X-ray pulse detected near event horizon as black hole devours star

Pulse pattern suggests distant black hole must be spinning at least at 50 percent the speed of light.

This artist's impression shows hot gas orbiting in a disk around a rapidly-spinning black hole. The elongated spot depicts an X-ray-bright region in the disk, which allows the spin of the black hole to be estimated. Image: NASA/CXC/M. Weiss
This artist's impression shows hot gas orbiting in a disk around a rapidly-spinning black hole. The elongated spot depicts an X-ray-bright region in the disk, which allows the spin of the black hole to be estimated. Image: NASA/CXC/M. Weiss

On Nov. 22, 2014, stargazers detected an uncommon vent in the night sky: A supermassive black hole at the center of a galaxy, almost 300 million light-years from Earth, tearing apart a passing star.

The event is known as a tidal disruption flare, for the black hole’s massive tidal pull that shreds a star, made a burst of X-ray movement close to the center of the galaxy. From that point forward, a large group of observatories has prepared their sights on the occasion, with expectations of adapting progressively about how black holes feed.

MIT scientists now have captured the pulse from a star that is being torn apart by a supermassive black hole. This is for the first time, scientists have observed a star being torn apart at the event horizon of a supermassive black hole—and have used the data to estimate how fast the black hole is spinning.

The signal seems to radiate from an area near the black hole’s event horizon — the point past which material is gulped inevitably by the black hole. The flag appears to occasionally light up and fade every 131 seconds and endures over no less than 450 days.

According to scientists, this signal must be orbiting the black hole, just outside the event horizon, near the Innermost Stable Circular Orbit, or ISCO — the smallest orbit in which a particle can safely travel around a black hole.

Based on the signal’s stable proximity and the black hole’s mass, scientists quantified that the black hole is spinning at about 50 percent the speed of light.

Dheeraj Pasham, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research said, “Most supermassive black holes are dormant and don’t usually emit much in the way of X-ray radiation. Only occasionally will they release a burst of activity, such as when stars get close enough for black holes to devour them.”

“Given the team’s results, such tidal disruption flares can be used to estimate the spin of supermassive black holes — a characteristic that has been, up until now, incredibly tricky to pin down.”

“Events where black holes shred stars that come too close to them could help us map out the spins of several supermassive black holes that are dormant and otherwise hidden at the centers of galaxies. This could ultimately help us understand how galaxies evolved over cosmic time.”

Scientists mainly focused their search on ASASSN-14li, an event that astronomers identified in November 2014. They examined datasets from three observatories that collected X-ray measurements of the event since its discovery: the European Space Agency’s XMM-Newton space observatory, and NASA’s space-based Chandra and Swift observatories.

For the detailed study, they even developed an algorithm for the detection of periodic patterns in astrophysical data. By applying the algorithm on three datasets for ASASSN-14li, the team observed a surprisingly strong, stable, and a periodic burst of X-ray radiation that appeared to come from very close to the edge of the black hole.

Pasham said, “At first I didn’t believe it because the signal was so strong. But we saw it in all three telescopes. So in the end, the signal was real.”

“That’s not super fast — there are other black holes with spins estimated to be near 99 percent the speed of light. But this is the first time we’re able to use tidal disruption flares to constrain the spins of supermassive black holes.”

After discovering periodic signal, scientists came up with various scenarios, but the one that seems the most likely to generate such a strong, regular X-ray flare involves not just a black hole shredding a passing star, but also a smaller type of star, known as a white dwarf, orbiting close to the black hole.

Such a white dwarf may have been circling the supermassive black hole, at ISCO — the innermost stable circular orbit — for some time. Alone, it would not have been enough to emit any sort of detectable radiation. For all intents and purposes, the white dwarf would have been invisible to telescopes as it circled the relatively inactive, spinning black hole.

Sometime around Nov. 22, 2014, a second star passed close enough to the system that the black hole tore it apart in a tidal disruption flare that emitted an enormous amount of X-ray radiation, in the form of hot, shredded stellar material.

As the black hole pulled this material inward, some of the stellar debris fell into the black hole, while some remained just outside, in the innermost stable orbit — the very same orbit in which the white dwarf circled.

As the white dwarf came in contact with this hot stellar material, it likely dragged it along as a luminous overcoat of sorts, illuminating the white dwarf in an intense amount of X-rays each time it circled the black hole, every 131 seconds.

Such a scenario would be incredibly rare and would only last for several hundred years at most — a blink of an eye in cosmic scales. The chances of detecting such a scenario would be exceedingly slim.

Pasham said, “The problem with this scenario is that, if you have a black hole with a mass that’s 1 million times that of the sun, and a white dwarf is circling it, then at some point over just a few hundred years, the white dwarf will plunge into the black hole. We would’ve been extremely lucky to find such a system. But at least in terms of the properties of the system, this scenario seems to work.”

The results’ overarching significance is that they show it is possible to constrain the spin of a black hole, from tidal disruption events. 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.

The paper describing the study is published in the journal Science.