Constraints on jet-driven disk accretion in Sagittarius A*

TL;DR

This paper explores how magnetic coupling and outflows can lead to the formation of a cooler, less luminous thin disk in Sagittarius A*, aligning theoretical models with observational constraints and suggesting lower accretion rates.

Contribution

It demonstrates that magnetic coupling and outflows can produce a compact, cool disk in Sgr A*, relaxing previous observational constraints on thin-disk accretion in low-luminosity galactic nuclei.

Findings

Formation of a cool, geometrically-thin disk at small radii in Sgr A*

Reduced disk luminosity due to magnetic coupling with jets

Compatibility of modified disk emission with NIR observations

Abstract

We revisit theoretical and observational constraints on geometrically-thin disk accretion in Sagittarius A* (Sgr A*). We show that the combined effects of mass outflows and electron energization in the hot part of the accretion flow can deflate the inflowing gas from a geometrically-thick structure. This allows the gas to cool and even thermalize on an inflow timescale. As a result, a compact, relatively cool disk may form at small radii. We show that magnetic coupling between the relativistic disk and a steady-state jet results in a disk that is less luminous than a standard relativistic disk accreting at the same rate. This relaxes the observational constraints on thin-disk accretion in Sgr A* (and by implication, other Low-Luminosity Active Galactic Nulcei, LLAGN). We find typical cold gas accretion rates of a few * 10^{-9} solar masses / yr. We also find that the predicted modified disk emission is compatible with existing near-infrared (NIR) observations of Sgr A* in its quiescent state provided that the disk inclination angle is > 87 degrees and that the jet extracts more than 75% of the accretion power.