Milky Way’s black hole reveals erratic nature
Bright bursts from Sagittarius A* have waxed and waned for years
The black hole at the heart of the Milky Way erupts unpredictably and chaotically, a new study looking at over 15 years of data has found.
Known as Sagittarius A* (Sgr A*), the black hole at the centre of our Galaxy lies more than 26,000 lightyears from Earth, and is around four million times the mass of the Sun. The hot gas and dust around the black hole means it glows brightly at radio, X-ray and gamma ray wavelengths. Astronomers have known for decades that over the timescale of a day the emissions will suddenly flare, increasing in brightness by ten to a hundred times, but weren’t sure how this flaring behaved over longer time spans.
To investigate this, Alexis Andrés, a postgraduate student from the University of Amsterdam, looked at data taken by the Neil Gehrels Swift Observatory, a telescope which has been looking for gamma ray bursts since 2006.
“The long dataset of the Swift Observatory did not just happen by accident,” says Nathalie Degenaar, who requested the specific measurements from Swift when she was a PhD student and became Andrés’s supervisor at the University of Amsterdam. “Since then, I’ve been applying for more observing time regularly. It’s a very special observing programme that allows us to conduct a lot of research.”
This data showed that Sgr A* had a high rate of flares from 2006 to 2008, which then dropped off before rising once again in 2012.
“How the flares occur remains unclear,” says Jakob van den Eijnden from 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 team hope to uncover more about the flare’s changing rate by monitoring Sgr A* using Swift over the next few years. This should reveal whether a passing cloud or an as yet unknown phenomenon is causing this fitful behaviour at our Galaxy’s heart. https://swift.gsfc.nasa.gov/