A star has survived approaching a supermassive black hole, only to face it again three years later. The same should happen in the coming years, giving astronomers the opportunity to study the central black hole of a distant galaxy.
When an unlucky star passes too close to a black hole, its tormentor’s gravitational forces suck up its material in a process known as a tidal disruption event (TDE).
During TDE, the star is spaghetified — its plasma is torn apart, shredded and stretched into filaments that begin to swirl around the black hole, forming an accretion disk. This glowing disk of heated gas can be detected by astronomers, appearing as a large explosion.
One such explosion, called AT2018fyk, occurred on September 8, 2018 and was detected by instruments such as the All-Sky Automated Survey for Supernovae (ASASSN), Swift, XMM-Newton and NICER.
The event occurred in the core of a galaxy located 893 million light-years away and was unusually bright. Typically, TDEs exhibit a smooth decline in brightness over several years, but this was not the case for the AT2018fyk.
About 600 days after the AT2018fyk explosion, the X-rays emitted by the event disappeared much faster than predicted. Another 600 days passed and the black hole suddenly exploded again.
In a new study, astronomers demonstrate a model that may explain this unusual event: the star, part of a binary system split by the black hole, survived spaghettification on its first approach. Still trapped in the orbit of the cosmic titan, it returned to experience another tidal disruption.
The first approach occurred when the star had a binary companion. At the time, the black hole threw one of them away with a gravitational “kick”, giving it a speed of 1,000 km/s, out of the galaxy. The other star, in turn, was tightly bound to the black hole, in an elliptical orbit of 1,200 days.
Even trapped in the black hole’s orbit, the second star wasn’t quite within the radius where the TDE fully occurs. With that, only part of its material was stripped after the first approach, thus forming the bright accretion disk.
This accretion disk, however, took about 600 days to form around the black hole. When astronomers saw the explosion formed by the event, the star was already safe, close to the most distant point of its orbit.
However, after another 600 days, the star, now reduced to just a dense core, began to approach the black hole again and steal almost all the gas back from the accretion disk. This caused the X-ray emission to suddenly disappear.
Again, the black hole’s gravity soon robs the star of more material as it makes its closest pass, resulting in a new accretion disk after 600 days. This made the system glow again in X-rays.
This cycle of interactions helped astronomers determine that the black hole has a mass nearly 80 million times that of the Sun. Upcoming TDE events on this star could also contribute to our understanding of how supermassive black holes evolved.
In August, the star — or what’s left of it — will approach the black hole again, stealing material from the accretion disk and dimming the X-ray emissions. Meanwhile, the black hole will suck more matter out of the star, and after 600 days (March 2025), the new accretion disk will make the black hole shine again.
Of course, this won’t last forever: At some point, the star will eventually lose enough mass to spaghetify completely. When will this happen? According to the study, it depends on how much mass the star loses with each approach. If the loss is just 1%, the star will survive for many more encounters, but if it’s 10%, it may already be destroyed.
Source: The Astrophysical Journal Latters, Syracuse University, College of Arts and Sciences
