Ages ago, in a galaxy some 300 million light years away, an unwitting star veered fatally close to a powerful black hole.
The research, published on January 10 in Nature, provides some tantalizing new evidence about the way a black hole evolves once it gobbles up a star. The researchers analyzed data gathered by a number of instruments, including NASA's Chandra X-ray Observatory and Neil Gehrels Swift space telescopes, as well as the European Space Agency's XMM-Newton spacecraft. These X-rays were coming from an area extremely close to the black hole's event horizon, leading the team to suggest the signal is orbiting the black hole. The signal appears to periodically brighten and fade every 131 seconds, and persists over at least 450 days.
However, the boundary of a black hole-the event horizon, which is the point of no return-offers some opportunity for study.
Although the study only looked at a stellar-mass black hole, at 10 times the mass of the sun, it may provide clues as to how black holes evolve to become "supermassive" and how that may influence the galaxies that swirl around them.
The findings, reported today in the journal Science, are the first demonstration of a tidal disruption flare being used to estimate a black hole's spin.
Black holes are massive beasts that annihilate anything that dares to cross them. The original sighting in 2014 still holds up: a black hole lured in a passing star and tore it to pieces. Some of the remains of the star are pulled into an X-ray-bright disk where they circle the black hole before passing over the "event horizon", the boundary beyond which nothing, including light, can escape.
"This system is exciting because we think it's a poster child for tidal disruption flares", Pasham says. The event, he said, appears to match theoretical predictions. He chose to apply his code to the three datasets for ASASSN-14li, to see if any common periodic patterns would rise to the surface.
Astronomers received this ancient signal, made of x-rays, in 2014. "But we saw it in all three telescopes".
"Yet the black hole is spinning so fast it completes one rotation in about two minutes, compared to the 24 hours it takes our planet to rotate", Remillard added. Estimating the spin has been tough to do until now. The victory here is the ability to use tidal disruption flares to constrain the spin.
Once Pasham discovered the periodic signal, it was up to the theorists on the team to find an explanation for what may have generated it. These X-rays can be seen every time the star orbits the black hole, which is once every 131 seconds. Others, though, remained in the innermost stable circular orbit (ISCO) - the closest spot where objects can orbit a black hole without being devoured by it. Alone, it would not have been enough to emit any sort of detectable radiation. "That gives us information about the spin rate of the supermassive black hole itself". This marks the first time that astronomers used X-rays, which orbit the black hole every 131 seconds, to calculate its incredible speed. The chances of detecting such a scenario would be exceedingly slim.
As the black hole strips away the star, this volatile matter heats up to 1 million degrees Celsius as it gets pulled into a disk around the black hole, explained Pasham. "But at least in terms of the properties of the system, this scenario seems to work". This pulse allows scientists to try to define certain properties of the black hole in question, such as its mass and spin. Going forward, he hopes to identify similar stable patterns in other star-shredding events, from black holes that reside further back in space and time. Steiner hypothesizes that as the gaseous accretion disk begins falling into the black hole, incredibly high pressures squeeze the corona's particles and thus that leads to the cosmic shrinkage we see.