Gas bubble swirls around our galaxy’s supermassive black hole and reveals its secrets

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As the Event Horizon Telescope was collecting data for its remarkable new image of the Milky Way’s supermassive black hole, Sagittarius A*, a host of other telescopes, including NASA’s three space-based X-ray observatories, were also observing it (this was in April 2017). . But it was with the Earth-based Atacama Large Millimeter/submmillimeter Array (ALMA) telescope that astronomers noticed signs of a “hot spot” orbiting a supermassive black hole. This would be a bubble of very hot gas. An analysis of its behavior should help to better understand the dynamic environment of the object.

When the Event Horizon Telescope (EHT) observed Sagittarius A* in April 2017 for a newly discovered new image, the collaboration scientists also scanned the black hole with 8 instruments that detect different wavelengths of light.

Thus, they collected X-ray data from: NASA’s Chandra X-ray Observatory, Network of Nuclear Spectroscopic Telescopes (NuSTAR), and Neil Gerels Swift Observatory; radio from East Asia Very Long Baseline Interferometer (VLBI) and global 3mm VLBI array; infrared images from the Atacama Large Millimeter/submmillimeter Array (ALMA) telescope at the European Southern Observatory in Chile.

To calibrate the EHT data, Vilgus and his colleagues, members of the EHT collaboration, used ALMA data from the Sagittarius A* satellite, recorded simultaneously with the EHT observations. To the team’s surprise, ALMA measurements alone contained many more clues to the nature of the black hole. Their discovery is the subject of a publication in the journal Astronomy & Astrophysics.

Watch at the right time

While some supermassive black holes can be extremely active – “devouring” large amounts of gas and dust and glowing intensely in X-rays – Sagittarius A* is pretty quiet by comparison.

So by chance, the ALMA telescope, during this EHT observing campaign, caught a burst or burst of X-ray energy emitted by the center of the galaxy between April 6 and 12, 2017. Scientists believe that this type of eruptions, previously observed with X-ray and infrared telescopes, is associated with so-called “hot spots”, bubbles of hot gas that rotate very quickly near a black hole.

Recording of an X-ray burst between April 6 and 12, 2017. © NASA/CXC/A. Hobart

Wilgus, of the Nicolaus Copernicus Astronomy Center in Poland and the Black Hole Initiative at Harvard University in the US, said in a statement: “What is really new and interesting is that such flares have so far only been clearly present in the X-ray and X-ray bands. infrared observations of Sagittarius A*. Here, for the first time, we see a very strong indication that orbital hotspots are also present in radio observations.”

Jesse Vos, a doctoral student at Radboud University (Netherlands), who also participated in this study, hypothesizes that explaining this radio observation, that the behavior of these hot spots may be similar to the manifestation of a known physical phenomenon: “When hot spots emitting infrared radiation are cold, they become visible at longer wavelengths, such as those observed by ALMA and EHT.”

A bubble of cosmic gas explains the mysteries of black holes

Remember that black holes are objects in which gravity is so strong that nothing, not even light, can escape. The event horizon, or “surface” of a black hole, marks this boundary of no return, while the accretion disk is shaped by matter orbiting it.

Astronomers have long believed that flares are due to magnetic interactions within the matter of this accretion disk, including very hot gas with a magnetic field surrounding the black hole. New data support this idea. Co-author Monika Moskibrodska of Radboud University notes: “Now we are finding strong evidence for the magnetic origin of these eruptions, and our observations give us clues to the geometry of the process. The new data is extremely useful for constructing a theoretical interpretation of these events.”

ALMA allows astronomers to study Sagittarius A* polarized radio emission, which can be used to reveal the black hole’s magnetic field. The team used these observations with theoretical models to learn more about the formation of the hot spot and the environment in which it resides, including the magnetic field around the black hole. This study provides much stronger constraints on the shape of this magnetic field than previous observations, helping astronomers understand the nature of the Milky Way’s central black hole and its surroundings.

Maciek Wilgus of the Max Planck Institute for Radio Astronomy in Bonn, Germany, who led the study, explains: “We think we are dealing with a hot gas bubble orbiting Sagittarius A* in an orbit similar in size to that of the planet Mercury. , but completing a full cycle in about 70 minutes. This requires a breathtaking speed of about 30% of the speed of light! “.

Indeed, the observations confirm some of the previous findings made with ESO’s Very Large Telescope (VLT) GRAVITY instrument, which makes infrared observations. The GRAVITY and ALMA data suggest that the eruption was due to this accumulation of gas swirling clockwise around the black hole across the sky, with the hotspot’s orbit almost pointing forward.

Yvan Martí-Vidal, from the University of Valencia in Spain, co-author of the study, adds: “In the future, we will be able to track hotspots at different frequencies using coordinated observations at multiple wavelengths with GRAVITY and ALMA – the success of such an attempt would be a real milestone in our understanding physics of flares in the galactic center”.

The team now plans to try to directly observe gas clusters in orbit with the EHT, in order to study the supermassive black hole as accurately as possible and determine its dynamic characteristics, which will allow to predict its evolution. Vilgus concludes, “I hope one day we can say we ‘know’ what’s going on in Sagittarius A*.”

Astronomy and Astrophysics.

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