A sun-like star located in a galaxy about 500 million light-years away is being gradually swallowed by a black hole. Each time it approaches a black hole, it will shed material equivalent to three Earth masses.
A massive burst of X-rays detected by astronomers at the University of Leicester suggests that three times the mass of Earth is burning in a black hole. In a nearby galaxy, a sun-like star is being slowly devoured by a small, ravenous black hole, losing three Earth masses of material with each approach.
The discovery by astronomers at the University of Leicester, published in the journal Nature Astronomy on September 7, provides a "missing link" in our understanding of how black holes disrupt orbiting stars. It suggests that a large number of stars are being devoured but have yet to be discovered. The research team received support from the UK Space Agency and the UK Science and Technology Facilities Council (STFC).
discover details
Astronomers were awakened by a bright X-ray flash that appeared to come from the center of the nearby galaxy 2MASXJ02301709+2836050, about 500 million light-years from the Milky Way. Named Swift J0230, it was first discovered by scientists using a new tool developed at the Neil Gehrels Swift Observatory. They quickly arranged further Swift observations of it and found that it did not gradually decay as expected, but would shine brightly for 7-10 days and then suddenly go out, repeating this process approximately every 25 days.
Optical image of a galaxy experiencing a new event, taken from archived PanSTARRS data. The X-ray object is located somewhere within the white circle, about the size of a pinhead 100 meters away. Also shown is the location of a 2-year-old supernova. Source: Daniele B. Malesani/PanSTARRS
Connect the missing pieces
Similar behavior has been observed in so-called quasi-periodic bursts and periodic nuclear transients, in which a star's material is torn apart by a black hole as it orbits close to it, but they differ in the frequency of the bursts and in whether the bursts occur primarily in X-rays or optical light. The regularity of Swift J0230's radiation is somewhere in between, suggesting it is the "missing link" between the two burst types.
Using models proposed for both types of events as a guide, the scientists concluded that Swift J0230's outburst represented a star similar in size to our sun orbiting in an elliptical orbit around a low-mass black hole at the center of the galaxy. As the star's orbit approaches the black hole's powerful gravitational pull, material equivalent to three Earth masses is ripped out of the star's atmosphere and heated as it falls into the black hole. The high temperature of about 2 million degrees Celsius releases a large amount of X-rays, which were first captured by the Swift satellite.
X-ray images of the same location in the sky before (left) and after (right) the explosion of Swift J0230. The images were taken with the X-ray telescope on board the Swift satellite. Image credit: Phil Evans (University of Leicester)/NASASwift
Expert Insights
Lead author Dr Phil Evans, from the School of Physics and Astronomy at the University of Leicester, said: "This is the first time we have seen a star like the Sun being repeatedly torn apart and swallowed up by a low-mass black hole. The so-called 'repeated, partial tidal disruption' event itself is a "It's a fairly new discovery, and it seems to fall into two categories: those that erupt every few hours, and those that erupt every year or so. This new system falls right in between those two categories, and when you run the numbers, you see that the types of objects involved also fit very well."
Dr Rob Eyles-Ferris, who works with Dr Evans on the Swift satellite, recently completed his PhD at the University of Leicester, which included studying the destruction of stars by black holes. "In most systems we've seen in the past, the star was completely destroyed," he explained. "Swift J0230 is an exciting new addition to the category of partially destroyed stars because it shows us the true connection that has been discovered between these two classes of objects, and our new system provides us with the missing link."
Illustration of the Neil Gales-Swift Observatory. Source: NASA
Looking forward to discovering more
Dr. Kim Page from the University of Leicester was involved in data analysis for this study: "Given that we discovered Swift J0230 within a few months of launching our new transient hunting tool, we expect there will be many more such objects waiting to be discovered."
Dr Chris Nixon is a theoretical astrophysicist who recently transferred to the University of Leeds from the University of Leicester. He led efforts to theoretically explain the event. His research has been funded by the UK Science and Technology Facilities Council and the Leverhulm Trust.
They estimate that the black hole is about 10,000 to 100,000 times the mass of the Sun, which is quite small for the supermassive black holes typically found in the centers of galaxies. The black hole at the center of our galaxy is thought to have a mass of 4 million solar masses, while most black holes are around 100 million solar masses.
This is the first discovery using the Swift satellite's new transient detector, which was developed by the University of Leicester team and ran on their computers. When an extreme event occurs, it causes bursts of X-rays in areas of the sky that were previously devoid of X-rays, which astronomers call astronomical X-ray transients. Although these transient events herald the onset of extreme events, they are not easy to detect, at least not quickly, so this new tool was developed to look for new types of transient events in real time.
Dr Evans added: "This type of object was largely undetectable until we built this new facility and very quickly discovered this brand new, never-before-seen event. Swift is almost 20 years old and it's suddenly discovering brand new events that we never knew existed. I think that shows that every time you find a new way of looking at space, you learn something new and find that there's something out there that you didn't know existed before."
International cooperation and future prospects
Dr Caroline Harper, head of space science at the UK Space Agency, said: "This is another exciting discovery from the world-leading Swift mission - every time a sun-like star orbits close enough, a low-mass black hole bites it."
The UK Space Agency has been working with NASA on this mission for many years; the UK led the development of the hardware for two key science instruments and provided funding for the Swift Science Data Centre, which we continue to support. We look forward to Swift's future understanding of gamma-ray bursts throughout the universe and the large-scale events that cause them.