Why Do Repeated Black Hole Flares Fade? Star Spin May Hold the Answer

Some stars that stray near supermassive black holes survive repeated close passes, each one stripping away material in what astronomers call a repeating partial tidal disruption event, or rpTDE, producing a flare of light. Scientists have long puzzled over why these flares dim with each new encounter. Researchers led by Ananya Bandopadhyay of Syracuse University say they may have solved the mystery.

When a star passes close enough, tidal forces can stretch it into a strand of plasma in a process called spaghettification, feeding it entirely to the black hole in a full tidal disruption event, or TDE. A star in an rpTDE instead survives repeated bites, flaring each time.

The team earlier found this partly depends on a star's structure: a low-mass star is like fluffy meringue, losing material from deep within, while a high-mass star is layered like an onion, with only outer layers stripped. That alone could not explain dimming in four of the ten known rpTDEs, since the black hole spins up a passing star, shortening its orbit and keeping flares bright.

The missing piece may be a star already spinning fast before its first encounter. Then the black hole's torque adds little more spin, so the orbit does not shorten, and less material stripped per pass shows up as dimming. This fast spin could come from "Hills capture": most stars orbit in binary pairs, and a black hole can rip apart a tight binary nearby, ejecting one star while capturing the other onto a tight, months-long orbit that leaves it spinning fast.

Team member Eric Coughlin of Syracuse University said the pattern traces to one underlying cause: the tidal destruction of a binary system and the capture of one of the stars. The findings may also explain fast-orbiting stars around the Milky Way's black hole, Sagittarius A* (Sgr A*), though not currently in an rpTDE. The research was published Aug. 18 in The Astrophysical Journal.