Astronomers may have discovered a new kind of survival story: a star that had a brush with a giant black hole and lived to tell the tale via bursts of X-rays.
Data from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton uncovered the event, caused by a red giant star passing too close to a supermassive black hole in a galaxy dubbed GSN 069, about 250 million light-years from Earth. The black hole has a mass about 400,000 times that of the Sun, putting it on the small end of the scale for such cosmic phenomena.
Once the red giant was captured by the black hole’s gravity, the outer layers of the star containing hydrogen were stripped off and careened toward the black hole, leaving the core of the star – known as a white dwarf – behind.
“In my interpretation of the X-ray data the white dwarf survived, but it did not escape,” said Andrew King, of the University of Leicester in England, who performed the study. “It is now caught in an elliptical orbit around the black hole, making one trip around about once every nine hours.”
As the white dwarf makes its nearly thrice-daily orbit, the black hole pulls material off at its closest approach, some 15 times the radius of the event horizon – or the point of no return – away from the black hole. The stellar detritus enters into a disk surrounding the black hole and releases a burst of X-rays that Chandra and XMM-Newton can detect. In addition, King predicts gravitational waves will be emitted by the black hole and white dwarf pair, especially at their nearest point.
King anticipates that the combined effect of gravitational waves and an increase in the star’s size as it loses mass should cause its orbit to become wider and more circular, slowing its loss of mass and eventually sending the white dwarf spiraling slowly away from the black hole.
“It will try hard to get away, but there is no escape. The black hole will eat it more and more slowly, but never stop,” King said. “In principle, this loss of mass would continue until, and even after, the white dwarf became a planet, with a mass similar to Jupiter, in about a trillion years. This would be a remarkably slow and convoluted way for the universe to make a planet!”
Astronomers have found many stars that have been completely torn apart by encounters with black holes – dubbed tidal disruption events – but there are very few reported cases of near misses, in which the star likely survived.
Grazing encounters like this should be more common than direct collisions, given the statistics of cosmic traffic patterns, but they could easily be missed for a couple of reasons, King said. First, it can take a surviving star too long to complete an orbit around a black hole for astronomers to see repeated bursts. Black holes much more massive than the one in GSN 069 also may swallow a star whole. In these cases, astronomers wouldn’t observe anything.
“In astronomical terms, this event is only visible to our current telescopes for a short time – about 2,000 years,” King said. “So unless we were extraordinarily lucky to have caught this one, there may be many more that we are missing. Such encounters could be one of the main ways for black holes the size of the one in GSN 069 to grow.”
A paper on the findings appears in the March 2020 issue of the Monthly Notices of the Royal Astronomical Society.
Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science and flight operations from Cambridge and Burlington, Massachusetts.