Towards Self-Consistent Modelling of the Sgr A* Accretion Flow: Linking Theory and Observation

TL;DR

This paper presents a self-consistent 2-D hydrodynamic model of Sgr A*'s accretion flow, fitting observational data to constrain flow properties, and distinguishes between different emission components to improve understanding of SMBH accretion physics.

Contribution

It introduces the first self-consistent 2-D hydrodynamic simulation fitting Chandra data, constraining angular momentum and flow structure of Sgr A*'s accretion flow.

Findings

Less than 1% of inflowing gas accretes onto SMBH.

Point-like emission is likely a combination of synchrotron and inverse-Compton processes.

The angular momentum of the gas is constrained, with a centrifugal radius of about 8×10^{-3} pc.

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 2-D hydrodynamic simulations to Chandra observations of Sgr A* with Markov Chain Monte Carlo sampling, self-consistently modelling the 2-D inflow-outflow 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$_c$ $\approx$ 0.056 r$_b$ $\approx8\times10^{-3}$ pc, where r$_b$ is the Bondi radius. Less than 1\% of the inflowing gas accretes onto the SMBH, the remainder being ejected in a polar outflow. For the first time, we decouple the quiescent point-like emission from the spatially extended flow. We find this point-like emission, accounting for $\sim4$\% 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 powerlaw 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.