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Employing the XMM-Newton X-ray observatory, astronomers have observed a supermassive black hole already recognized for its enigmatic behavior exhibiting yet another peculiar phenomenon.
The team from the Massachusetts Institute of Technology (MIT) hypothesizes that a deceased stellar remnant, or white dwarf, precariously balanced on the brink of the black hole is responsible for the increasingly frequent eruptions of high-energy light.
The black hole referred to is 1ES 1927+654, situated approximately 270 million light-years from Earth, possessing a mass nearly 1 million times that of the sun. 1ES 1927+654 first revealed its oddities to astronomers in 2018 when the swirling plasma cloud surrounding it, identified as the corona, disappeared and subsequently reappeared. Such an event had never been witnessed around a black hole until then.
The situation became even more bizarre when the MIT team detected flares of X-rays emanating from 1ES 1927+654 with increasing frequency. Within two years, the occurrence of these high-energy bursts escalated from once every 18 minutes to once every 7 minutes. This behavior is also unprecedented for a black hole.
If these peculiar events are a consequence of an orbiting white dwarf, a category of stellar remnant left after a star of similar mass to the sun ceases to exist, then this deceased star is executing an extraordinary balancing act.
“This would be the closest instance we are aware of around any black hole,” stated team co-leader and MIT researcher Megan Masterson. “This indicates that entities like white dwarfs might endure very near to an event horizon for a comparatively prolonged period.”
If the origin of these unusual phenomena is a finely tuned white dwarf, the researchers speculate that it may be identifiable through the ripples in space and time known as gravitational waves generated by the system.
Current gravitational wave detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO) lack the sensitivity to detect such emissions. Nonetheless, upcoming gravitational wave observatories such as NASA’s space-based detector LISA (Laser Interferometer Space Antenna) might have the precision required to achieve such a discovery.
“These new detectors are engineered to identify oscillations on the scale of minutes, so this black hole system lies perfectly within that range,” mentioned team member and MIT physics professor Erin Kara.
The unusual history of 1ES 1927+654
Both Kara and Masterson have an extensive background with 1ES 1927+654. They were part of the team that observed seven years ago when the supermassive black hole’s corona became obscured. They also witnessed its revival following its disappearance.
For a short period, the newly formed corona of 1ES 1927+654 was the brightest X-ray source in the sky visible from Earth. The remarkable characteristics of 1ES 1927+654 prompted the team to continue monitoring it.
“It remained exceptionally luminous, even though it was dormant for a couple of years and was somewhat fluctuating. Yet we felt compelled to keep tracking it because of its astonishing beauty,” Kara explained. “Then we noticed something that had never truly been observed before.”
To examine 1ES 1927+654 in further detail, the researchers consulted data gathered by the European Space Agency (ESA) X-ray spacecraft XMM-Newton.
This unveiled the rising pulse frequency of X-rays from this black hole, a phenomenon termed “quasi-periodic oscillations” that has previously been observed around black holes. What makes 1ES 1927+654 distinctive is that this flickering appeared to consistently increase from every 18 minutes to every 7 minutes over two years.
“We have never witnessed such a dramatic volatility in the rate at which it flashes,” Masterson clarified. “This appeared nothing like a typical supermassive black hole.”
The detection of the flashing of 1ES 1927+654 in X-rays provided the MIT team with a crucial indication regarding the cause behind this unusual behavior.
Residing on the edge
X-rays are predominantly generated from a chaotic and turbulent sea of swiftly moving plasma in the immediate vicinity of black holes. This high-energy light is considerably less likely to be produced from farther away from black holes where the cooler plasma moves at a slower pace.
“Witnessing something in the X-rays indicates that you’re quite close to the black hole,” Kara remarked. “When you observe variability on a timescale of minutes, that is near the event horizon, and the initial consideration is circular movement and whether something could be revolving around the black hole.”
The team concluded that whatever is causing these X-rays resides merely a few million miles from the outer boundary or “event horizon” of the supermassive black hole. The event horizon represents the threshold around each black hole where gravity escalates to the extent that not even light can escape its grasp.
The MIT researchers propose two primary theories to account for the bizarre behavior of 1ES 1927+654, the first being associated with the structure of the black hole’s corona.
“One hypothesis is that this corona is oscillating, perhaps moving back and forth, and if it begins to contract, those oscillations accelerate as the dimensions lessen,” Masterson suggested. “However, we are only in the initial stages of understanding coronal oscillations.”
A significantly clearer explanation would suggest a cosmic thrill-seeker: a tightrope-walking white dwarf with roughly 10% of the sun’s mass.
In this situation, the white dwarf would be producing gravitational waves while orbiting 1ES 1927+654. This would result in the deceased star moving closer to the black hole, which would accelerate its speed, thereby boosting the frequency of X-ray emissions.
Despite the fact that this white dwarf is nearly at the threshold of no return concerning its closeness to the supermassive black hole, the MIT group believes that this deceased star is unlikely to plunge into the black hole any time soon. That’s due to the black hole pulling the white dwarf inward while the dead star sheds material, which propels it backward and keeps the white dwarf away from the event horizon.
“Since white dwarfs are small and dense, they are quite challenging to disintegrate, allowing them to approach a black hole closely,” Kara clarified. “If this situation is accurate, this white dwarf is right at the turning point, and we may observe it moving further away.”
The researchers plan to persist in their observations of 1ES 1927+654 and will transition to ever more advanced telescopes to achieve this. They also aim to utilize LISA, slated for launch in the 2030s, to “hear” gravitational waves emanating from the potentially audacious white dwarf evading certain demise around this black hole.
“What I’ve learned with this source is to never cease observing it as it will likely provide us with new insights,” Masterson concluded. “The next step is to remain vigilant.”
The team showcased their findings at the 245th assembly of the American Astronomical Society in National Harbor, Maryland on Monday (Jan. 13). Their conclusions are set to be published in the journal Nature.
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