"Invisible" black holes of intermediate mass can manifest themselves, temporarily "reviving" white dwarfs flying at a short distance from them. This is the conclusion reached by scientists who published an article in the Astrophysical Journal.
“It is extremely important to understand how many of these black holes exist in the universe. The answer to this question will help us uncover the secrets of the birth of the largest and oldest supermassive black holes in the Universe. If we ever see how black holes 'resurrect' white dwarfs, we will take a big step towards this,”says Christopher Fragile of the Kavli Institute for Theoretical Physics in Santa Barbara (USA).
Life and death in space
Observations of the first epochs of the life of the Universe show that supermassive black holes with a mass of several billion Suns already existed in the first galaxies of the universe, which would be impossible if they began to grow from "zero", as scientists believed earlier. For this reason, astronomers have been looking for so-called "intermediate-mass black holes" for a long time, which could arise in the course of direct collapse of matter and serve as "seeds" for the birth of supermassive holes.
Today astronomers know about the existence of four black holes of this kind - the X-2 object in the M82 galaxy, the HLX-1 source in the Phoenix constellation "catapulted" from the ESO 243-49 galaxy, as well as the NGC2276-3c black hole in the NGC2276 galaxy and the GCIRS object 13E in the Milky Way.
Many scientists suspect that these objects are not actually black holes, since their discoverers cannot yet accurately measure their mass and other physical properties. Fragile and his colleagues have come up with an ingenious technique for testing the properties of such objects using "dead stars".
Luminaries, similar in size to the Sun, do not turn into black holes or pulsars in the last stages of their life, but gradually "burn out". In their place, a cloud of hot gas and a white dwarf appear - the former super-hot core of a star, consisting almost entirely of helium and heavier elements.
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It continues to glow due to residual heat and gravitational compression, but thermonuclear reactions inside it completely stop, since the temperatures and pressures inside the white dwarf are too low for the nuclei of heavy elements to start merging with each other.
Space "lazars"
White dwarfs, as Fragile notes, often arise in close proximity to black holes in the center of galaxies and in large globular clusters. Studying the consequences of their random rendezvous, his team discovered an unusual property of white dwarfs that could be used to search for intermediate-mass black holes.
The fact is that black holes generate tidal forces that make objects approaching them stretch and contract, which should increase the temperatures and density of matter inside them to tens of billions of Kelvin degrees and several tons per cubic centimeter. In some cases, as scientists have suggested, this will be enough to restart thermonuclear reactions inside the "dead star".
“In order for the interior of the white dwarf to flare up again, it is necessary that the black hole has a sufficiently 'average' size. In this case, tidal forces will manifest themselves at large distances, but the black hole will not immediately swallow the white dwarf or disappear inside it if its mass is small,”continues the astrophysicist.
The "reborn" star, as shown by the calculations of scientists, will synthesize new elements differently from ordinary stars and supernovae. In particular, it will produce unusually high amounts of nickel-56 and other elements associated with iron, which will allow them to be uniquely identified by observing supernovae with anomalous spectrum.
So far, as Fragile admits, scientists have not been able to find traces of such black holes and "reborn" white dwarfs, but no one has purposefully searched for them in the past. Long-term observations and analysis of data already collected by telescopes, as well as connecting gravitational observatories to searches for such objects, he said, will help find them in the coming years and decades.
