On November 22, 2014, astronomers spotted a rare event in the night sky: a supermassive black hole in the center of a galaxy, about 300 million light years from Earth, destroying a passing star. The event, known as an outbreak of tidal disruption due to the massive force of the black hole tide ripping a star, has created an explosion of X-ray activity near the center of the galaxy. Since then, a series of observatories have trained their visions at the event, hoping to learn more about how black holes feed.
Now, researchers at MIT and elsewhere have looked at observations from multiple telescopes at the event and found a curiously intense, stable, and periodic pulse or X-ray signal in all data sets. The signal seems to emanate from an area very close to the event horizon of the black hole – the point beyond which the material is swallowed inescapably by the black hole. The signal appears to clear and disappear periodically every 131 seconds and persists for at least 450 days.
Researchers believe that whatever is emitting the periodic signal must be in orbit of the black hole, near the event horizon, near the Closest Circular Orbit, or ISCO – the smallest orbit in which a particle can safely travel around of a black hole.
Given the steady proximity of the signal to the black hole, and the mass of the black hole, which the researchers estimate to be about 1 million times greater than that of the Sun, the team calculated that the black hole is spinning around 50% of the speed of light.
The results, reported today in the journal Science, are the first demonstration of an outbreak of tidal disruption being used to estimate the rotation of a black hole.
First author of the study, Dheeraj Pasham, a postdoctoral fellow at the Kavli Institute for Astrophysics and Space Research at MIT, says that most supermassive black holes are dormant and do not normally emit much in the way of X-ray radiation. Only occasionally do they release an explosion of activity, like when the stars get close enough for the black holes to devour them. Now he says that given the team's results, these tidal bursts can be used to estimate the spinning of supermassive black holes – a feature that has heretofore been incredibly difficult to define.
"Events where black holes destroy nearby stars can help us map the spins of several supermassive black holes that are inactive and hidden in galaxy centers," says Pasham. "This could help us understand how galaxies have evolved over time."
Pasham's co-authors include Ronald Remillard, Jeroen Homan, Deepto Chakrabarty, Frederick Baganoff, and James Steiner of MIT; Alessia Franchini, University of Nevada; Chris Fragile of the College of Charleston; Nicholas Stone of Columbia University; Eric Coughlin of the University of California at Berkeley; and Nishanth Pasham, of Sunnyvale, California.
A real sign
Theoretical models of tidal explosions show that when a black hole breaks up a star, some of the star's material may be outside the event horizon, circulating, at least temporarily, in a stable orbit such as ISCO, and emitting periodic flashes of lightning- X before finally being fed by the black hole. The periodicity of the X-ray flashes therefore encodes important information about the size of the ISCO, which is dictated by the rapidity with which the black hole is rotating.
Pasham and his colleagues thought that if they could see these regular flashes very close to a black hole that had passed a recent tidal-breaking event, these signals could give them an idea of how fast the black hole was spinning.
They focused their search on ASASSN-14li, the tidal-breaking event that astronomers identified in November 2014, using the ASASSN based on the ground.
"This system is exciting because we think it is an example for tidal-bursts," says Pasham. "This particular event seems to coincide with many of the theoretical predictions."
The team analyzed the archived datasets from three observatories that have collected X-ray measurements of the event since its inception: the European Space Agency's XMM-Newton space observatory and NASA-based Chandra and Swift observatories. Pasham has previously developed a computer code to detect periodic patterns in astrophysical data, though not specifically for tidal-breaking events. He decided to apply his code to the three ASASSN-14li data sets to see if any common periodic patterns would surface.
What he observed was a surprisingly strong, steady, periodic explosion of X-ray radiation that seemed to come from very close to the edge of the black hole. The signal pulsed every 131 seconds, over 450 days, and was extremely intense – about 40% above the average black-ray brightness of the black hole.
"At first I did not believe because the signal was so strong," says Pasham. "But we saw that in the three telescopes. Then, in the end, the signal was real.
Based on the signal properties and mass and size of the black hole, the team estimated that the black hole is spinning at least 50% of the speed of light.
"This is not super fast – there are other black holes with spins estimated at about 99% of the speed of light," says Pasham. "But this is the first time we can use tidal burst flags to restrict supermassive black hole spins."
Lighting the Invisible
As soon as Pasham discovered the periodic signal, it was up to the team theorists to find an explanation for what might have generated it. The team presented several scenarios, but what appears to be the most likely to generate such a strong and regular explosion of X-rays involves not only a black hole ripping a passing star but also a smaller type of star, known as white. dwarf, orbiting near the black hole.
Such a white dwarf may have circled the supermassive black hole in the ISCO – the innermost and stable circular orbit – for some time. Alone, it would not have been enough to emit any detectable radiation. For all intents and purposes, the white dwarf would have been invisible to the telescopes while circling the relatively inactive and rotating black hole.
Around November 22, 2014, a second star passed close enough to the system that the black hole destroyed it in a surge of tidal disruption that emitted an enormous amount of X-ray radiation in the form of hot stellar material and crushed As the black hole drew this material in, some of the stellar debris fell into the black hole, while some remained outside, in the innermost stable orbit – the same orbit in which the white dwarf circled. When the white dwarf came in contact with this hot stellar material, it probably dragged him like a luminous overcoat, lighting the white dwarf in an intense amount of X-rays each time the black hole circled every 131 seconds.
Scientists admit that such a scenario would be incredibly rare and would last only a few hundred years at most – a blink of an eye on cosmic scales. The odds of spotting this scenario would be extremely small.
"The problem with this scenario is that if you have a black hole with a mass that is 1 million times larger than that of the sun, and a white dwarf is circling, then at some point for a few hundred years the dwarf "Pasham says:" We would be very lucky to find this system, but at least in terms of system properties, this scenario seems to work. "
The general meaning of the results is that they show that it is possible to restrict the turning of a black hole, from tidal breaking events, according to Pasham. Going forward, he hopes to identify similar stable patterns in other star-destroying events, of black holes residing more in space and time.
"Over the next decade, we expect to see more of these events," says Pasham. "Estimating the turns of several black holes since the beginning of time until now would be valuable in terms of estimating whether there is a relationship between the turning and the age of the black holes."
This research was supported, in part, by NASA.