On November 22, 2014, astronomers spotted a rare event in the night sky: a supermassive black hole in the center of the galaxy, nearly 300 million light-years from Earth, tearing off the star that passes. The event, known as the flame caused by tidal tsunami, caused an explosion of X-ray activity near the center of the galaxy, due to the large tide of a black hole that tears the star. Since then, many observers have trained their sights on the event, hoping to learn more about how black holes are kept.
MIT researchers and others have deepened according to data from several telescopic observations of the event and discovered an unusually intense, stable and periodic pulsation of X-rays in all data sets. The signal appears to come from an area that is very close to the horizon of a black hole event – the point at which the material is inevitably swallowed by a black hole. It appears that the signal periodically illuminates and fades every 131 seconds and lasts at least 450 days.
Researchers believe that anything that emits a periodic signal circulates around a black hole just before the horizon of events, close to the farthest stable circular orbit or ISCO – the smallest orbit in which particles can travel safely around a black hole.
Given the steady proximity of the signal, the black hole and the mass of the black hole, which the researchers previously estimated to be about one million times greater than the sunshine, calculated that the black hole was rotating at approximately 50% light velocity.
The findings were reported today in the magazine Science, are the first proof that tidal disturbances occur in order to evaluate the rotation of the black hole.
The first author of the study, Dheeraj Pasham, a post at the MIT Institute for Astrophysics and Space Research, Kavli, says that most supermassive black holes are stationary and usually do not emit much in X-ray mode. Occasionally they will relax the rest of the activity, such as when the stars are close enough to make them black holes. Now, according to the results of the team, such tidal disturbances can be used to evaluate the rotation of supermassive black holes – a characteristic that has so far been incredibly demanding.
"Events where black holes tear off the stars that come too close to them can help us map the spins of several supermassive black holes that are stationary and otherwise hidden in centers of galaxies," says Pasham. "This could ultimately help us understand how galaxies developed during the cosmic time."
Pasham's co-authors include Ronald Remillard, Jeroen Homan, Deepto Chakrabarty, Frederick Baganoff and James Steiner from MIT; Alessia Franchini of the University of Nevada; Chris Fragile from College of Charleston; Nicholas Stone of Columbia University; Eric Coughlin of the University of Berkeley; and Nishanth Pasham from Sunnyvale, California.
Theoretical tide disruptions show that when a black hole cuts an asterisk, some material of this star can remain outside the horizon of events, circulates at least temporarily in a stable orbit like ISCO, and emits an occasional pulsation of the X-ray before it is finally powered by a black hole. The X-ray flash periodicity thus encodes key information about the size of the ISCO, which itself dictates how quickly the black hole rotates.
Pasham and his colleagues thought that if they could see such regular flashlights that were very close to the black hole that experienced the recent tidal event, they showed how quickly the black hole swung.
They focused on finding the ASASSN-14li, a tidal event that was discovered by astronomers in November 2014 using the SuperNovae Automated Surveying System (ASASSN).
"This system is exciting, because we think it's a poster for children that cause tides in the tide," says Pasham. "This event seems to fit many theoretical predictions."
The group reviewed the archives of three observatories that collected X-ray measurements of the event since its discovery: the Space Observatory of the XMM-Newton of the European Space Agency and the NASA Observatory in Space Chandra and Swift. Pasham has previously developed a computer code to detect periodic patterns in astrophysical data, although not for events related to tidal disorders. He decided to use his code for three data sets for ASASSN-14li to determine whether normal surface patterns would appear on the surface.
He observed a surprisingly strong, stable and periodic outbreak of X-ray radiation, which seemed very close to the edge of the black hole. The signal was pulsating every 131 seconds, more than 450 days, and was extremely intense – about 40 percent above the average brightness of x-rays in a black hole.
"At first, I did not believe that the signal was so powerful," says Pasham. "But we saw this in all three telescopes. So the signal was real. "
Based on the characteristics of the signal and the mass and size of the black hole, the team estimated that the black hole rotates at at least 50 percent of the speed of light.
"It's not super fast – there are other black holes with estimated rotations of around 99 percent of the speed of light," says Pasham. "But this is the first time that we can use outbursts of tidal disturbances to limit the rotation of supermassive black holes."
Illumination of the invisible
When Pasham discovered a periodic signal, it was up to the theoreticians in the team to find an explanation for what he might have created. The team has prepared various scenarios, but one that seems most likely to create such a strong, regular X-ray, does not only involve a black hole that destroys passers-by stars, but also a smaller type of star known as a white dwarf circling around the black hole.
Such a white dwarf may have circled for some time a supermassive black hole on the ISCO – the most internal stable circular orbits. Sam, it would not be enough to transmit any detectable radiation. A white dwarf would be for all purposes and an invisible telescope, as it surrounded the relatively inactive, the spinning of the black hole.
Somewhere around November 22, 2014, the second star went sufficiently close to the system that the black hole torn it into a tide that triggered vast X-ray radiation in the form of a hot, broken star material. When the black hole pulled this material inward, some stellar waste fell into a black hole, while some remained just outside, in the deepest stable orbit – the very same orbit in which the white circle surrounded. When the white dwarf came into contact with this hot star material, he probably dragged it like a kind of light coating, which illuminated the white dwarf in an intense X-rays each time it encircled the black hole every 131 seconds.
Scientists admit that such a scenario would be extremely rare and would last only a few hundred years – the biggest moment in cosmic scales. The possibilities for detecting such a scenario would be extremely small.
"The problem with this scenario is that if you have a black hole with a mass that is one million times larger than the sun and circled by white dwarfs, then at some point in a few hundred years, a white dwarf will dive into a black hole," says Pasham. "We would be very happy to find such a system. But at least with regard to the features of the system, this scenario seems to work.
The most important significance of the results is that it is possible to limit the rotation of the black hole due to events due to tidal disorders, says Pasham. In the future, he hopes to identify similar stable patterns in other events, such as black holes, located in space and time.
"In the next decade, we hope to discover more of these events," says Pasham. "Evaluating the rotation of several black holes from the beginning of time to now would be valuable in terms of estimating whether there is a connection between the rotation and the age of black holes."
This research was partly supported by NASA.