A bubble of gas swirls around our galaxy’s supermassive black hole, revealing its secrets

A bubble of gas swirls around our galaxy’s supermassive black hole, revealing its secrets

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As the Event Horizon Telescope gathered data for its remarkable new image of the Milky Way’s supermassive black hole, Sagittarius A*, it was also observed by a legion of other telescopes, including NASA’s three X-ray Space Observatories (this was in April 2017). . But it was using the ground-based Atacama Large Millimeter/submillimeter Array (ALMA) telescope that astronomers spotted signs of a “hot spot” orbiting the supermassive black hole. It would be a bubble of very hot gas. 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 to obtain the recently revealed new image, the collaboration’s scientists also scanned the black hole using 8 instruments that detect different wavelengths of light.

So they collected X-ray data from: NASA’s Chandra X-ray Observatory, the Nuclear Spectroscopic Telescope Network (NuSTAR), and the Neil Gehrels Swift Observatory; radio from the East Asian Very Long Baseline Interferometer (VLBI) array and the Global 3-millimeter VLBI array; infrared images from the Atacama Large Millimeter/submillimeter Array (ALMA) telescope at the European Southern Observatory in Chile.

To calibrate the EHT data, Wielgus and his colleagues, members of the EHT collaboration, used ALMA data from Sagittarius A* recorded at the same time as the EHT observations. To the team’s surprise, there were many more clues to the nature of the black hole in the ALMA measurements themselves. Their discovery is the subject of a journal publication Astronomy and astrophysics.

Watch at the right time

While some supermassive black holes can be extremely active—eating up large amounts of gas and dust and glowing intensely in X-rays—Sagittarius A* is rather quiet by comparison.

It is therefore a coincidence that during this observing campaign for the EHT, the ALMA telescope was surprised by a burst or eruption of X-ray energy emitted from the center of the galaxy between April 6 and 12, 2017. Scientists believe that this type of eruption, previously observed by X-ray and infrared telescopes, is connected with so-called “hot spots”, bubbles of hot gas that orbit very quickly near the black hole.

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

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

Jesse Vos, PhD student at Radboud University (Netherlands), who also participated in this study, postulates an explanation for this radio observation that the behavior of these hotspots could be similar to the manifestation of a known physical phenomenon: ” As infrared-emitting hot spots cool, they become visible at longer wavelengths, such as those observed by ALMA and the EHT “.

A bubble of cosmic gas to explain 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 point of no return, while the accretion disk is made up of the matter orbiting around it.

Astronomers have long thought that the flares come from magnetic interactions in the mass of this accretion disk, including very hot gas with a magnetic field surrounding the black hole. New findings support this idea. Co-author Monika Mościbrodzka from Radboud University points out: We now find strong evidence for a magnetic origin for these eruptions, and our observations give us clues to the geometry of the process. The new data are extremely useful in creating a theoretical interpretation of these events. “.

ALMA allows astronomers to study polarized radio emissions from Sagittarius A*, 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 hotspot and the environment in which it is embedded, 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 Wielgus of the Max Planck Institute for Radio Astronomy in Bonn, Germany, who led the study, explains: We believe we are dealing with a bubble of hot gas orbiting Sagittarius A* in an orbit similar in size to the planet Mercury, but completing a full loop in just 70 minutes. This requires a staggering speed of about 30% the speed of light! “.

Indeed, the observations confirm some of the previous discoveries made with ESO’s Very Large Telescope (VLT) GRAVITY, which observes in the infrared. Data from GRAVITY and ALMA suggest that the eruption originated from this clump of gas swirling around the black hole clockwise across the sky, with the hotspot orbiting nearly opposite each other.

Ivan Marti-Vidal of the University of Valencia in Spain, co-author of the study, adds: In the future, we should be able to track hot spots across frequencies with coordinated multi-wavelength observations with GRAVITY and ALMA – the success of such an effort would be a real milestone in our understanding of the physics of galactic center flares. “.

The team now plans to try to directly observe the gas clumps in orbit using the EHT to probe the supermassive black hole as closely as possible and determine its dynamical characteristics, which will allow predictions of its evolution. Wielgus concludes: Hopefully one day we’ll be able to say we ‘know’ what’s going on in Sagittarius A* “.

Source: Astronomy & Astrophysics

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