Low angular momentum flow model for Sgr A*

Abstract

We examine the low angular momentum flow model for Sgr A* using two-dimensional hydrodynamical calculations based on the parameters of the specific angular momentum and total energy estimated in the recent analysis of stellar wind of nearby stars around Sgr A*. The accretion flow with the plausible parameters is non-stationary and an irregularly oscillating shock is formed in the inner region of a few tens to a 160 Schwarzschild radii. Due to the oscillating shock, the luminosity and the mass-outflow rate are modulated by several per cent to a factor of 5 and a factor of 2-7, respectively, on time-scales of 1 h to 10 d. The flows are highly advected and the radiative efficiency of the accreting matter into radiation is very low, ∼10 −5 to 10 −3 , and the input accretion rateṀ input = 4.0 × 10 −6 M yr −1 results in the observed luminosities ∼10 36 erg s −1 of Sgr A* if a two-temperature model and the synchrotron emission are taken into account. The mass-outflow rate of the gas originating in the post-shock region increases with the increasing input specific angular momentum and ranges from a few to 99 per cent of the input accreting matter, depending on the input angular momentum. The oscillating shock is necessarily triggered if the specific angular momentum and the specific energy belong to or are located just nearby the range of parameters responsible for a stationary shock in rotating inviscid and adiabatic accretion flow. The time variability may be relevant to the flare activity of Sgr A*.

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

“…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).…”

“…Referring to this work and Okuda & Molteni (2012), we consider here a set of parameters λ = 1.35, ǫ = 1.98 × 10 −6 and a mass accretion rateṀ = 4.0 ×10 −6 M ⊙ yr −1 and examine the time-variations of the magnetized low angular momentum flow, focusing on the long-term flares of Sgr A*.InFig. 2, we show the analytical transonic solution corresponding to the above λ and ǫ, where flow after crossing the outer critical point 'a' continues to proceed along the supersonic branch 'ab' and enters the event horizon of the black hole(Okuda & Molteni 2012), where the outer critical point R a is 1.68 ×10 4 R g . However, the flow chooses to jump from point 'b' to 'c'…”

Low angular momentum flow model II for Sgr A

“…In this paper, we consider two models of λ = 1.68 and ǫ = 3.97 × 10 −6 (model A) and λ = 1.35 and ǫ = 1.98 × 10 −6 (model B) in the usual nondimensional units. The Mach number versus radius relation in the models is given in figs 1 and 2 in the previous paper (Okuda & Molteni 2012). In model A, the particle which passes through the outer sonic point falls down supersonically inwards but never attains the event horizon since it makes a closed loop of the Mach number curve.…”

“…With these flow parameters for λ and ǫ, they showed analytically that there is no continuous flow solution which attains to the event horizon, and the resulting flow would be non-stationary, but that, for the case of the angular momentum λ= 1.55 lower than the best estimates for Sgr A*, there exists a standard stationary shock solution. Motivated by their suggestion and results, we examined the low angular momentum flow model for Sgr A* using 2D time-dependent hydrodynamical calculations and discussed the results on the activity of Sgr A* (Okuda & Molteni 2012). However, in the initial model used, we assumed a constant ratio of the ion temperature Ti to the electron temperature Te throughout the region.…”

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

“…We consider three representative cases of the accretion flow with no angular momentum (case A), the low angular momentum flow (case B) and the advection-dominated accretion flow (case C), respectively. In the previous work, we have studied the low angular momentum flow model in the inviscid limit (Okuda & Molteni 2012;Okuda 2014) but we intend to re-examine here considering the effect of viscosity. The case A is studied for initial flow with flow parameters same as case B.…”