Selections from 2020: A Twinkling Black Hole

Editor’s note: In these last two weeks of 2020, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA

Published April 2020

Main takeaway:

Recent observations show that the supermassive black hole at the center of the Milky Way, Sagittarius A*, flickers on a regular timescale of 30 minutes, with slower variations on hour-long timescales. The observations of this twinkling are presented in a publication led by Yuhei Iwata (Keio University).

Why it’s interesting:

The twinkles around this 4-million-solar-mass black hole, detected at millimeter wavelengths within 700 minutes of ALMA data across 10 days, may be caused by radio hot spots in the inner disk of material falling onto Sgr A*. Because this material is moving at incredibly high speeds, emitted photons from hot spots get a boost that make them shine brightly enough for us to detect. If this picture is correct, we can use our observations of these flickers to learn more about the strange physics that occurs close in around black holes.

Why this matters for imaging:

M87 EHT image

The first detailed image of a black hole, M87*, taken with the Event Horizon Telescope. [Adapted from EHT collaboration et al 2019]

Wondering why the Event Horizon Telescope released a picture of the black hole at the center of the galaxy M87, yet they haven’t announced images of the much closer supermassive black hole within our own galaxy? It’s largely due to the difference in scales: since M87* is much larger than Sgr A* (more than a thousand times more massive!), the motions around M87* occur on substantially longer timescales than the motions around Sgr A*. The steady nature of the black hole in M87 makes it much easier to photograph than the twinkling, ever-changing Sgr A* — as the timescales for flicking measured here by Iwata and collaborators demonstrate. Because Sgr A* is so tricky to capture, we’ll have to be patient as we wait for images of it!


Yuhei Iwata et al 2020 ApJL 892 L30. doi:10.3847/2041-8213/ab800d