Gas is approaching the horizon of events

Astronomers have detected three eruptions near the central black hole of our galaxy that looked like gas in orbit around the invisible object.

Welcome to the threshold of a black hole.

An international team of astronomers has captured the movement of hot, magnetized gas near the horizon of the central supermassive black hole events in our galaxy, Sagittarius A * (pronounced "star A"). This gas is probably part of an inflated disk that Sgr A * feeds lazily. The disc provides a glittering light source, its constant glow sometimes flares that observers see in wavelengths ranging from X-rays to radio.

While they were watching the S2 star spinning near the black hole this summer, astronomers working with the GRAVITY instrument on the Very Large Telescope interferometer in Chile saw three bright bursts – two of them almost as bright as S2 in infrared – near Sgr A *. Each rocket lasts between 30 and 90 minutes and does not stay in place. Instead, everyone seemed to run around the black hole at 30% of the speed of light, tracing about two-thirds of a clockwise loop that (the uncertainties in the data set aside) has the black hole at its center.

The behavior looks a lot like what a hot spot on the disk would do. In 2005, Avery Broderick (now the University of Waterloo, Canada) and Abraham Loeb (Harvard) predicted that hot spots in the charged gas might be detectable, their orbits exploring the gravitational landscape near the most stable circuit around black hole.

The polarization changes of the flares corroborate this image. The light is polarized because of the magnetic fields in the disc, which serve as shepherd dogs to the charged particles that emit light. Broderick explains that these magnetic fields cross the disc, like big hoops around the size of the black hole. A hotspot only illuminates a small part of the accretion flow, so that when turning around it, the direction of the magnetic field in the illuminated region seems to turn – that is. is what the GRAVITY team has seen.

As reported by the team in October Astronomy and astrophysics, the data points to hot spots in orbit very close to the horizon of events of the black hole, making a passage every 45 minutes or so.

"It's an incredible measure," Broderick says. If astronomers see more flares doing the same thing, then "this represents an extraordinary opportunity to do accurate gravity testing in its most extreme environments: about the horizons of black holes".

Participants in a galactic center workshop in Germany last week discussed the analysis at length, but are cautious about interpreting hot spots. "Everyone believes that there is some kind of movement in a flare near the black hole," says Sera Markoff (University of Amsterdam, the Netherlands), who specializes in physics of physics. ; accretion. But not everyone is sold on the hotspot solution. "Most of us think that more complicated things are possible, such as movements associated with flares or magnetic jets." (Astronomers searched for Sgr A * jets but did not see anything definitive.)

Another concern is the angle at which we could see the black hole disk. The GRAVITY team thinks the record is pretty face to face. Others think that the probabilities for this are statistically low, however, and radio observations have favored a sharper view. Future radio results can clarify the tilt of the disc.

A more precise view could also create a better silhouette for the Event Horizon telescope, a global project to capture the "shadow" of Sgr A * and other supermassive black holes in the glow of their surrounding gas.

Reference:

GRAVITY Collaboration. "Detection of orbital movements near the last stable circular orbit of the solid black hole Sgr A *." Astronomy and astrophysics. October 2018.