Towards self-consistent modelling of the Sgr A* accretion flow: linking theory and observation

Abstract

The interplay between supermassive black holes (SMBHs) and their environments is believed to command an essential role in galaxy evolution. The majority of these SMBHs are in the radiative inefficient accretion phase where this interplay remains elusive, but suggestively important, due to few observational constraints. To remedy this, we directly fit 2D hydrodynamic simulations to Chandra observations of Sgr A* with Markov chain Monte Carlo sampling, self-consistently modelling the 2D inflowoutflow solution for the first time. We find the temperature and density at flow onset are consistent with the origin of the gas in the stellar winds of massive stars in the vicinity of Sgr A*. We place the first observational constraints on the angular momentum of the gas and estimate the centrifugal radius, r c ≈ 0.056 r b ≈ 8 × 10 −3 pc, where r b is the Bondi radius. Less than 1% of the inflowing gas accretes on to the SMBH, the remainder being ejected in a polar outflow. We decouple the quiescent point-like emission from the spatially extended flow. We find this point-like emission, accounting for ∼ 4% of the quiescent flux, is spectrally too steep to be explained by unresolved flares, nor bremsstrahlung, but is likely a combination of a relatively steep synchrotron power law and the high-energy tail of inverse-Compton emission. With this self-consistent model of the accretion flow structure, we make predictions for the flow dynamics and discuss how future X-ray spectroscopic observations can further our understanding of the Sgr A* accretion flow.

The Chandra Dust-scattering Halo of Galactic Center Transient Swift J174540.7–290015

“…Extend this to our understanding of SgrA * , with the fiducial Bondi radius of 3 7 (Wang et al 2013). The brightness from the inner scattering halo is comparable to the fraction of quiescent SgrA * flux that has been attributed to unresolved flares (10%, Neilsen et al 2013) and to point-like emission (4%, Roberts et al 2017). …”

“…Furthermore, the ISM column obtained from X-ray absorption does not match that obtained from extinction measurements in the infrared (Fritz et al 2011); they differ by a factor of two. Finally, Chandra observations have revealed that the X-ray emission from SgrA * is extended, allowing us to test theoretical models that trace the flow of hot gas as it falls into the supermassive black hole (Baganoff et al 2003;Shcherbakov & Baganoff 2010;Wang et al 2013;Roberts et al 2017). This situation is complicated by the fact that SgrA * is variable, flaring on the order of once per day for hours at a time (e.g., Neilsen et al 2013) and impacting neutral gas in the vicinity of the GC (e.g., Krivonos et al 2016).…”

A Nonthermal Radio Filament Connected to the Galactic Black Hole?

“…Consequently, most of the material accreting through the Bondi radius must be ejected in an outflow of some kind (e.g., Sadowski et al 2013;Yuan & Narayan 2014;Yuan et al 2015;Roberts et al 2017). Both jets and broadly collimated winds have been considered, but the nature of the outflow remains to be observationally determined.…”

Colliding winds in and around the stellar group IRS 13E at the galactic centre

“…Could such a radiatively inefficient IMBH explain the possible point-like component of the X-ray emission? In the hot accretion case, the spatial distributions of the plasma density and temperature of the accretion flow can be scaled with their values at the so-called Bondi radius r b ∝ M IMBH (e.g., Roberts et al 2017). This radius characterizes the size of the region in which the plasma of certain thermal and/or kinematic properties can be initially captured by the gravity of the black hole.…”

“…This radius characterizes the size of the region in which the plasma of certain thermal and/or kinematic properties can be initially captured by the gravity of the black hole. Although the Bondi accretion model itself is typically too simplistic to describe the complex accretion process, the X-ray emission from the hot gas accretion is qualitatively dominated by the outer accretion flow (e.g., Wang et al 2013;Roberts et al 2017). One may then expect that L x ∝ r 3 b n 2 e ∝ M 3 IMBH n 2 e .…”