Long-term James Webb observations reveal dynamic changes in the plasma disk surrounding Sagittarius A*
The black hole at the heart of the Milky Way is not in a state of rest or sleep. In contrast, the ring of plasma surrounding it is constantly flickering, with particularly bright bursts, as observations have shown.
Astronomers used the James Webb Space Telescope (JWST) to track Sagittarius A* and its surrounding disk for hours on end over the course of a year, from April 2023 to April 2024. These were the longest continuous observations to date of the supermassive black hole at the center of our galaxy.
The telescope revealed a "constant bubbling" of light intensity in the disk, changing every few seconds or minutes, as explained by astrophysicist Farhad Yousef-Zeda of Northwestern University in Evanston, Illinois. Several times a day, and seemingly at random, the disk emits an exceptionally bright burst, Yousef-Zeda and his colleagues report. In a letter published on February 20 in the Astrophysical Journal Letters.
The supermassive black hole, called Sagittarius A* or Sgr A* for short, weighs about 4 million times the mass of the Sun and is located about 26,000 light-years from our solar system. Most of the time, the black hole is fairly quiet, consuming matter in its vicinity only occasionally and emitting bursts of light and energy.
But that doesn't mean it's just sitting there, inactive. Previous observations, including the first image of the black hole, suggested that the white-hot disk of plasma that accumulates around it is constantly flickering. Computer simulations of the flow of material in the disk indicate that the disk's brightness can vary over minutes to years. The new observations not only confirm these ideas, but also provide insights into how the flickering occurs.
The James Webb Space Telescope (JWST) has advantages over other telescopes that have allowed it to capture the disk variations in action. Because the telescope is not in orbit around Earth, there is no interference from Earth, allowing it to observe for longer, continuous periods of time. In addition, it can observe objects at two wavelengths simultaneously.
"We can see things in color, not just black and white," says Youssef-Zeda.
The researchers believe that two processes are at work simultaneously, adds Youssef-Zeda. The turbulence and fluctuations in the disk itself cause the bubbling, and at the same time, the process that leads to the large outbursts may be similar to magnetic recovery events, in which magnetic field lines collide and release bursts of energy. These events also occur during solar flares.
The team requested 24 hours of continuous observation with the James Webb Telescope to find out more details.
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