Star's Own Spin Explains Fading Black Hole Flares

Stars that survive a close encounter with a supermassive black hole and return for more passes lose part of their mass each time, but in four of the roughly ten known systems of this kind, every successive flash of light was fainter than the one before, and existing models could not explain why. A team of astrophysicists at Syracuse University now says the answer lies in a property nobody had accounted for: how fast the star was already spinning before the black hole first caught it. Sources differ on the publication date: the paper appeared in the journal on August 19, the university’s statement followed on August 20, and it was reported more widely on August 23.

When a star passes too close to a supermassive black hole, the difference in gravitational pull between its near and far side, known as a tidal force, can tear it apart completely. That is a classic tidal disruption event. Part of the shredded material then falls, or accretes, onto the black hole, releasing light over a period ranging from a few days to several months, which lets astronomers indirectly study an object that does not radiate on its own. If the star does not come close enough to be destroyed outright, it loses only part of its mass, and the surviving core keeps orbiting the black hole, returning for more close passes every few months to a few years. Scientists call this a repeating partial tidal disruption. The same team had already shown in earlier work that a black hole’s tidal force does more than strip away matter, it also applies torque that spins the star up further with every pass.

The puzzle that stumped the team for two years

So far, about 10 systems have been found in which the same star repeatedly returns to a black hole and loses part of its material each time instead of being destroyed all at once. In four of them, each successive flare is fainter than the last, and for a long time that looked impossible to explain. A shrinking amount of stripped material seemed like the obvious cause, but earlier hydrodynamic simulations showed the opposite: even as the star loses less and less mass, the predicted flare stays roughly the same brightness, because the star’s faster spin shortens the time it takes for the debris to fall back, making up for the shortfall.

“We’ve been stumped by this for two years,” said Ananja Bandopadaj, a PhD candidate in the Department of Physics at Syracuse University, in a university statement.

What was missing, as Bandopadaj put it, was one new ingredient: a star that was already spinning very fast before its first encounter. The new study shows that such a star cannot be sped up much further on later passes, so the time it takes for the stripped gas to fall back onto the black hole stays roughly the same from one pass to the next. Since the star loses less material each time, the peak inflow of gas drops, and with it the brightness of the flare, exactly as telescopes record it.

Why would a star already be spinning that fast before its first encounter?

That leaves the question of why a star would already be spinning so fast before the black hole first reached it.

“It’s also extremely hard to bind a star to a supermassive black hole tightly enough that it orbits in just a few months, and yet that seems to be what’s happening in these repeating events,” said Erik Koflin, an associate professor in the Department of Physics at Syracuse University, in a university statement.

The answer may be what’s known as a Hills breakup. In this mechanism, a binary system of two stars orbiting tightly around each other passes by a supermassive black hole, whose gravity splits the pair apart: one star gets flung out of the system, while the other is captured into its own orbit. In a tight binary, stars often become tidally locked, meaning each spins on its own axis at the same rate the pair orbits each other. The tighter the binary, the shorter its orbital period and the faster the tidally locked star spins, so a binary tight enough to leave behind the kind of short orbit seen in these repeating events would also have to leave behind a star that is already spinning fast.

How much material a star loses on each pass also depends on its internal structure. Bandopadaj compares a low mass star to a soft, loose meringue, increasingly vulnerable to the black hole’s tidal forces with every pass. A more massive star has a denser core, layered like an onion, so it sheds its outer layers while the core stays relatively intact, meaning it loses less and less mass with each successive pass. These structural differences explain why repeating tidal disruption events do not all behave the same way.

“Ananja’s work shows that each of these oddities can be explained by the same underlying phenomenon: the tidal breakup of a binary system and the capture of one of the stars. From a theoretical standpoint, this is a major step forward in our understanding of the physics at work in these systems,” Koflin said.

Koflin adds that a Hills breakup could also explain the origin of some of the stars currently orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way, in what he calls “our own cosmic backyard.”

What this study does not prove

The paper, whose lead author is Bandopadaj along with Amend, Koflin and collaborators from other institutions, was written by researchers in the Department of Physics at Syracuse University and published in The Astrophysical Journal under identifier 10.3847/1538-4357/ae8f31. Its conclusions rest on hydrodynamic simulations, not on direct observation of the star’s own rotation, since a star’s spin before its first encounter cannot be measured directly at that distance. The sample of roughly ten known repeating partial tidal disruptions, only four of which show fading flares, remains too small for any statistical generalization. The study also does not prove that a Hills breakup actually produced the stars observed, it remains only a mechanism that could explain both their fast spin and their unusually tight orbit at the same time. Whether that is really what happened will require further observations and, ideally, the discovery of new repeating events in future sky surveys.