Supermassive black holes are mysterious bodies. Scientists aren’t entirely sure how these beating hearts at the centers of most large galaxies formed. That includes Sagittarius A* (Sgr A*), the supermassive black hole at the center of our own Milky Way galaxy.
Now a new preprint study is shedding light on Sagittarius A* by studying what happens as material falls toward the black hole.
Typically, as dust, gas and other material sink toward a supermassive black hole, the black holes emit an “absolute torrent of light,” says Steve DiKerby, a postdoctoral researcher at the department of physics and astronomy at Michigan State University and co-author of the new paper. Sagittarius A*, however, is pretty dim. “It’s emitting only a tiny trickle of radiation,” DiKerby says.
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Yet that wasn’t always the case. Rather DiKerby and his colleagues’ work suggests that the disk of material swirling around Sagittarius A* once emitted much, much brighter x-rays—as much as 10,000 times brighter than those it emits today. Incredibly, that may have been the case as recently as a few hundred years ago, the research suggests.
The findings were presented at a meeting of the American Astronomical Society earlier this month and have been accepted for publication in Astrophysical Journal Letters.
The work is “very important,” says Joseph Michail, a postdoc at the Center for Astrophysics | Harvard & Smithsonian. It “tells us about the ‘recent’ history” of Sagittarius A*—something that had been missing from the research record, he says.
“This work shows something completely different from the Sgr A* we know and love—it was incredibly bright,” Michail says.
DiKerby and his colleagues used a powerful new x-ray telescope called XRISM (X-Ray Imaging and Spectroscopy Mission) to look at clouds of molecular gas surrounding the black hole. These clouds are thought to reflect x-rays coming from around the black hole, acting as a “cosmic mirror” into its past, DiKerby explains.
“We can not only know how bright Sagittarius A* is today but also how bright it appeared 100 years ago and 1,000 years ago,” he says.
To put its change in brightness into perspective, the brightest observed x-ray flare from Sagittarius A* happened in 2013, but that event had only 1 percent of the brightness of what the black hole may have emitted perhaps as recently as a few hundred years ago, Michail says.
“Effectively, XRISM is telling us that something substantial happened” to the black hole sometime in the past few centuries, Michail says. What, however, remains a mystery—for now.
