“This tells us that objects like white dwarfs may be able to live very close to an event horizon for a relatively extended period of time.”
Astronomers have observed another odd occurrence from a supermassive black hole that is already known for its enigmatic behavior using the XMM-Newton X-ray instrument.
A team from the Massachusetts Institute of Technology (MIT) believes that the reason for the growing frequency of high-energy light explosions is a white dwarf, or dead stellar core, that is boldly perched on the edge of the black hole.
This particular black hole, 1ES 1927+654, is around 270 million light-years away from Earth and has a mass of about one million times that of the sun. Astronomers first became aware of 1ES 1927+654’s peculiarity in 2018 when the corona, the swirling cloud of plasma that surrounds it, disappeared and then resurfaced. Astronomers had never witnessed anything like this occur around a black hole before.
When the MIT crew noticed X-ray bursts exploding from 1ES 1927+654 with increasing regularity, the situation became even more bizarre. The frequency of these high-energy flashes rose from once every 18 minutes to once every 7 minutes over a two-year period. Additionally, this behavior is unheard of for a black hole.
This dead star is doing an amazing balancing act if these strange episodes are caused by an orbiting white dwarf, a kind of stellar remnant left behind when a star with roughly the mass of the sun dies.
“This would be the closest thing that we know of around any black hole,” Megan Masterson, a researcher at MIT and team co-leader, posted in a statement. “This tells us that objects like white dwarfs may be able to live very close to an event horizon for a relatively extended period of time.”
The researchers hypothesize that gravitational waves, which are ripples in space and time emitted from the system, might be used to discover a delicately balanced white dwarf, which is the source of these odd episodes.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) and other gravitational wave detectors are currently insufficiently sensitive to detect such an emission. Future gravitational wave observatories, such as NASA’s space-based LISA (Laser Interferometer Space Antenna) detector, might be accurate enough to detect such waves, though.
“This black hole system is in that sweet spot,” said Erin Kara, a team member and physics professor at MIT. “These new detectors are designed to detect oscillations on the scale of minutes.”
The peculiar past of 1ES 1927+654
1ES 1927+654 has a lengthy history with both Kara and Masterson. Both were on the team that observed the darkening of the corona of the supermassive black hole seven years ago. Additionally, they saw its self-regeneration following its removal.
The newly formed corona of 1ES 1927+654 briefly held the title of brightest X-ray source in the sky above Earth. The team decided to keep looking into 1ES 1927+654 because of its remarkable character.
Even though it hadn’t done anything new for a few years and was just gurgling away, it was still incredibly bright. However, because it was so lovely, we felt compelled to continue keeping an eye on it,” Kara said. “Then we noticed something that has never really been seen before.”
The researchers used data gathered by the European Space Agency (ESA) X-ray probe XMM-Newton to conduct a more thorough analysis of 1ES 1927+654.
This showed that the X-rays from this black hole were emitting more frequent pulses, a phenomenon known as “quasi-periodic oscillations” that has been observed around black holes in the past. The peculiarity of 1ES 1927+654 is that, over the course of two years, this flickering appeared to gradually rise from every 18 minutes to every 7 minutes.
Masterson clarified, “We’ve never seen this dramatic variability in the rate at which it’s flashing.” “This looked absolutely nothing like a normal supermassive black hole.”
The MIT team had a crucial hint as to the reason for this peculiar behavior since the flashing of 1ES 1927+654 was visible in X-rays.
Living on the brink
In the immediate neighborhood of black holes, a violent and chaotic sea of quickly moving plasma is most likely the source of X-rays. Further from black holes, where cooler plasma travels more slowly, this high-energy light is considerably less likely to be released.
“Seeing something in the X-rays is already telling you you’re pretty close to the black hole,” Kara stated. “When you see variability on the timescale of minutes, that’s close to the event horizon, and the first thing your mind goes to is circular motion and whether something could be orbiting around the black hole.”
The scientists discovered that the source of these X-rays is only a few million kilometers from the supermassive black hole’s outer border, or “event horizon.” Every black hole has an event horizon, which is the point at which gravity gets so powerful that not even light can escape.
The first of the two main theories put up by the MIT researchers to explain the peculiar behavior of 1ES 1927+654 has to do with the corona of the black hole.
“One idea is that this corona is oscillating, maybe bobbing back and forth, and if it starts to shrink, those oscillations get faster as the scales get smaller,” added Masterson. “But we’re in the very early stages of understanding coronal oscillations.”
A cosmic daredevil—a white dwarf with a mass of roughly 10% of the sun’s—would provide a far more compelling explanation.
In this case, the white dwarf would be circling 1ES 1927+654 and releasing gravitational waves. The X-ray emissions would occur more often as a result of the dead star getting closer to the black hole and moving faster.
The MIT team does not believe that this dead star will plunge into the supermassive black hole anytime soon, despite the fact that it is almost at the point of no return in terms of its proximity to the black hole. This is because material is being released by the dead star as the black hole pulls the white dwarf inward. Keeping the white dwarf away from the event horizon means kicking it backward.
“Because white dwarfs are small and compact, they’re very difficult to shred apart, so they can be very close to a black hole,” Kara said. “If this scenario is correct, this white dwarf is right at the turnaround point, and we may see it get further away.”
In order to continue observing 1ES 1927+654, the researchers want to use ever-more-advanced telescopes. Additionally, they plan to “hear” gravity waves rippling away from the possible bold white dwarf braving certain death near this black hole using LISA, which is scheduled to launch in the 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 said. “The next step is just to keep our eyes open.”
On Monday, January 13, the team gave a presentation at the American Astronomical Society’s 245th meeting in National Harbor, Maryland. The journal Nature will publish their findings.
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