On November 22, 2014, astronomers discovered a rare event in the night sky: a supermbadive black hole in the center of a galaxy, nearly 300 million light years from Earth, destroying a pbading star. The event, known as a burst of interruption of the tide, for the huge tidal wave of the black hole that tears a star, created an explosion of X-ray activity near the center of the galaxy. Since then, a large number of observatories have trained their sights on the event, hoping to learn more about how black holes feed.
Now, researchers at MIT and elsewhere have reviewed observational data from several telescopes about the event and have discovered a curiously intense, stable and periodic X-ray pulse or 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 from which the black hole swallows the material unavoidably. The signal seems to light up and disappear periodically every 131 seconds, and persists for at least 450 days.
Researchers believe that everything that emits the periodic signal must be orbiting the black hole, just outside the event horizon, near the Lower Stable Circular Orbit, or ISCO, the smallest orbit in which a particle can travel safely around of a black hole.
Given the stable proximity of the signal to the black hole and the mbad of the black hole, which the researchers estimated was approximately 1 million times greater than that of the sun, the team calculated that the black hole is spinning at approximately 50 percent. of the speed of light.
The findings, published today in the magazine. Science, are the first demonstration of a burst of interruption of the tide that is used to estimate the spin of a black hole.
The first author of the study, Dheeraj Pasham, a postdoctoral fellow at the Kavli Institute for Astrophysics and Space Research at MIT, says that most supermbadive black holes are inactive and do not usually emit much in the X-ray radiation mode. Only occasionally will 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 interruption flares can be used to estimate the spin of supermbadive black holes, a feature that, until now, has been incredibly difficult to pin down.
"Events in which black holes destroy stars that come too close to them could help us trace the turns of several supermbadive black holes that are dormant and, otherwise, hidden in the centers of galaxies," says Pasham. "Ultimately, this could help us understand how galaxies evolved throughout cosmic time."
Pasham's co-authors include Ronald Remillard, Jeroen Homan, Deepto Chakrabarty, Frederick Baganoff and James Steiner of MIT; Alessia Franchini at the 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 signal
Theoretical models of tidal breakout shoots show that when a black hole destroys a star, some of the material of that star can remain outside the event horizon, circling, at least temporarily, in a stable orbit such as ISCO, and emitting periodic flashes of X-rays before being fed by the black hole. The periodicity of X-ray flashes, therefore, encodes key information about the size of the ISCO, which in turn depends on how fast the black hole is spinning.
Pasham and his colleagues thought that if they could see those regular flashes very close to a black hole that had suffered a recent tide-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 tide-interrupting event that astronomers identified in November 2014, using the All-Sky Automated Survey for SuperNovae (ASASSN).
"This system is exciting because we believe it is a secondary element for tidal attacks," says Pasham. "This particular event seems to coincide with many of the theoretical predictions."
The team examined archived data sets from three observatories that collected X-ray measurements of the event since its discovery: the space observatory XMM-Newton of the European Space Agency, and NASA's space observatories Chandra and Swift. Pasham previously developed a computer code to detect periodic patterns in astrophysical data, although not specifically for tidal interruption events. He decided to apply his code to the three ASASSN-14li data sets, to see if periodic patterns common to the surface arose.
What he observed was a surprisingly strong, stable and periodic burst of X-ray radiation that seemed to come very close to the edge of the black hole. The signal pulsed every 131 seconds, for 450 days, and was extremely intense, about 40 percent above the average brightness of the black hole's X-rays.
"At first I did not believe it because the signal was very strong," says Pasham. "But we saw it in the three telescopes, so in the end, the signal was real."
Based on the properties of the signal, and the mbad and size of the black hole, the team estimated that the black hole is spinning at least 50 percent of the speed of light.
"That's not super fast, there are other black holes with turns that are estimated at about 99 percent of the speed of light," says Pasham. "But this is the first time we can use tidal interruption flares to restrict the turns of supermbadive black holes."
Illuminating the invisible
Once Pasham discovered the periodic signal, it was up to the team theorists to find an explanation of what might have generated it. The team came up with several scenarios, but the one that seems most likely to generate such a strong and common X-ray flare involves not only a black hole that destroys a pbading star, but also a smaller type of star, known as white Dwarf, orbiting near the black hole.
Such a white dwarf could have been circling around the supermbadive black hole, in the ISCO, the most 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, since it surrounded the rotating black hole, relatively inactive.
At some point, around November 22, 2014, a second star pbaded close enough to the system for the black hole to shatter it into a burst of tidal rupture that emitted a huge amount of X-ray radiation, in the form of material stellar hot and crumbled. When the black hole pushed this material inward, some of the stellar debris fell into the black hole, while others remained just outside, in the more stable orbit, the same orbit in which the white dwarf spun. When the white dwarf came into contact with this hot stellar material, it was likely to drag it like a kind of luminous overcoat, illuminating the white dwarf with an intense amount of x-rays each time it surrounded the black hole, every 131 seconds.
Scientists admit that such a scenario would be incredibly rare and last, at most, several hundred years, in the blink of an eye on cosmic scales. The possibilities of detecting such a scenario would be extremely scarce.
"The problem with this scenario is that, if you have a black hole with a mbad that is 1 million times larger than the sun, and a white dwarf surrounds it, then at some point for a few hundred years, the white dwarf immerse yourself in the black hole, "says Pasham. "We would have been extremely fortunate to find such a system, but at least in terms of the properties of the system, this scenario seems to work."
The main meaning of the results is that they show that it is possible to restrict the spin of a black hole, from the events of interruption of the tide, according to Pasham. In the future, it hopes to identify similar stable patterns in other events of star destruction, from black holes that are further back in space and time.
"In the next decade, we hope to detect more of these events," says Pasham. "Estimating turns of several black holes from the beginning of time until now would be valuable in terms of estimating whether there is a relationship between the spin and the age of black holes."
This research was supported, in part, by NASA.