For 500 days, powerful X-ray signals came to Earth from a remote galaxy. The most surprising thing is that they were periodic. They repeated exactly every 131 seconds. To reach that galaxy and know the origin of these signals would have to travel for almost three centuries at 300,000 kilometers per second – the speed of light – something totally impossible with current technology. Now, thanks to several space telescopes, a team of astronomers has managed to explain the phenomenon and, in passing, clarify how the black holes.
The theory of relativity Einstein predicts the existence of these bodies, corpses of large stars whose huge mass is concentrated on a reduced spherical surface so that nothing that crosses its threshold can escape the force of gravity, not even light. They are invisible to telescopes, but thanks to the observation of their surroundings, the different kinds of holes and their behavior can be better known.
In November 2014, several telescopes captured a burst of X-rays coming from a black hole with a mass a million times greater than that of the Sun at the center of the galaxy in question. It is a body similar to the one at the center of our own galaxy, the Milky Way. The flash occurred when the hole engulfed a star that crossed the event horizon, the limit beyond which nothing can escape its attraction.
An event like this happens only once every 50,000 years in a galaxy. "
"An event like this happens only once every 50,000 years in a galaxy," says Dheeraj Pasham, a physicist at the Massachusetts Institute of Technology (MIT) in the United States. His team analyzed the observations of this black hole made by the space telescopes XMM Newton of the European Space Agency and Chandra and Swift of NASA. They all caught the same X-ray signal that arrived just from the event horizon.
Thanks to these signals, the team was able to estimate for the first time the speed of rotation of a black hole: 150,000 kilometers per second, that is, half the speed of light, as explained in a study posted today in Science and presented at the congress of the Astronomical Society of the United States, which is held in Seattle.
The team's hypothesis is that part of the star was not devoured, but disintegrated in a cloud of gas and dust that remained orbiting just at the horizon of the hole. The periodic pulses are due to another star in the same orbit, a white dwarf, which dragged the dust cloud with it and produces periodic X-ray emissions. It is an extremely rare phenomenon that will last only a few hundred years before that the hole swallows this other star, explain the people in charge of the study.
The work of Pasham will allow exploring regions of the cosmos impossible to visit and clarify the evolution of this type of black holes, fundamental for the evolution of the galaxies that form around them. "Using these same principles," Pasham explains, one could infer the rotation of other supermassive black holes and even create "a distribution function to explain how supermassive black holes have evolved from the beginning of time until now," he explains. On the periodic pulses, it may not be known again. "The signal was active for 500 days. After the brightness decreased radically and is no longer detectable with any telescope, "explains the astrophysicist. Only another case of this type of signals is known, he adds.
The work will allow exploring regions of the cosmos impossible to visit
Last year, another powerful X-ray burst came from a place barely 10,000 light years from Earth. It was a black hole of 10 solar masses that had just swallowed a large amount of dust and gas that came from a nearby star. A team of astronomers from the US and Europe turned to the Nicer experiment, which began operating last year aboard the International Space Station, to map the black hole based on its light emanations. Around these bodies a disc of gas and dust forms to hundreds of thousands of degrees subjected to the great speed of rotation, which ends up decomposing the atoms. The protons and neutrons remain in this accretion disk while the electrons form a cloud just above the hole called the corona. The study, published on the cover of the magazine Nature, has captured how this crown contracts tens of kilometers when the hole sticks to a binge of star matter and spits out the powerful bursts of X-rays.
It is the first time that something like that is observed in a nearby black hole and of small size, the most violent. Scientists believe that they can use these bodies as analogues of supermassive holes to study their effects on the evolution of distant galaxies. The team has already captured four other similar events with the instrument on board the ISS, according to Phil Uttley, co-author of the study. "We are about to get breakthrough discoveries," he says.
In the universe there are two large classes of black holes. "The stellar masses are the size of a city and masses of up to 10 soles and are born of explosions of huge stars," writes Daryl Haggard, from the Space Institute of McGill University in Canada, in a commentary published by Nature. "Supermassive holes are the size of the solar system, they concentrate millions or billions of times the mass of the Sun and reside in the center of galaxies." What is still impossible to know is what happens with what falls into a hole. "According to Einstein's theory of relativity, no information can escape from inside a black hole, because it would have to travel faster than light [y la relatividad deja claro que nada puede ser más rápido que la luz]", Explains Teo Muñoz Darias, of the Institute of Astrophysics of the Canary Islands. Only thanks to new theories yet to be demonstrated how quantum gravity could begin to answer this question.