The team was supported by the UK Space Agency and the UK Science and technology Facilities Council (STFC).
The astronomers were alerted to the star by a bright X-ray flash that seemed to come from the centre of the nearby galaxy 2MASX J02301709+2836050, around 500 million light-years away from the Milky Way. Named Swift J0230, it was spotted the moment it happened for the first time using a new tool developed by the scientists for the Neil Gehrels Swift Observatory. They rapidly scheduled further Swift observations of it, finding that instead of decaying away as expected, it would shine brightly for 7-10 days and then abruptly switch off, repeating this process roughly every 25 days.
Similar behaviour has been observed in what are termed quasi-periodic eruptions and periodic nuclear transients, where a star has material ripped away by a black hole as its orbit takes it close by, but they differ in how often they erupt, and in whether it is in X-rays or optical light that the explosion is predominant. The regularity of Swift J0230's emissions fell between the two, suggesting that it forms the 'missing link' between the two types of outburst.
Using the models proposed for these two classes of event as a guide, the scientists concluded that the Swift J0230 outburst represents a star of a similar size to our own sun in an elliptical orbit around a low-mass black hole at the centre of its galaxy. As the star's orbit takes it close to the intense gravitational pull of the black hole, material equivalent to the mass of three Earths is wrenched from the atmosphere of the star and heated up as it falls into the black hole. The intense heat, around 2 million degrees Celsius, releases a huge amount of X-rays which were first picked up by the Swift satellite.
Lead author Dr Phil Evans of the University of Leicester School of Physics and Astronomy said: "This is the first time we've seen a star like our Sun being repeatedly shredded and consumed by a low mass black hole. So-called 'repeated, partial tidal disruption' events are themselves quite a new discovery and seem to fall into two types: those that outburst every few hours, and those that outburst every year or so. This new system falls right into the gap between these, and when you run the numbers, you find the types of objects involved fall nicely into place too."
Dr Rob Eyles-Ferris, who works with Dr Evans on the Swift satellite, recently completed his PhD at Leicester, which included the study of stars being disrupted by black holes. He explains: "In most of the systems we've seen in the past the star is completely destroyed. Swift J0230 is an exciting addition to the class of partially-disrupted stars as it shows us that the two classes of these objects already found are really connected, with our new system giving us the missing link."
Dr Kim Page from the University of Leicester, who worked on the data analysis for the study, said: "Given that we found Swift J0230 within a few months of enabling our new transient-hunting tool, we expect that there are a lot more objects like this out there, waiting to be uncovered."
Dr Chris Nixon is a theoretical astrophysicist who recently moved from the University of Leicester to the University of Leeds. He led the theoretical interpretation of this event. His research is funded by the UK Science and Technology Facilities Council and the Leverhulme Trust.
They estimate that the black hole is around 10,000 to 100,000 times the mass of our sun, which is quite small for the supermassive black holes usually found at the centre of galaxies. The black hole at the centre of our own galaxy is thought to be 4 million solar masses, while most are in the region of 100 million solar masses.
It is the first discovery to be made using the new transient detector for the Swift satellite, developed by the University of Leicester team and running on their computers. When an extreme event takes place, causing an X-ray burst in a region of the sky where there were previously no X-rays, astronomers call it an astronomical X-ray transient. Despite the extreme events they herald, these events are not easy to find, or at least, not quickly - and so this new tool was developed to look for new types of transients in real time.
Dr Evans adds: "This type of object was essentially undetectable until we built this new facility, and soon after it found this completely new, never-before-seen event. Swift is nearly 20 years old and it's suddenly finding brand new events that we never knew existed. I think it shows that every single time you find a new way of looking at space, you learn something new and find there's something out there you didn't know about before."
Dr Caroline Harper, Head of Space Science at the UK Space Agency, said: "This is yet another exciting discovery from the world-leading Swift mission - a low mass black hole taking 'bites' from a Sun-like star whenever it orbits close enough.
"The UK Space Agency has been working in partnership with NASA on this mission for many years; the UK led on the development of hardware for two of the key science instruments and we provided funding for the Swift Science Data Centre, which we continue to support. We look forward to even more insights from Swift about gamma ray bursts throughout the cosmos, and the massive events that cause them, in the future."
Swift learns a new trick, spots a snacking black hole
by Jeanette Kazmierczak for GSFC News
Greenbelt MD (SPX) Sep 11 - Using NASA's Neil Gehrels Swift Observatory, which launched in 2004, scientists have discovered a black hole in a distant galaxy repeatedly nibbling on a Sun-like star. The object heralds a new era of Swift science made possible by a novel method for analyzing data from the satellite's X-ray Telescope (XRT).
"Swift's hardware, software, and the skills of its international team have enabled it to adapt to new areas of astrophysics over its lifetime," said Phil Evans, an astrophysicist at the University of Leicester in the United Kingdom and longtime Swift team member. "Neil Gehrels, the mission's namesake, oversaw and encouraged many of those transitions. Now, with this new ability, it's doing even more cool science."
Evans led a study about the unlucky star and its hungry black hole, collectively called Swift J023017.0+283603 (or Swift J0230 for short), which was published on Sept. 7 in Nature Astronomy.
When a star strays too close to a monster black hole, gravitational forces create intense tides that break the star apart into a stream of gas. The leading edge swings around the black hole, and the trailing edge escapes the system. These destructive episodes are called tidal disruption events. Astronomers see them as flares of multiwavelength light created when the debris collides with a disk of material already orbiting the black hole.
Recently, astronomers have been investigating variations on this phenomena, which they call partial or repeating tidal disruptions.
During these events, every time an orbiting star passes close to a black hole, the star bulges outward and sheds material, but survives. The process repeats until the star loses too much gas and finally breaks apart. The characteristics of the individual star and black hole system determine what kind of emission scientists observe, creating a wide array of behaviors to categorize.
Previous examples include an outburst that occurred every 114 days, potentially caused by a giant star orbiting a black hole with 78 million times the Sun's mass. Another recurred every nine hours around a black hole with 400,000 times the Sun's mass, likely caused by an orbiting stellar cinder called a white dwarf.
On June 22, 2022, the XRT captured Swift J0230 for the first time. It lit up in a galaxy around 500 million light-years away in the northern constellation Triangulum. Swift's XRT observed nine additional outbursts from the same location roughly every few weeks.
Evans and his team propose that Swift J0230 is a repeating tidal disruption of a Sun-like star orbiting a black hole with over 200,000 times the Sun's mass. They estimate the star loses around three Earth masses of material on each pass. This system provides a bridge between other types of suspected repeating disruptions and allowed scientists to model how interactions between different star types and black hole sizes affect what we observe.
"We searched and searched for the event brightening in the data collected by Swift's Ultraviolet/Optical Telescope," said Alice Breeveld, a research fellow at the University College London's Mullard Space Science Laboratory (MSSL) who has worked on the instrument since before the satellite launched. "But there wasn't any sign of it. The galaxy's variability was entirely in X-rays. That helped rule out some other potential causes."
Swift J0230's discovery was possible thanks to a new, automated search of XRT observations, developed by Evans, called the Swift X-ray Transient Detector.
After the instrument observes a portion of the sky, the data is transmitted to the ground, and the program compares it to previous XRT snapshots of the same spot. If that portion of the X-ray sky has changed, scientists get an alert. In the case of Swift J0230, Evans and his colleagues were able to rapidly coordinate additional observations of the region.
Swift was originally designed to study gamma-ray bursts, the most powerful explosions in the cosmos. Since the satellite launched, however, scientists have recognized its ability to study a whole host of celestial objects, like tidal disruptions and comets.
"Swift J0230 was discovered only about two months after Phil launched his program," said S. Bradley Cenko, the mission's principal investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It bodes well for the detector's ability to identify other transient events and for Swift's future exploring new spaces of science."
Goddard manages the Swift mission in collaboration with Penn State, the Los Alamos National Laboratory in New Mexico, and Northrop Grumman Space Systems in Dulles, Virginia. Other partners include Leicester, MSSL, Brera Observatory in Italy, and the Italian Space Agency.
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