About 3 billion years ago, a hapless star got caught in a twisted tug-of-war between two gigantic black holes — and now, we are seeing the faint screams of X-rays emanating from this violent event. If confirmed, it could be the most distant episode of two black holes attacking a star ever seen.
An international team of astronomers reported their decades-long observation of the faintest known variable X-ray flare in a paper accepted for publication in the journal The Innovation in November.
The X-ray source, named XID 925, was first spotted in 1999 within the Chandra X-ray Observatory's Deep Field South survey, the deepest and most complete X-ray survey ever taken. Since then, astronomers have kept a close eye on it, watching as what was initially a bright pinprick of radiation fell dimmer and dimmer, reaching just a paltry one-fortieth of its initially observed peak.
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Making stellar spaghetti
A bright surge in X-rays followed by a long span of dimming is exactly what astronomers expect from violent encounters called tidal disruption events (TDEs), which happen when a star wanders too close to a supermassive black hole. Before the star is swallowed by the monster's event horizon (the point of no return), the black hole's enormous gravity rips the star to shreds — a process cutely dubbed "spaghettification," as if the star were being pulled into a thin strand of pasta.
The stellar material then settles into a thin, rapidly rotating disk just outside the black hole. The energy released by this process makes the gas so hot that it emits X-ray radiation that's visible even from the other side of the universe. Then, the material funnels its way to the gaping maw of the black hole itself, and the disk loses brightness.
XID 925 was already remarkable, as it was one of the most distant and faintest known TDEs ever recorded. But in 1999, it all went haywire.
Between January and March of that year, XID 925 rapidly and unexpectedly brightened by a factor of 27. Then, the X-ray brightness collapsed just as quickly as it appeared, and XID 925 continued to fade from the scene.
A back-to-back black hole attack
Now, the astronomers behind the new study believe there is another culprit behind this strange brightening. This is no simple case of a TDE around a single supermassive black hole. This is a case of a TDE around two supermassive black holes.
They argue that the unlucky star was caught in the gravitational embrace of a central gigantic black hole and another, smaller (but still large in its own right) companion black hole. The larger black hole tore apart the star and transformed it into an accretion disk. But then, the second black hole swung close to the disk, or even plowed right through it, and this disruption led to a furious burst of energies, the scientists explained.
Like a hapless car crashing into the scene of an accident, the event made a messy situation even messier — in this case, by triggering the release of even more X-rays. Once the smaller black hole moved on, the system returned to normal.
While the astronomers cautioned that this story doesn't perfectly explain all of the data, they argued that it's the most compelling scenario given what we know. If it is true, it would be the most distant known binary black hole tidal disruption event, giving us a crucial and exciting window into the complex relationships between stars and black holes in the hearts of young galaxies.
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