Sagittarius A*, the 4.3-million-solar-mass black hole at the center of our Milky Way Galaxy, is a strong source of radio, X-rays and gamma rays. It also displays flaring emission in the X-ray band atop a steady, quiescent level, as well as in other wavebands, most prominently in the near-infrared. The physical process producing the flares is not fully understood and it is unclear if the flaring rate varies, although some recent works suggest it has reached unprecedented variability in recent years. Using over a decade of regular X-ray monitoring of NASA’s Neil Gehrels Swift Observatory, astronomers found that Sagittarius A* not only flares irregularly from day to day but also in the long term.
This X-ray image of the Milky Way’s center merges all Swift observations from 2006 through 2013; Sagittarius A* is at the center; low-energy (300 to 1,500 electron volts) X-rays appear red; green are medium-energy (1,500 to 3,000 eV); blue are high-energy (3,000 to 10,000 eV). Image credit: NASA / Swift / N. Degenaar.
In February 2006, Swift started to monitor the Milky Way’s center with the on-board X-Ray Telescope (XRT) with the aim of studying Sagittarius A* as well as numerous X-ray binary systems located in the region.
Apart from a handful of interruptions and Sun constraints, Swift/XRT has pointed at the Galactic center every 1-3 days since 2006.
In the current study, Nacional Autonoma de México astronomer Alexis Andrés and colleagues used all available Swift/XRT data that covered Sagittarius A* and spanned the period between February 24, 2006 and August 6, 2019.
The analysis of the data showed high levels of activity from 2006 to 2008, with a sharp decline in activity for the next four years.
After 2012, the frequency of flares increased again — the researchers had a difficult time distinguishing a pattern.
In the next few years, they expect to gather enough data to be able to rule out whether the variations in the flares from Sagittarius A* are due to passing gaseous clouds or stars, or whether something else can explain the irregular activity observed from our galaxy’s central black hole.
“The long dataset of the Swift observatory did not just happen by accident,” said Dr. Nathalie Degenaar, an astronomer at the University of Amsterdam.
“I’ve been applying for more observing time regularly. It’s a very special observing program that allows us to conduct a lot of research.”
“How the flares occur exactly remains unclear,” said Dr. Jakob van den Eijnden, an astronomer at the University of Oxford.
“It was previously thought that more flares follow after gaseous clouds or stars pass by the black hole, but there is no evidence for that yet.”
“And we cannot yet confirm the hypothesis that the magnetic properties of the surrounding gas play a role either.”
The results were published in the Monthly Notices of the Royal Astronomical Society.
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A. Andrés et al. 2022. A Swift study of long-term changes in the X-ray flaring properties of Sagittarius A. MNRAS 510 (2): 2851-2863; doi: 10.1093/mnras/stab3407
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