Free Astronomy Magazine January-February 2019

14 JANUARY-FEBRUARY 2019 SPACE CHRONICLES mense mass is known as the inner- most stable orbit, and it is from here that the observed flares originate. “It’s mind-boggling to actually wit- ness material orbiting a massive black hole at 30% of the speed of light,” marvelled Oliver Pfuhl, a scientist at the MPE. “GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented de- tail.” These measurements were only possible thanks to international collaboration and state-of-the-art instrumentation. The GRAVITY instrument which made this work possible combines the light from four telescopes of ESO’s VLT to create a virtual super- telescope 130 metres in diameter, and has already been used to probe the nature of Sagittarius A*. Earlier this year, GRAVITY and SINFONI, an- other instrument on the VLT, al- lowed the same team to accurately measure the close fly-by of the star S2 as it passed through the extreme gravitational field near Sagittarius A*, and for the first time revealed T he central parts of our Galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope. By following the motions of the most central stars over more than 16 years, astronomers were able to de- termine the mass of the su- permassive black hole that lurks there. [ESO/S. Gillessen et al.] T his video starts with a wide view of the Milky Way and then zooms into a visualization of data from simulations of orbital motions of gas swirling around at about 30% of the speed of light on a circular orbit around the su- permassive black hole Sagittarius A*. [ESO/Gravity Consortium/L. Calçada/N. Risinger (skysurvey.org )] the effects predicted by Einstein’s general relativity in such an extreme environment. During S2’s close fly- by, strong infrared emission was also observed. “We were closely moni- toring S2, and of course we always keep an eye on Sagittarius A*,” ex- plained Pfuhl. “During our observa- tions, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!” This emission, from highly energetic electrons very close to the black hole, was visible as three prominent bright flares, and exactly matches theoretical predictions for hot spots orbiting close to a black hole of four million solar masses. The flares are thought to originate from magnetic interactions in the very hot gas or- biting very close to Sagittarius A*. Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, who led the study, explained: “This al- ways was one of our dream projects but we did not dare to hope that it would become possible so soon.” Referring to the long-standing as- sumption that Sagittarius A* is a su- permassive black hole, Genzel con- cluded that “the result is a resound- ing confirmation of the massive black hole paradigm” . !