In 2017, the Atacama Large Millimeter / submillimeter Array (ALMA) telescope looked into the dark heart of our system where Sgr A *, a 4 million solar mass black hole, is located. There they found a flash of light, dangling, in an amazing orbit. These were quasi-periodic millimeter wave blinks from Sagittarius A *.
“It is known that Sgr A * sometimes bursts at millimeter wavelengths. This time, using ALMA, we obtained high-quality data of the variation of the intensity of radio waves from Sgr A * over 10 days, 70 minutes per day. Then we found two trends: quasi-periodic variations with a typical time scale of 30 minutes and slow variations of an hour, “explains Yuhei Iwata of Keio University and lead author of the article published in the Astrophysical Journal Letters.
That is, astronomers are fully aware that the supermassive black hole at the center of the Milky Way occasionally flares up due to the sudden absorption of clouds of material. However, these are not the only emissions detected near the black hole. This new flicker at millimeter wavelengths is different. These newly discovered “flashes” are quasi-periodic variations that are less intense than previously detected flares around the black hole we call Sagittarius A *. The team suggests that the emission is consistent with material circulating in the innermost stable orbit, which is less than 10 million kilometers from the black hole’s surface.
What causes it?
Taking into account that the black hole itself does not produce any type of emission and that the source of the emission is the fiery gaseous disk around the black hole, said gas around the black hole does not go directly to the gravitational well, but rather rotates around the black hole to form an accretion disk.
The researchers focused on small time scale variations and found that the 30 minute period of variation is comparable to the orbital period of the innermost edge of the accretion disk with a radius of 0.2 astronomical units (1 astronomical unit corresponds to the distance between the Earth and the Sun: 150 million kilometers). For comparison, Mercury, the innermost planet in the solar system, revolves around the Sun at a distance of 0.4 astronomical units. Considering the colossal mass at the center of the black hole, its effect of gravity is also extreme on the accretion disk.
” This emission could be related to some exotic phenomena that occur in the vicinity of the supermassive black hole, ” says Tomoharu Oka, professor at Keio University and co-author of the work.
The cause of these variations is unclear, but experts propose a scenario that involves the formation of hot spots of material within the disk. According to Einstein’s theory of special relativity , the emission is greatly amplified when the source moves towards the observer with a speed comparable to that of light. The incredible gravitational pull of the black hole, causing the disk to rotate at more than a third of the speed of light, could create the amplified emission seen by ALMA.
But there is a ‘but’.
“In general, the faster the movement, the more difficult it is to take a photo of the object,” said co-author Professor Tomoharu Oka, also at Keio University, referring to last year’s milestone of capturing the first image. of a black hole. Instead, the variation of the emission itself provides a convincing view of the movement of the gas . We can witness the very moment of gas absorption by the black hole with a long-term monitoring campaign with ALMA ”.
Astronomers plan to extract independent information to understand the puzzling environment around this supermassive black hole.
Referencia: Yuhei Iwata, Tomoharu Oka, Masato Tsuboi, Makoto Miyoshi, Shunya Takekawa. Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA. The Astrophysical Journal, 2020; 892 (2): L30 DOI: 10.3847/2041-8213/ab800d
