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A chance discovery sheds light on exploding stars

Despite the increasing number of archived data on general astronomical surveys, astronomical discoveries still depend to a large extent on pure chance. And theoretical aspects of these discoveries can guide (and sometimes derail them). However, there is no “standard model” of the universe and all its elements. Therefore, progress is often driven by unexpected factors. However, observing paranormal phenomena and explaining their significance requires a good observer. In a paper recently published in the journal Nature, Translated by Oli Koenig and his research team1 A serendipitous discovery was successful, by accurately characterizing an X-ray flux, which was observed for only 35.8 seconds, showing that this spectrum of radiation was caused by a white dwarf exploding star, which contributes to enhancing our understanding of the physics of those stars.

The rays were spotted by an instrument called eROSITA, an instrument that scans vast areas of the sky for X-ray emissions every four hours, from an orbit around Earth’s second Lagrangian Point in space (L2), a point that also contains an orbit The James Webb Space Telescope, the second of Earth’s five Lagrangian points. These points are areas in which the gravitational forces of the Sun and the Earth are balanced by the acceleration of the central attraction that is generated by the influence of the orbits of the satellites, so that these satellites settle in their orbit.

The E Rosetta instrument takes six months to build an X-ray map image of the sky. It should be noted that before the joint German-Russian space mission came to a halt as a result of the Russian invasion of Ukraine, the instrument had completed four full scans. The data generated by the instrument allows us to study variations in astronomical X-ray sources over time scales from several seconds to several years.

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Koenig and his research team noticed that a very bright X-ray source, which was only observed once in a separate survey conducted on July 7, 2020, had coincided in space and time with raging explosion Classic, known as “YZ Reticuli”2. From here, the research team concluded that the prediction was correct3 It dates back several decades to the physics of these supernovae.

Classical supernova explosions can be defined as thermonuclear reactions (primarily similar to hydrogen bombs), which suddenly accelerate to an enormous degree, and arise on the surfaces of stars known as white dwarfs; That is, the amounts of ash left after low-mass stars deplete the nuclear energy that feeds them. The term white dwarfs refer to inert bodies that have a mass close to the mass of the Sun, and have sizes similar to the size of the Earth. Its density is about a million times the density of ordinary matter.

When a white dwarf’s mass increases in a binary astronomical system by accumulating material from its companion star in that system (Fig. 1), the enormous pressure at the base of the added hydrogen gas layer pushes the gas into a state in which its pressure is determined by the quantum properties of its electrons, and not by the quantum properties of its electrons. heat pressure. This means that the accumulated hydrogen does not expand, even as its temperature continues to rise. A thermonuclear reaction occurs when that layer gets hot enough to trigger fusion reactions.

Figure 1 |  The start of a raging explosion.  When hydrogen gas accumulates on a white dwarf star from a companion star in a binary astronomical system (the image is not according to a specific scale), the pressure and temperature in the accumulating layer increases, but the gas does not expand.  As the temperature continues to rise, a thermonuclear reaction is triggered, which suddenly accelerates.  Some amount of energy ejects the outer layers of the white dwarf, resulting in a supernova explosion (not shown) that can eventually be seen with the naked eye.  However, it was previously predicted3 that some energy emitted by the explosion immediately seeps through the accretion layer and can be detected as a low-energy X-ray flash, which Koenig and his research team 1 recently verified.

Figure 1 | The start of a raging explosion.
When hydrogen gas accumulates on a white dwarf star from a companion star in a binary astronomical system (the image is not according to a specific scale), the pressure and temperature in the accumulating layer increases, but the gas does not expand. As the temperature continues to rise, a thermonuclear reaction is triggered, which suddenly accelerates. Some amount of energy ejects the outer layers of the white dwarf, resulting in a supernova explosion (not shown) that can eventually be seen with the naked eye. However, a prediction previously reported3That some of the energy released by the explosion immediately seeps through the accretion layer and can be detected as a low-energy X-ray flash, which was possible for Koenig and his research team.1 Check it out recently.

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Since the rates of these fusion reactions increase very rapidly with an increase in temperature, and because this increase in temperature does not relieve pressure, the reaction quickly gets out of control. In an immediate response, a large portion of the layer accumulating on the star explodes, dramatically increasing its radius and relieving the pressure.

It should be noted that in 1990, a prediction reported3 That this suddenly accelerated reaction would produce a bright flash when the energy of the explosion reached the photosphere (the outermost layer of the white dwarf). At this point, the star’s brightness reaches its maximum possible degree, in what is known as Eddington’s radia, because the acceleration of external reactions in the star’s material due to the pressure of its radiation matches the acceleration of internal reactions due to gravity. Before the accretion layer can be ejected, when its radius is close to that of the white dwarf, the star’s temperature will have reached several million degrees.3But that heat would cool so quickly that the electromagnetic radiation would move from the X-ray frequency range with the accumulating material radius increasing at a rate of several thousand kilometers per second, so the supernova would be visible to the naked eye after a period ranging from several hours to several days, when the temperature was The heat has decreased and the emitted radiation has shifted to optical frequencies.

Later, days to months after the explosion, the expanding ejecta thins out to allow low-energy X-rays (those with less than one kiloelectronvolt) to pass through. In principle, this process should make the white dwarf’s surface visible again, because it still emits light close to Eddington’s, but the low-energy X-ray flash has never been seen before.

Astronomical observations require patience, as well as ingenuity in their interpretation. For example, when a fast X-ray flash was detected in the E Rosetta data, Konig and his research team realized that this flash coincided with its presence at the site of the YZ Reticuli explosion, at the expected timing of that explosion, which allowed researchers after a long wait to verify correctness of the prediction made in 1990.

It is worth noting that in addition to the fact that this discovery, which occurred by chance, confirms the theory developed in 1990, it also raises two new conclusions. The first is an accurate estimation of the duration of the thermonuclear reaction. By calculating the time difference between this reaction and the start of the explosion at light frequencies, we can reach the dynamics of the expanding photosphere. The second conclusion is that the surface temperature at the time of the explosion limits the mass of the white dwarf.

Perhaps the lesson learned from these observations is that observing a brief and unexpected astronomical event can allow members of a brilliant research team to deduce the occurrence of a rare astronomical phenomenon and extract benefits from it. In order to avoid the perception of the ease of making this discovery, it should be noted that the position of the X-ray emission was in fact too bright to be discerned by the E Rosetta observing instrument, as its observations were severely affected by the “accumulated” photons that were arriving at it at a speed that exceeded its ability to detect it. Count or count these photons. This greatly complicated data analysis. But by overcoming that problem, Koenig and his research team bridged a knowledge gap in our understanding of how classic supernova explosions occur. In conclusion, it should be noted that all these results were the result of careful observation that took 35.8 seconds.