Sagittarius A*, the 4.3-million-solar-mass black hole at the center of our Milky Way Galaxy, was observed with the Atacama Large Millimeter/submillimeter Array (ALMA) as a part of the Event Horizon Telescope (EHT) campaign in 2017.
“We think we’re looking at a hot bubble of gas zipping around Sagittarius A* on an orbit similar in size to that of the planet Mercury, but making a full loop in just around 70 minutes,” said Dr. Maciek Wielgus, a researcher at the Max Planck Institute for Radio Astronomy and a member of the EHT Collaboration.
“This requires a mind blowing velocity of about 30% of the speed of light.”
In April 2017, the EHT linked together eight existing radio telescopes worldwide, including ALMA, resulting in the recently released first ever image of Sagittarius A*, which is about 27,000 light-years away from Earth.
To calibrate the EHT data, Dr. Wielgus and colleagues used ALMA data recorded simultaneously with the EHT observations of Sagittarius A*.
To their surprise, there were more clues to the nature of the black hole hidden in the ALMA-only measurements.
By chance, some of the observations were done shortly after a burst or flare of X-ray energy was emitted from the center of our Galaxy, which was spotted by NASA’s Chandra Space Telescope.
These kinds of flares, previously observed with X-ray and infrared telescopes, are thought to be associated with so-called ‘hot spots’ — hot gas bubbles that orbit very fast and close to the black hole.
“What is really new and interesting is that such flares were so far only clearly present in X-ray and infrared observations of Sagittarius A*,” Dr. Wielgus said.
“Here, we see for the first time a very strong indication that orbiting hot spots are also present in radio observations.”
“Perhaps these hot spots detected at infrared wavelengths are a manifestation of the same physical phenomenon: as infrared-emitting hot spots cool down, they become visible at longer wavelengths, like the ones observed by ALMA and the EHT,” said Jesse Vos, a Ph.D. student at Radboud University.
The flares were long thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*, and the new findings support this idea.
“Now we find strong evidence for a magnetic origin of these flares and our observations give us a clue about the geometry of the process,” said Dr. Monika Mościbrodzka, an astronomer at Radboud University.
“The new data are extremely helpful for building a theoretical interpretation of these events.”
The data from ALMA and the GRAVITY instrument at ESO’s Very Large Telescope both suggest the flare originates in a clump of gas swirling around the black hole at about 30% of the speed of light in a clockwise direction in the sky, with the orbit of the hot spot being nearly face-on.
“In the future we should be able to track hot spots across frequencies using coordinated multiwavelength observations with both GRAVITY and ALMA — the success of such an endeavor would be a true milestone for our understanding of the physics of flares in the Galactic center,” said Dr. Ivan Marti-Vidal, an astronomer at the University of València.
The findings appear in the journal Astronomy & Astrophysics.
M. Wielgus et al. 2022. Orbital motion near Sagittarius A*. Constraints from polarimetric ALMA observations. A&A 665, L6; doi: 10.1051/0004-6361/202244493
Source link: https://www.sci.news/astronomy/sagittarius-a-hot-spot-11224.html