Astronomers have witnessed flashes of X-rays shooting out of a black hole and believe that a dead star, or white dwarf, passing close by the black hole could be causing the eruptions. The most plausible outcome is this brazen white dwarf (the spent core of a sun-like star), which is about one-tenth the mass of our sun.
X-ray Flashes from a Nearby Supermassive Black Hole Accelerate Mysteriously
Astronomers have witnessed flashes of X-rays shooting out of a black hole and believe that a dead star, or white dwarf, passing close by the black hole could be causing the eruptions. The most plausible outcome is this brazen white dwarf (the spent core of a sun-like star), which is about one-tenth the mass of our sun.
The Unexpected Behavior of 1ES 1927+654
In 2018, astronomers at MIT and elsewhere observed that the black hole’s corona — a cloud of whirling, white-hot plasma — suddenly disappeared, before reassembling months later. The brief though dramatic shut-off was a first in black hole astronomy. Recently, members of the same team have caught the same black hole exhibiting more unprecedented behavior.
The astronomers have detected flashes of X-rays coming from the black hole at a steadily increasing clip. Over a period of two years, the flashes, at millihertz frequencies, increased from every 18 minutes to every seven minutes. This dramatic speed-up in X-rays has not been seen from a black hole until now.
A White Dwarf as the Culprit?
The researchers explored a number of scenarios for what might explain the flashes. They believe that the most likely culprit is a spinning white dwarf — an extremely compact core of a dead star that is orbiting around the black hole and getting precariously closer to its event horizon, the boundary beyond which nothing can escape the black hole’s gravitational pull.
The white dwarf must be pulling off an impressive balancing act, as it could be coming right up to the black hole’s edge without actually falling in. “This would be the closest thing we know of around any black hole,” says Megan Masterson, a graduate student in physics at MIT, who co-led the discovery.
Gravitational Waves and the White Dwarf’s Fate
If a white dwarf is at the root of the black hole’s mysterious flashing, it would also give off gravitational waves, in a range that would be detectable by next-generation observatories such as the European Space Agency‘s Laser Interferometer Space Antenna (LISA).
“These new detectors are designed to detect oscillations on the scale of minutes, so this black hole system is in that sweet spot,” says co-author Erin Kara, associate professor of physics at MIT. The team predicts that the white dwarf will not fall into the black hole’s gravity, but rather shed part of its outer layer into the black hole.
A New Window into Black Hole Physics
The team plans to continue observing the system, with existing and future telescopes, to better understand the extreme physics at work in a black hole’s innermost environments. They are particularly excited to study the system once the space-based gravitational-wave detector LISA launches — currently planned for the mid 2030s.
“The one thing I’ve learned with this source is to never stop looking at it because it will probably teach us something new,” Masterson says. “The next step is just to keep our eyes open.
