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The black hole at the Milky Way’s heart neither slumbers nor sleeps. Instead, the ring of plasma surrounding it flickers constantly, punctuated by superbright flares, observations show.
Astronomers used the James Webb Space Telescope to observe Sgr A* and its disk for hours at a time over the course of a year, from April 2023 through April 2024. These were the longest continuous observations yet of our galaxy’s central supermassive black hole.
The telescope revealed a “constant bubbling” in the disk’s light that changed every few seconds or minutes, says astrophysicist Farhad Yusef-Zadeh of Northwestern University in Evanston, Ill. A few times a day, and seemingly at random, the disk would emit a blindingly bright flare, Yusef-Zadeh and colleagues report in the Feb. 20 Astrophysical Journal Letters.
The supermassive black hole, called Sagittarius A* or Sgr A* for short, is about 4 million times as heavy as the sun and lies about 26,000 light-years from our solar system. The black hole is fairly quiet most of the time, only occasionally gobbling up material in its vicinity and letting out bursts of light and energy.
But that doesn’t mean it’s just lying there. Earlier observations, including the first image of the black hole, had suggested that the white-hot disk of plasma that accretes around it is constantly flickering. Computer simulations of how material flows in the accretion disk predicted that the disk’s brightness should vary from minutes to years. The new observations not only confirm those ideas, but also shed light on how the flickering happens.
JWST has some advantages over other telescopes that allowed it to catch the disk’s variability in action. Because the telescope is not in Earth’s orbit, Earth never gets in its way, letting the telescope take longer continuous looks. It can also observe objects in two different wavelengths of light simultaneously.
“We can see things in color, rather than black-and-white,” Yusef-Zadeh says.
The researchers think there are two processes at work, Yusef-Zadeh says. Turbulence in the disk itself causes the bubbling. Meanwhile, the process behind the big flares could be analogous to magnetic reconnection events, in which two lines of magnetic field collide and release bursts of energy. These events also happen during solar flares.
The team has requested a full 24 hours of continuous observation time with JWST to find out more.
