A supermassive black hole, 1ES 1927+654, is emitting unusual, rhythmic X-ray pulses. These oscillations may be caused by a white dwarf star orbiting close to the black hole’s event horizon. The changing pulse frequency suggests intriguing physics at play, and future gravitational wave sensors like LISA could confirm this theory.
A supermassive black hole is sending out a mysterious pulsing beat
Author: Alex Wilkins
UNUSUAL vibrations emanating from a supermassive black hole seem to be growing more frequent and they could be caused by a white dwarf star orbiting perilously close to its event horizon.
In 2018, a supermassive black hole called 1ES 1927+654 took astronomers by surprise by changing from being relatively inactive to becoming extremely bright. It was the first time such a black hole had been observed changing in this way.
The black hole began to dim after this, but now, Megan Masterson at the Massachusetts Institute of Technology and her colleagues have discovered that it seems to be sending out X-ray radiation in a regular rhythm.
They first measured these oscillations in 2022, finding that the brightness of the X-rays fluctuated by 10 per cent roughly every 18 minutes. By 2024, this had decreased to every 7.1 minutes (arXiv, doi.org/g8zmw2).
“It was exciting in and of itself just to find these oscillations, because it’s only one of a handful [of supermassive black holes that do this],” says Masterson. “But I think the most exciting thing to us was that the oscillation period – how fast these oscillations were happening – was changing on human-observable timescales, which is not usually what we see around supermassive black holes.”
Everyone really wants to see a white dwarf get eaten by a supermassive black hole
This changing oscillation period implies that interesting physics is happening near the black hole’s event horizon, the boundary at which its gravitational pull is so strong that even light can’t escape.
One thing that could explain the oscillation is a white dwarf star that is occasionally having its matter siphoned off by the black hole. According to her team’s calculations, Masterson says this white dwarf would be millions of miles away from the black hole, but given that the black hole itself is millions of miles across, it puts it very close to the event horizon. “It’s actually extraordinarily close to the supermassive black hole,” says Masterson. “It’s just this tiny little object orbiting around this gargantuan beast.”
“It’s kind of amazing that such a small body can make such a significant impact on what we’re seeing from around the supermassive black hole,” she says.
However, other scenarios could also explain the strange signal. To confirm it is a white dwarf, astronomers will need to detect gravitational waves from the black hole. Current sensors can only detect gravitational waves at hertz or kilohertz frequencies, which correspond to black holes roughly the mass of our sun. But the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave observatory set to launch in 2035, should be able to detect millihertz gravitational waves, which is what supermassive black holes produce.
Knowing that a white dwarf might be orbiting a supermassive black hole is useful for a mission like LISA, says Matt Nicholl at Queen’s University Belfast in the UK. “Everyone really wants to see a white dwarf get eaten by a black hole with LISA,” says Nicholl. “It helps tremendously if we have a few candidates.”
Credits: TCA, LLC.