Variable accretion and emission from the stellar winds in the Galactic Centre

Abstract

We present numerical simulations of stellar wind dynamics in the central parsec of the Galactic Centre, studying in particular the accretion of gas on to Sgr A * , the supermassive black hole. Unlike our previous work, here we use state-of-the-art observational data on orbits and wind properties of individual wind-producing stars. Since wind velocities were revised upwards and non-zero eccentricities were considered, our new simulations show fewer clumps of cold gas and no conspicuous disc-like structure. The accretion rate is dominated by a few close 'slow-wind stars' (v w 750 km s −1 ), and is consistent with the Bondi estimate, but variable on time-scales of tens to hundreds of years. This variability is due to the stochastic infall of cold clumps of gas, as in earlier simulations, and to the eccentric orbits of stars. The present models fail to explain the high luminosity of Sgr A * a few hundred years ago implied by Integral observations, but we argue that the accretion of a cold clump with a small impact parameter could have caused it. Finally, we show the possibility of constraining the total mass-loss rate of the 'slow-wind stars' using near infrared observations of gas in the central few arcseconds.

The long-term millimeter activity of active galactic nuclei

“…Koyama et al 2008) and theoretical (e.g. Cuadra et al 2008) evidence suggests activity timescales of several hundred years. An even more extreme example is provided by recent observations of "Hanny's Voorwerp" which suggest flux variations of the galactic nucleus of IC 2497 by factors >100 on timescales <70 000 years (e.g., Schawinski et al 2010).…”

SGR A* and Its Environment: Low-Mass Star Formation, the Origin of X-Ray Gas and Collimated Outflow

“…The estimated mass accretion rate onto Sgr A * is several orders of magnitude smaller than the rate at which young, windy stars in the innermost 0.5 pc supply mass to the Bondi radius of Sgr A * (Coker & Melia 1997;Cuadra et al 2006Cuadra et al , 2008. Chandra observations have characterized the X-ray emission surrounding SgrA * is being spatially extended with a radius of ∼1 5 (Baganoff et al 2003;Wang et al 2013).…”

“…This gas is generally thought to be supplied by the combined winds of the mass-losing young stars in the central parsec of the Galaxy, which are estimated to supply material to Sgr A * at a rate of a few times 10 −6 - M yr 1 (Coker & Melia 1997;Rockefeller et al 2004;Cuadra et al 2006Cuadra et al , 2008. However, the discovery of populations of young stars within the S-cluster which consists of ∼16 B dwarfs and 3 O stars on highly eccentric orbits within 1″, begs the question whether mass loss from these stars plays a dominant role in supplying gas to the vicinity of Sgr A * .…”

Oscillating shocks in the low angular momentum flows as a source of variability of accreting black holes

“…Recently, several papers have been published where the low angular momentum model for Sgr A* accretion was proposed and the amount of angular momentum from wind of the Wolf-Rayet star IRS 13 E3 was estimated to have quite low values λ ∈ (1.68÷2.16) R Schw c ∼ (3.36÷4.32)M (Mościbrodzka et al 2006;Okuda & Molteni 2012). However, other 3D simulations show considerably higher values of angular momentum of stellar wind at the Bondi radius ∼ 10 4 M (Cuadra et al 2008;Yuan & Narayan 2014). It is also questionable if the two component advective model possessing both high and low angular momentum segment can apply to Sgr A* accretion.…”