The role of viscosity in AGN outflows in relation to jet periodicities

Abstract

Aims. Adopting the Smoothed Particle Hydrodynamics (SPH) numerical method, we performed a grid of evolving models of a 3D axially-symmetric, viscous accretion disc around a supermassive black hole (SMBH) of 10 6 ÷ 10 9 M . Such sort of simulations are typical of accretion discs in active galactic nuclei (AGN). In such disc models, we pay attention to the role of the specific angular momentum λ and the turbulent viscosity parameter α, according to the Shakura and Sunyaev prescription. One or two shock fronts in the radial inviscid flow develop, according to the assigned initial kinematic and thermodynamic conditions. Methods. By fixing the initial and boundary conditions on the flow at the disc outer edge, where the total energy, the specific angular momentum λ, and the initial velocity are concerned, we find pairs of (α, λ) values determining radial periodical oscillations in the shock front. In some cases, an outflow develops from the subsonic post-shock region, close to the black hole. Results. The link between the accretion disc and the jet refueling through the onset of a centrifugal shock front is evident. We also compare model variability periods with observed ones in the radio light curves of AGN.

Collective molecular dissipation on Navier–Stokes macroscopic scales: Accretion disc viscous modeling in SPH

“…For numerical simulations of AGN, α SS ≈ 10 −4 − 10 −3 is often adopted. 26,27 Values for α SS ≈ 10 −2 have also been found for the observed protostellar objects, 28 while for a fit of FU Orionis observed outburst, 29-31 α SS ≈ 10 −3 − 3 × 10 −3 . In fully ionized discs in dwarf novae, the best observed evidence suggests a typical α SS ∼ 10 −1 − 4 × 10 −1 , whilst the relevant numerical MHD simulations evaluate α SS ≈ 10 times smaller.…”

Periodic mass loss from viscous accretion flows around black holes

“…This issue need further in- vestigation. Furthermore, while hot, dissipative flows show single shock, low energy dissipative flow showed multiple shocks (Lanzafame et al 2008;Lee et al 2011), however, the effect of high α has not been investigated. The outflow also shows quasi-periodicity, however, blobs of matter are being ejected persistently with the oscillation of inner part of the disc, and therefore, such persistent activity will eventually give rise to a stream of matter and therefore a quasi-steady mildly relativistic jet.…”

“…We would like to find the condition of the shocked disc that produces weak or strong jets. Disc instability due to viscous transport has been shown before and has been identified with QPOs (Lanzafame et al 1998(Lanzafame et al , 2008Lee et al 2011), however, we would like to show how this instability might affect the shock induced bipolar outflows. Moreover, it has been shown theoretically that the energy and angular momentum for which steady shock exists in inviscid flow, will become unstable for viscous flow (Chakrabarti & Das 2004;.…”

General relativistic numerical simulation of sub-Keplerian transonic accretion flows on to black holes: Schwarzschild space–time

“…However, it has been shown that turbulence triggered by MRI produce the value of the alpha viscosity parameter ∼ 0.01 (Brandenburg et al 1995;Hawley et al 1995Hawley et al , 1996Arlt & Rüdiger 2001;Smak 1999;King, Pringle & Livio 2007;Kotko & Lasota 2012). In the literature, many authors from various groups published results of analytical viscous solutions as well as viscous hydro-dynamic simulations where it has been shown the shock is stable if the viscosity parameter is lower than a critical viscosity parameter (Chakrabarti 1990b(Chakrabarti , 1996dChakrabarti & Molteni 1995;Lanzafame, Molteni & Chakrabarti 1998;Lanzafame et al 2008;Lee, Ryu & Chattopadhyay 2011;Das et al 2014;Lee et al 2016). Also, there have been many stability studies of shock (Nakayama 1992(Nakayama , 1994Nobuta & Hanawa 1994;Gu & Foglizzo 2003;Gu & Lu 2006), but it was shown that even under non-axisymmetric perturbations, the shock tends to persist, albeit, as a deformed shock (Molteni, Tóth, & Kuznetsov 1999).…”