Detection of a Drag Force in G2's Orbit: Measuring the Density of the Accretion Flow onto Sgr A* at 1000 Schwarzschild Radii

Abstract

The Galactic Center black hole Sgr A* is the archetypical example of an underfed massive black hole. The extremely low accretion rate can be understood in radiatively inefficient accretion flow models. Testing those models has proven to be difficult due to the lack of suitable probes. Radio and submm polarization measurements constrain the flow very close to the event horizon. X-ray observations resolving the Bondi radius yield an estimate roughly four orders of magnitude further out. Here, we present a new, indirect measurement of the accretion flow density at intermediate radii. We use the dynamics of the gas cloud G2 to probe the ambient density. We detect the presence of a drag force slowing down G2 with a statistical significance of ≈ 9 σ. This probes the accretion flow density at around 1000 Schwarzschild radii and yields a number density of ≈ 4 × 10 3 cm −3 . Self-similar accretion models where the density follows a power law radial profile between the inner zone and the Bondi radius have predicted similar values.

The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds

“…First, our simulations reproduce the total X-ray luminosity observed by Chandra (Baganoff et al 2003), meaning that we capture at least a majority of the hot, diffuse gas at large radii. Second, our simulations reproduce the r −1 density scaling inferred from observations that were taken over a large radial range (Gillessen et al 2019), implying that we are capturing a majority of the gas at all radii and that our inflow/outflow rates have the right radial dependence. Third, our simulations can reproduce the magnitude of the RM of both the magnetar (produced at r 0.1 pc, Eatough et al 2013) and Sgr A* (produced at r 10 −4 pc, Marrone et al 2007), demonstrating that our calculated magnetic field strengths are reasonable at both small and large scales.…”

“…Several viable models have been proposed, including those that appeal to strong outflows (Blandford & Begelman 1999) and those that appeal to convective instabilities that trap gas in circulating eddies (Stone, Pringle & Begelman 1999;Narayan, Igumenshchev & Abramowicz 2000;Quataert & Gruzinov 2000b;Igumenshchev & Narayan 2002;Pen, Matzner & Wong 2003). The range of models corresponds to a dependence of density on radius between the two extremes of r −3/2 and r −1/2 , with the combination of multiple observational estimates at ∼ 7 different radii supporting r −1 in the inner regions of the flow (Gillessen et al 2019) with a potential break near the Bondi radius (Wang et al 2013). Another key consideration is the angular momentum of the gas being fed at large radii.…”

A population of dust-enshrouded objects orbiting the Galactic black hole

“…While G2 is tidally interacting during its closest approach to Sgr A* 10 , the dust component of G2 has remained unresolved. The emitting gas is unbound at closest approach 10 , but that is not inconsistent with the existence of a stellar mass keeping the dust emission compact 8 . Several models have been proposed to account for G2 in terms of an optically thick distribution of dust surrounding a star: a young, low-mass star (T Tauri star) that has retained a protoplanetary disk 25 (scenario 1) or that generates a mass-loss envelope 26 (scenario 2); or, the merger of a binary system 9,24,27 (scenario 3).…”

Monitoring dusty sources in the vicinity of Sagittarius A*

“…Considering the report of a drag force for the DSO/G2 by Gillessen et al (2019), it would be interesting to investigate the effect of the background subtraction.…”