Wednesday, May 11, 2022

'Runaway' star may explain unusual glow around black hole

At the center of a distant galaxy, a black hole is slowly consuming a disk of gas that swirls around it like water in a drain. As a steady stream of gas is drawn into the "mouth" of the "monster", ultra-hot particles gather near the black hole, above and below the disk, generating an X-ray glow that can be seen at 300 millions light years away from Earth. 

We know that these collections of ultra-hot gas, called coronas, exhibit visible changes in their luminosity, brightening or dimming by a factor of up to 100 times when a black hole feeds. But two years ago, astronomers watched in awe as X-rays from the black hole's corona in a galaxy known as 1ES 1927+654 completely disappear, shrinking by a factor of 10,000 in about 40 days. about 100 days later, it became nearly 20 times brighter than before the event.

 X-rays from a black hole's corona are direct by-products of the black hole's feeding, so the disappearance of this radiation from 1ES 1927+654 likely means its food supply has been cut off. In a new study published in The Astrophysical Journal Letters, scientists theorize that a runaway star may have gotten too close to the black hole and been destroyed. If that were the case, the star's moving debris could have collided with part of the disk, briefly dispersing the gas. 

University of Santiago, Chile, and lead author of the study. "It was so weird that at first we thought there was something wrong with our data. When we saw it was real, it was very exciting. But we also had no idea what we were dealing with; no one had ever seen anything like this." Almost every galaxy in the Universe can host a supermassive black hole at its center, like the one in 1ES 1927+654, millions or billions of times larger than our Sun. They grow by consuming the gas that surrounds them, also known as an accretion disk. Because black holes do not emit or reflect light, they cannot be seen directly, but the light from their coronas and accretion disks provides a way to learn more about them. dark objects. 

The authors' stellar hypothesis is also supported by the fact that a few months before the X-ray signal disappeared, observatories on Earth saw the disk greatly increase in brightness at visible wavelengths (those that can be seen by the human eye). . This could have resulted from the initial collision of stellar debris with the disk.

The disappearance event at 1ES 1927+654 is unique not only because of the dramatic change in brightness, but also because of the depth at which astronomers were able to study it. The burst of visible light prompted Ricci and colleagues to request follow-up observations of the black hole using NASA's NICER (Neutron star Interior Composition Explorer), an X-ray telescope aboard the International Space Station. In total, NICER observed the system 265 times over 15 months. Additional monitoring in X-rays was obtained with NASA's Swift Neil Gehrels Observatory - which also observed the system under ultraviolet light - as well as NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) and ESA's XMM-Newton Observatory.

As the coronal X-rays disappeared, NICER and Swift observed less energetic X-rays from the system, so collectively these observatories provided a continuous stream of information throughout the event.

While a rogue star seems the most likely culprit, the authors note that there may be other explanations for the unprecedented event. A notable feature of the observations is that the overall drop in brightness was not a smooth transition: day by day, the low-energy X-rays that NICER detected showed dramatic variation, sometimes changing brightness by a factor of 100 by as little as 8. hours . In extreme cases, black hole coronas are known to become 100 times brighter or dimmer, but on much longer timescales. These rapid changes, occurring continuously for months, are extraordinary.

“This dataset contains a lot of puzzles,” said Erin Kara, an assistant professor of physics at MIT (Massachusetts Institute of Technology) and co-author of the new study. "But this is exciting, because it means we're learning something new about the Universe. We think the stellar hypothesis is good, but I also think we're going to keep looking at this event for a long time to come." and next-generation observatories are designed to look for short-term changes in cosmic phenomena, a practice known as "time domain astronomy," that could reveal more events like this.

"This new study is a great example of how flexibility in the timing of observations allows NASA and ESA missions to study relatively fast-evolving objects and look for long-term changes in their average behavior," said Michael Loewenstein, co-author of the paper. and astrophysicist for the NICER mission at the University of Maryland in College Park and NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Is this active black hole going to return to the state it was in before the disturbance event? Or is the system fundamentally altered? We are continuing our observations to find out."

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