Free Astronomy Magazine May-June 2025

12 MAY-JUNE 2025 ASTRO PUBLISHING To conduct the study, Yusef-Zadeh and his team used Webb’s NIRCam (Near-Infrared Camera) to observe Sagittarius A* for a total of 48 hours in 8- to 10-hour increments across one year. This enabled them to track how the black hole changed over time. While the team expected to see flares, Sagittarius A* was more active than they anticipated. The observations revealed ongoing fire- works of various brightnesses and durations. The accretion disk sur- rounding the black hole generated five to six big flares per day and sev- eral small sub-flares or bursts in be- tween. Although astrophysicists do not yet fully understand the processes at play, Yusef-Zadeh suspects two sep- arate processes are responsible for the short bursts and longer flares. He posits that minor disturbances within the accretion disk likely gen- erate the faint flickers. Specifically, turbulent fluctuations within the disk can compress plasma (a hot, electrically charged gas) to cause a temporary burst of radiation. Yusef- Zadeh likens these events to solar flares. “It’s similar to how the Sun’s magnetic field gathers together, compresses, and then erupts a solar flare,” he explained. “Of course, the processes are more dramatic be- cause the environment around a black hole is much more energetic and much more extreme. But the Sun’s surface also bubbles with ac- tivity. ” Yusef-Zadeh attributes the big, bright flares to occasional mag- netic reconnection events — a process where two magnetic fields collide, releasing energy in the form of accelerated particles. Traveling at velocities near the speed of light, these particles emit bright bursts of radiation. “A magnetic reconnection event is like a spark of static elec- tricity, which, in a sense, also is an ‘electric reconnection,’” Yusef- Zadeh said. Because Webb’s NIRCam T his timelapse video shows observations of the Milky Way’s central black hole, known as Sagittarius A* (A-star), by NASA’s James Webb Space Telescope. At left, NIRCam (Near-Infrared Camera) images, created by combining data at wave- lengths of 2.1 and 4.8 microns, show a point of light that brightens and dims ran- domly. At right, the brightness is plotted as a function of time. Webb detected both faint flickers and brighter flares (one of which is seen near the end of the video). This video compresses about 9 hours of observations into 30 seconds. [NASA, ESA, CSA, Farhad Yusef-Zadeh (Northwestern), Howard Bushouse (STScI), Leah Hustak (STScI), Alyssa Pagan (STScI)] can observe two separate wave- lengths at the same time (2.1 and 4.8 microns in the case of these observa- tions), Yusef-Zadeh and his collabo- rators were able to compare how the flares’ brightness changed with each wavelength. Yet again, the re- searchers were met with a surprise. They discovered events observed at the shorter wavelength changed brightness slightly before the longer-wavelength events. “This is the first time we have seen a time delay in measurements at these wavelengths,” Yusef-Zadeh said. “We observed these wave- lengths simultaneously with NIRCam and noticed the longer wavelength lags behind the shorter one by a very small amount — maybe a few seconds to 40 seconds.” This time delay provided more clues about the physical processes occur- ring around the black hole. One ex- planation is that the particles lose energy over the course of the flare — losing energy quicker at shorter wavelengths than at longer wave- lengths. Such changes are expected for particles spiraling around mag- netic field lines. To further explore these questions, Yusef-Zadeh and his team hope to use Webb to observe Sagittarius A* for a longer period of time, such as 24 uninterrupted hours, to help reduce noise and enable the re- searchers to see even finer details. “When you are looking at such weak flaring events, you have to compete with noise,” Yusef-Zadeh said. “If we can observe for 24 hours, then we can reduce the noise to see features that we were unable to see before. That would be amaz- ing. We also can see if these flares repeat themselves or if they are truly random.” !