The Galactic Center Weather Forecast

TL;DR

This paper models how increased accretion rates onto Sgr A* affect its observable emission features, predicting changes in the black hole's silhouette visibility and multi-wavelength emission as the accretion rate varies.

Contribution

It extends existing accretion models to explore the impact of increased accretion rates on Sgr A*'s observable properties, including the black hole shadow and multi-wavelength emission.

Findings

Black hole silhouette becomes brighter and more extended with modest increases in accretion rate.

At high accretion rates, the silhouette is hidden beneath the photosphere at 230 GHz and 345 GHz.

Increased accretion leads to persistent X-ray and infrared emission exceeding current flare brightness.

Abstract

In accretion-based models for Sgr A* the X-ray, infrared, and millimeter emission arise in a hot, geometrically thick accretion flow close to the black hole. The spectrum and size of the source depend on the black hole mass accretion rate $\dot{M}$. Since Gillessen et al. have recently discovered a cloud moving toward Sgr A* that will arrive in summer 2013, $\dot{M}$ may increase from its present value $\dot{M}_0$. We therefore reconsider the "best-bet" accretion model of Moscibrodzka et al., which is based on a general relativistic MHD flow model and fully relativistic radiative transfer, for a range of $\dot{M}$. We find that for modest increases in $\dot{M}$ the characteristic ring of emission due to the photon orbit becomes brighter, more extended, and easier to detect by the planned Event Horizon Telescope submm VLBI experiment. If $\dot{M} \gtrsim 8 \dot{M}_0$ this "silhouette of the black hole will be hidden beneath the synchrotron photosphere at 230 GHz, and for $\dot{M} \gtrsim 16 \dot{M}_0$ the silhouette is hidden at 345 GHz. We also find that for $\dot{M} > 2 \dot{M}_0$ the near-horizon accretion flow becomes a persistent X-ray and mid-infrared source, and in the near-infrared Sgr A* will acquire a persistent component that is brighter than currently observed flares.