Free Astronomy Magazine November-December 2022

32 NOVEMBER-DECEMBER 2022 ASTRO PUBLISHING the Black Hole Initiative at Harvard University, USA. “Perhaps these hot spots detected at infrared wave- lengths are a manifestation of the same physical phenomenon: as in- frared-emitting hot spots cool down, they become visible at longer wave- lengths, like the ones observed by ALMA and the EHT,” adds Jesse Vos, a PhD student at Radboud University, the Netherlands, who was also in- volved in this study. The flares were long thought to originate from magnetic interac- tions 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 interpreta- tion of these events,” says co-author Monika Mo ś cibrodzka from Radboud Univer- sity. ALMA allows as- tronomers to study polarised radio emis- sion from Sagittarius A*, which can be used to unveil the black hole’s magnetic field. The team used these observations together with theoretical mod- els to learn more about the formation of the hot spot and the envi- ronment it is embed- ded in, including the magnetic field around Sagittarius A*. Their research provides stronger constraints on the shape of this magnetic field than previous observations, helping as- tronomers uncover the nature of our U sing ALMA, astronomers have found a hot bub- ble of gas that swirls around Sagittarius A*, the black hole at the centre of our galaxy, at 30% of the speed of light. This video summarizes the discovery. [ESO, EHT Collaboration, M. Kornmesser, L. Calçada (Acknowledgment: M. Wielgus), Jordy Davelaar et al./Radboud University/BlackHoleCam, ALMA (ESO/ NAOJ/NRAO) and C. Malin (christophmalin.com )] T his image shows the Atacama Large Millimeter/submillimeter Array (ALMA) looking up at the Milky Way as well as the location of Sagittarius A*, the su- permassive black hole at our galactic centre. Highlighted in the box is the image of Sagittarius A* taken by the Event Horizon Telescope (EHT) Collaboration. Located in the Atacama Desert in Chile, ALMA is the most sensitive of all the observatories in the EHT array, and ESO is a co-owner of ALMA on behalf of its European Mem- ber States. [ESO/José Francisco Salgado (josefrancisco.org) , EHT Collaboration] black hole and its surroundings. The observations confirm some of the previous discoveries made by the GRAVITY instrument at ESO’s Very Large Telescope (VLT), which ob- serves 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 suc- cess of such an endeavour would be a true milestone for our understand- ing 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. The team is also hoping to be able to di- rectly observe the orbiting gas clumps with the EHT, to probe ever closer to the black hole and learn more about it. “Hopefully, one day, we will be comfortable saying that we ‘know’ what is going on in Sagit- tarius A*,” Wielgus concludes. !