Black holes can swallow stars
New insights into space - Black holes devour entire stars
Black holes are among the most mysterious objects in the cosmos. They can unite the mass of millions or sometimes billions of suns, and their extreme gravity means that not even light can escape from them. The gravity monsters owe their name to this fact, and for precisely this reason they cannot be observed directly with telescopes. However, if a black hole absorbs new matter, it first lights up before it is swallowed. And from observing this cosmic meal one can learn a lot about the properties of the heavyweights. On the occasion of the annual meeting of the American Astronomical Society AAS in Seattle, researchers presented new results of such "nutritional studies".
Black holes tear stars apart
A supermassive black hole sleeps in the hearts of most galaxies, which has a major impact on the evolution of the entire galaxy. At a distance of usually a few hundred thousand years it awakens to new activity: If a star comes too close, it is torn to pieces by the gigantic force of gravity and finally devoured.
One such “tidal disruption event” was registered by an automatic telescope system in November 2014 in a galaxy 290 million light years away. A light year is the distance that light travels in one year - around 9.5 trillion kilometers. The star had been torn apart by the strong tidal forces of the black hole: Its gravity pulls on the near side of the star more than on the far side, tearing the star apart.
Researchers determine the rotation of the black hole
Several observatories followed the cosmic drama. From the observations, a group led by Dheeraj Pasham from the Massachusetts Institute of Technology (MIT) has now determined the rotation of the black hole, the so-called spin. This spin is next to the mass, which can be derived from the effect of gravity on other celestial bodies, the most important property of the gravity monsters, which cannot be easily determined.
"It is very difficult to isolate the spin of a black hole because spin effects only appear very close to the hole itself, where gravity is extremely strong and it is difficult to get a clear view," explains Pasham in a press release the European Space Agency Esa. Their X-ray satellite "XMM-Newton" had targeted the event.
Matter becomes red hot and then swallowed
The matter of the disrupted star does not fall directly into the black hole, but collects in a kind of vortex, the so-called accretion disk, before it is finally swallowed. The matter on this disk heats up to millions of degrees and emits bright X-rays. According to Pasham, the researchers' assumption should be able to be determined from the observation of the innermost area of the accretion disk. "However, observations of such events have not been sensitive enough to examine this region of strong gravity in detail - until now."
In the data from the European X-ray satellite and the space telescopes "Chandra" and "Swift" of the US space agency NASA, the scientists discovered a regular fluctuation of the signal from the black hole with a rhythm of 131 seconds. The signal could be observed for 450 days, as the team reports in the journal “Science”.
Detailed observation is a first
"This is an extraordinary find: Such a bright signal, which remains stable for so long, has never before been observed in the vicinity of any black hole," emphasizes co-author Alessia Franchini from the University of Milan. "And then there is the signal from the immediate vicinity of the event horizon of the black hole - beyond this point we cannot observe anything because the force of gravity is so strong that not even light can escape."
From the rhythm of the X-ray pulses, the researchers deduced the dimensions of the so-called smallest stable circular orbit (ISCO; innermost stable circular orbit) around the black hole. These in turn result from how fast the black hole rotates. Result of the analysis: The examined black hole must rotate at least 50 percent of the speed of light.
"It's not super-fast - there are other black holes whose spin is estimated to be close to 99 percent the speed of light," explains Pasham. "But it is the first time that we have been able to use the flare-up of a tidal star-rupturing event to contain the spin of a supermassive black hole."
Important step in understanding galaxies
The study thus demonstrates a new method for determining the spin of supermassive black holes. The researchers hope to discover more such events in the next decade. Estimating the spins of several black holes from the beginning of time until today could help, for example, with investigating the question of whether there is a connection between the age and spin of a black hole, they explain.
"Events in which black holes tear apart stars that come too close to them could help us map the spins of several black holes that are not active and otherwise hidden in the centers of galaxies," explains Pasham. "That could ultimately help us understand how galaxies evolved over cosmic time."
But it is not just supermassive black holes in the centers of large galaxies that are incorporating new matter. A team led by Erin Kara from the University of Maryland observed a relatively small black hole with about ten times the mass of our sun as it sucked in material from a companion star. The study, which was also presented at the Astronomers' Congress in Seattle, provides the clearest picture yet of how such small, stellar black holes consume matter and emit energy, the researchers report.
A black hole next to the constellation Leo
The team used the “Nicer” (Neutron Star Interior Composition Explorer) instrument, which was actually built to study so-called neutron stars, from the International Space Station ISS to examine the suddenly flaring black hole, which is about 10,000 light-years away in the constellation Leo. It was registered on March 11, 2018 by the Japanese instrument "Maxi", which searches the entire sky for sudden bursts of radiation in the X-ray range, and therefore bears the catalog number MAXI J1820 + 070.
The previously unknown black hole, known for short as J1820, had developed into one of the brightest sources of X-rays in just a few days and offered researchers a perfect observation perspective. "This glistening bright black hole appeared completely unobstructed on the screen, so that we got a completely unadulterated view of what was going on," reports Kara's colleague Jack Steiner from the Massachusetts Institute of Technology (MIT). This enabled the scientists to follow exactly how the black hole developed over the course of its meal.
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