Astronomers have found evidence of a “hot spot” orbiting Sagittarius A*, the black hole at the centre of our galaxy, using the Atacama Large Millimeter/submillimeter Array (ALMA).
This discovery provides crucial insight into the mysterious and ever-changing surroundings of our supermassive black hole.
“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,” remarks lead researcher Maciek Wielgus of the Max Planck Institute for Radio Astronomy in Bonn, “but making a full loop in just around 70 minutes. This requires a mind blowing velocity of about 30% of the speed of light!”
The observations were made using ALMA, a radio telescope co-owned by the European Southern Observatory (ESO), in the Chilean Andes as part of the Event Horizon Telescope (EHT) Collaboration’s quest to picture black holes. In April of 2017, the EHT connected eight existing radio telescopes around the world, including ALMA, resulting in the first-ever image of Sagittarius A*, which was just revealed.
Wielgus and his colleagues from the EHT Collaboration used ALMA data taken concurrently with the EHT observations of Sagittarius A* to calibrate the EHT data. Somewhat unexpectedly, the ALMA-only observations revealed much more information about the black hole’s composition.
By chance, some of the observations were made right after the Chandra Space Telescope saw a burst or flare of X-ray energy coming from the centre of our galaxy. These kinds of flares have been seen before with X-ray and infrared telescopes. They are thought to be caused by so-called “hot spots,” which are bubbles of hot gas that orbit the black hole very quickly and close to it.
“What is really new and interesting is that such flares,” adds Wielgus, “were so far only clearly present in X-ray and infrared observations of Sagittarius A*. Here we see for the first time a very strong indication that orbiting hot spots are also present in radio observations.”
According to Jesse Vos, a PhD candidate at Radboud University in the Netherlands who was also involved in this study, “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.”
The latest findings are consistent with the long-held theory that the flares are caused by magnetic interactions in the extremely hot plasma orbiting extremely near to Sagittarius A*.
Now, researchers have solid proof that these flares have magnetic origins, and our observations help us understand the geometry of the process. According to co-author Monika Mocibrodzka of Radboud University, the new data are very beneficial for developing a theoretical analysis of these events.
ALMA lets astronomers study the polarised radio waves coming from Sagittarius A, which can be used to find out what the magnetic field of the black hole is like. Together with theoretical models, the team used these observations to learn more about how the hot spot was made and how it fits into its surroundings, such as the magnetic field around Sagittarius A. Their research tells us more about the shape of this magnetic field than what we knew before. This helps astronomers figure out what our black hole and its surroundings are like.
The observations from ESO’s Very Large Telescope (VLT) in the infrared validate some of the prior discoveries made by the GRAVITY instrument. The data from GRAVITY and ALMA both indicate that the flare originates in a mass of gas that is orbiting the black hole almost face-on and moving at a speed of around 30% of the speed of light in a clockwise direction.
Ivan Marti-Vidal of the University of València in Spain, a co-author of the study, says that 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 endeavour would be a true milestone for our understanding of the physics of flares in the Galactic centre.”
With the EHT, the team also hopes to be able to look directly at the gas clouds that are orbiting the black hole. This will allow them to get closer to the black hole and learn more about it. Wielgus says, “Hopefully, one day, we will feel confident declaring that we ‘know’ what is going on in Sagittarius A*.
Source: Orbital motion near Sagittarius A∗
Image Credit: EHT Collaboration, ESO/M. Kornmesser (Acknowledgment: M. Wielgus)
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