Astronomers have discovered evidence of a “hot spot” around Sagittarius A*, the black hole at the center of our galaxy, using the Atacama Large Millimeter/submillimeter Array (ALMA). The discovery aids in our understanding of the mysterious and dynamic 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, but making a full loop in just around 70 minutes. This requires a mind blowing velocity of about 30% of the speed of light!” says Maciek Wielgus of the Max Planck Institute for Radio Astronomy in Bonn, Germany, who led the study published today in Astronomy & Astrophysics.
The observations were carried out with the Event Horizon Telescope (EHT) Collaboration’s attempt to picture black holes using the ALMA radio telescope in the Chilean Andes, which is co-owned by the European Southern Observatory (ESO). 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*.
To calibrate the EHT data, Wielgus and his colleagues, who are members of the EHT Collaboration, used ALMA data recorded simultaneously with the EHT observations of Sagittarius A*. To the team’s surprise, there were more clues to the nature of the black hole hidden in the ALMA-only measurements.
By happenstance, some of the observations were made just after the Chandra Space Telescope of NASA observed an X-ray flare or burst coming from the center of our galaxy. These flares, which have been previously seen with X-ray and infrared instruments, are likely to be connected to heated gas bubbles known as “hot spots” that orbit the black hole very quickly.
“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*. Here we see for the first time a very strong indication that orbiting hot spots are also present in radio observations,” says Wielgus, who is also affiliated with the Nicolaus Copernicus Astronomical Centre, Poland and the Black Hole Initiative at Harvard University, USA.
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.
Ivan Marti-Vidal
“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,” adds Jesse Vos, a PhD student at Radboud University, the Netherlands, who was also involved in this study.
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. The new data are extremely helpful for building a theoretical interpretation of these events,” says co-author Monika Mo?cibrodzka from Radboud University.
ALMA allows astronomers to study polarised radio emission from Sagittarius A*, which can be used to unveil the black hole’s magnetic field. To understand more about the creation of the hot spot and the environment it is enmeshed in, particularly the magnetic field surrounding Sagittarius A*, the team used these data in conjunction with theoretical models. Their work helps astronomers understand the characteristics of our black hole and its environs by offering tighter restrictions than earlier measurements on the form of this magnetic field.
The observations confirm some of the previous discoveries made by the GRAVITY instrument at ESO’s Very Large Telescope (VLT), which observes in the infrared. The data from GRAVITY and ALMA 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 endeavour would be a true milestone for our understanding of the physics of flares in the Galactic centre,” says Ivan Marti-Vidal of the University of València in Spain, co-author of the study.
In order to go closer to the black hole and understand more about it, the team is also aiming to be able to directly see the orbiting gas clumps with the EHT.
“Hopefully, one day, we will be comfortable saying that we ‘know’ what is going on in Sagittarius A*,” Wielgus concludes.
More Information
This research was presented in the paper “Orbital motion near Sagittarius A* Constraints from polarimetric ALMA observations” to appear in Astronomy & Astrophysics.
The team is composed of M. Wielgus (Max-Planck-Institut für Radioastronomie, Germany [MPIfR]; Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Poland; Black Hole Initiative at Harvard University, USA [BHI]), M. Moscibrodzka (Department of Astrophysics, Radboud University, The Netherlands [Radboud]), J. Vos (Radboud), Z. Gelles (Center for Astrophysics | Harvard & Smithsonian, USA and BHI), I. Martí-Vidal (Universitat de València, Spain), J. Farah (Las Cumbres Observatory, USA; University of California, Santa Barbara, USA), N. Marchili (Italian ALMA Regional Centre, INAF-Istituto di Radioastronomia, Italy and MPIfR), C. Goddi (Dipartimento di Fisica, Università degli Studi di Cagliari, Italy and Universidade de São Paulo, Brazil), and H. Messias (Joint ALMA Observatory, Chile).
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile.
ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
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