numerical solution agrees well with the analytical one in terms of shock position. However, if the outer boundary is located further away, the difference between the numerical and analytical shock locations becomes significant (Okuda & Molteni 2012;Okuda & Das 2015). This difference appears not because of the numerical scheme, but mainly attributed due to the assumption of the vertical hydrostatic equilibrium in analytical approach which is not strictly valid and eventually leads to the incorrect transonic solutions.…”

“…Also, the standing shock models of the low angular momentum flow have been investigated and applied to Sgr A* (Chakrabarti 1996;Mościbrodzka et al 2006;Czerny & Mościbrodzka 2008). Motivated by their works, we examined the low angular 3 momentum flow model for Sgr A* using 2D time-dependent hydrodynamic calculations and discussed the implication of their results on the activity of Sgr A* (Okuda & Molteni 2012;Okuda 2014;Okuda & Das 2015). On the other hand, the observational spectra of Sgr A* show a synchrotron emission component which is presumably driven by the magnetic field around Sgr A* (Ponti et al 2017).…”

Dynamical structure of magnetized dissipative accretion flow around black holes

“…According to the second law of thermodynamics, accretion solutions containing shock waves are preferred as they possess high entropy content (Becker & Kazanas 2001). Presence of shock waves in an accretion disc around black hole has been confirmed both the-oretically (Fukue 1987;Chakrabarti 1989Das et al 2001a;Lu et al 1999) as well as numerically (Molteni et al 1994(Molteni et al , 1996Das et al 2014;Okuda 2014;Okuda & Das 2015). Due to shock compression, the post-shock matter, equivalently post-shock corona (PSC), becomes hot and dense compared to the pre-shock matter and eventually PSC behaves like an effective boundary layer of the black hole.…”

“…Solutions particularly of this kind are potentially interesting as they may possess centrifugally supported shock waves. The presence of shock wave in an accretion flow has profound implications as it satisfactorily delineates the spectral and temporal behaviour of numerous black hole sources (Chakrabarti 1989(Chakrabarti , 1990Molteni et al 1994Molteni et al , 1996Becker & Kazanas 2001;Lu et al 1999;Das et al 2001a;Le & Becker 2004;Gu & Lu 2004;Le & Becker 2005;Becker et al 2008;Nagakura & Yamada 2009;Nandi et al 2012;Das et al 2009Das et al , 2014Okuda 2014;Iyer et al 2015;Okuda & Das 2015;Aktar et al 2015;Suková & Janiuk 2015). Thus, in this work we intend to study the properties of magnetically supported accretion solutions that possesses shock waves.…”

Standing shocks in magnetized advection accretion flows onto a rotating black hole

“…In the theoretical front, the shock induced global accretion solution around black hole and its implications are extensively studied by the numerous groups of workers (Fukue 1987;Chakrabarti 1989Chakrabarti , 1996bLu, Gu, & Yuan 1999;Gu & Lu 2001;Das et al 2001b;Gu & Lu 2004;Fukumura & Tsuruta 2004;Chakrabarti & Das 2004;Das 2007;Becker, Das, & Le 2008;Das, Becker, & Le 2009;Das, Chakrabarti, & Mondal 2010;Sarkar & Das 2015;Aktar, Das, & Nandi 2015;Sarkar & Das 2016;Aktar et al 2017;Sarkar, Das, & Mandal 2018). In addition, the existence of shock in accretion flow is also examined numerically considering hydrodynamics (Chakrabarti & Molteni 1993;Molteni, Lanzafame, & Chakrabarti 1994;Ryu, Chakrabarti, & Molteni 1997;Okuda 2014;Okuda & Das 2015;Suková & Janiuk 2015;Suková, Charzyński, & Janiuk 2017) as well as magnetohydrodynamic (MHD) environment (Nishikawa et al 2005;Takahashi et al 2006;Fukumura, Takahashi, & Tsuruta 2007;Fukumura et al 2016).…”