They swallow everything that has the misfortune to reach their horizon. The black holes never cease to amaze researchers. Difficult to detect and even more difficult to observe, researchers rely for this on the influence they exert on their immediate environment. being stars particularly massive and dense, their gravitational field deforms and attracts all that approaches, matter but also light ! The approaching material twists and then forms what is called a accretion disk : it is heated to extreme temperatures and then emits X-rays which are then captured by our detectors. We can also “see” black holes by looking at the distortion of the light that comes from behind them: their mass so gigantic creates an effect of gravitational lensit bends the light rays as they pass near the black hole.
But the most famous black hole, after that of our Milky Way, remains of course M87*. Located in the galaxy of the same name at 55 millionlight years of us and with a mass of 6.5 million solar masses, it emits waves radio : very low waves frequency. An image of M87* was constructed in 2019 using an array of eight radio telescopes, located around the world. The use of this network has made it possible to create the equivalent of a radio telescope with a diameter similar to that of the Earth, therefore several thousand kilometers. With such a diameter, the resolution reached made it possible to reconstruct between 2017, when the measurements were made, and 2019 for M87*, an image of the black hole.
For such an observation to be possible, however, the size of the black hole must be compatible with the resolution: the latter being proportional to the mass of the black hole, only supermassive stars can be observed. In a study published in the journal Physical Review Letters, two researchers from the University of columbia described a new method for detecting smaller but also farther black holes.
This new method could detect stellar black holes
The goal: to detect black holes smaller than M87* or Sagittarius A*, the black hole in the center of the Milky Way. In the case of stellar black holesthey are often located in binary systems so with a star, so it is the latter that is detected. The researchers then deduce from its movement the presence of a second star, then differentiate it from a neutron star. the accretion disk also allows the detection of small black holes, thanks to the X-rays that it emits. In their study, the researchers explain that their method only applies to binary systems, but this time formed by two black holes.
But it only works when two black holes are in the process of merge : they are then in orbit with each other. Another condition of the method: look at the phenomenon laterally. In this way, when a black hole passes in front of the other, then occurs this famous effect of gravitational lens. But that’s not all: the team also detected a drop in brightness due to the “shadow” of the black hole located at the back! The variation lasts between a few hours and a few days, and depends on the mass and the proximity of the black holes. ” If you measure the duration from the hollow, you can estimate the size and shape of the shadow cast by theevent horizon of the black hole, the point of no exit, where nothing escapes, not even light”, note the researchers in the study.
Their goal: to verify the theory of relativity
Until now, the shadow of black holes was only visible by the method used for M87* and Sgr A*, theinterferometry very long base or VLBI. This technique makes it possible to increase the resolution to have a diameter equivalent to that of the Earth, but even with such a resolution, few black holes are visible, and remain relatively close because the further they are, the smaller their apparent diameter, the more they require immense resolution. However, to discover how they are formed, it would be necessary to be able to look very far, at the beginnings ofUniverse, to unearth the very first such black holes. What the researchers are trying to solve by carrying out numerous modelingalways more realistic: in their study, the two researchers specifically simulated binary systems of black holes in a variety of different configurations.
They thus found, by numerous estimates of the uncertainties and the noise of the instruments, that the luminosity dip created by the passage of a black hole behind the other should be detectable for approximately 1% of the binary systems of supermassive black holes identified so far. A figure that seems to represent little, but could pay off big, because a single additional detection could be enough to bring many of the elements to these gigantic stars! In particular, this would make it possible to determine the mass and the spin of the black hole in question. Subsequently, the two scientists intend to test their predictions by varying their modeling, particularly in terms of the hypotheses on the thermodynamics of gas contained in the accretion disk.
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