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 submillimeter 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.

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