An international team of researchers led by graduate student Alexis Andrés has discovered that Sagittarius A*, the black hole at the center of the Milky Way, is not only flaring erratically every day, but also over the long term. The team came to this conclusion after analyzing 15 years of data. The research was initiated by Andres in 2019 when he was a summer student at the University of Amsterdam. In the years that followed, he continued his research, which will now be published in the Monthly Notices of the Royal Astronomical Society.
Sagittarius A* is a strong source of radio, X-rays and gamma rays (visible light is blocked by gas and dust). For decades, astronomers have known that Sagittarius A* flickers every day, emitting bursts of radiation 10 to 100 times brighter than the normal signal observed from black holes. To learn more about these mysterious flares, Andres looked for patterns in 15 years of data provided by NASA’s Neil Gehrels Swift Observatory, an Earth-orbiting satellite dedicated to detecting gamma-ray bursts. The Swift Observatory has been observing gamma rays from black holes since 2006. Analysis of the data shows that activity levels were high from 2006 to 2008, with a sharp drop in activity over the next four years. After 2012, the frequency of flares increased again—researchers struggled to distinguish a pattern.
Over the next few years, the team of astronomers hopes to collect enough data to rule out whether changes in Sagittarius A* flares are due to passing gas clouds or stars, or if there is something else that could explain the observed irregular activity From the black hole at the center of our galaxy.
“The Swift Observatory’s long datasets did not happen by chance,” said co-author and former director Dr. Nathalie Degenaar of Andres at the University of Amsterdam. When she was a doctoral student, her request for the Swift satellite to make these specific measurements was granted. “I’ve been regularly requesting more observation time since then. It’s a very special observation program that allows us to do a lot of research.”
Co-author Dr Jakob van den Eijnden from the University of Oxford commented on the team’s findings: “Exactly how flares occur is still unclear. It was previously thought that more flares would occur after gas clouds or stars pass through a black hole, but it is not yet known. There’s no evidence for that. And we haven’t been able to confirm the hypothesis that the magnetism of the surrounding gas also plays a role.”
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