The origin of hotspots around Sgr A*: Orbital or pattern motion?

Understand

The Gravity Collaboration detected a near-infrared hotspot moving around Sgr A* during the 2018 July 22 flare.

• They fitted the partial loop the hotspot made on the sky with a circular Keplerian orbit of radius $\simeq7.5\,r_{\rm g}$ around the supermassive black hole (BH), where $r_{\rm g}$ is the gravitational radius.
• However, because the hotspot traversed the loop in a short time, models in which the hotspot tracks the motion of some fluid element tend to produce a best-fit trajectory smaller than the observed loop.
• This is true for a circular Keplerian orbit, even when BH spin is accounted for, and for motion along a RIAF streamline.