After decades of observations, it has finally been confirmed that a neutron star is hidden inside the nebula surrounding the place where a massive star exploded in 1987 (the supernova known as SN1987A). Although theoretical models predict that the supernova explosions that end the life of some stars give light to neutron stars or black holes, the impenetrability of the debris cloud that surrounds the area had made its detection difficult.
The discovery, made by a large international group of astronomers led by Stockholm University, and using observations made with the James Webb space telescope, is especially relevant since it can contribute to improving the knowledge we have of these extraordinary phenomena.
SN1987A is the most studied supernova in history and the closest known since the year 1604. The explosion took place in the Large Magellanic Cloud, a satellite galaxy of the Milky Way located 168,000 light years away, and could even be observed with the naked eye. Furthermore, its discovery represented a unique scientific milestone when, on February 23, 1987, a stream of neutrinos (fundamental subatomic particles extraordinarily difficult to detect) reached Earth preceding the arrival of light from the burst.
For years, a multitude of instruments have been able to follow the evolution of SN1987A, a fact that has been possible thanks to the fact that the supernova was detected very early, already from the first moments of the explosion. Thus, this object has become a true laboratory in which to test the theoretical models that predict the behavior of this type of cataclysmic events.
According to current knowledge about the life and death of stars, supernovae similar to SN1987A occur when stars much more massive than the Sun exhaust the nuclear fuel inside them. At that moment, the star runs out of the necessary energy to counteract the compression exerted by its own gravity and the star collapses, collapsing towards the center at high speed. There, in the depths of the dying star, the matter is extremely compressed and, depending on the mass of the star, the result is the creation of a neutron star or a black hole.
In the case of SN1987A, the first studies suggested a mass, for the progenitor star, of about 15 to 20 times that of the Sun. A scenario that, according to theory, should lead to the birth of a neutron star (black holes would be created from even more massive stars). However, since it is such a recent explosion, the object is still hidden inside the large cloud formed by the material ejected in the cataclysm.
The authors of the study, which is published in the journal Science, have observed the supernova remnant SN1987A in the infrared range and with the high level of precision offered by the James Webb space telescope. In particular, they have analyzed the emissions generated by argon and sulfur atoms that have lost part of their electrons.
Atoms such as argon and sulfur are synthesized inside massive stars in the last phases of their life (along with other chemical elements such as iron, silicon, calcium or oxygen). Subsequently, they are expelled into space by the supernova explosion and become part of the nebula that expands away from the central point at high speed.
Any atom tends to remain in its ground state, with all its electrons occupying the most favorable energy levels, but a powerful radiation source can strip away some of these electrons through a process called ionization. The resulting atom, generically called an ion, generates very characteristic traces in the light spectrum in the form of bright lines. These emission lines are characteristic of each type of ion and used by scientists for their identification.
The detection of the emission lines of argon and sulfur ions indicates that the cloud of ejecta must still be receiving, after 37 years, a level of radiation sufficient to expel electrons from these atoms. But although the aforementioned emissions had already been found previously, the existing precision did not allow us to discern the specific location, within the expanding nebula, from which they came.
Now, with the capabilities of the James Webb, scientists have been able to verify that the emissions come from the innermost part of the cloud and, therefore, the radiation that causes the ionization of the atoms must come from the center of the explosion.
In the study, the researchers have been able to rule out various possibilities to explain the origin of the ionizing radiation, such as the existence of a surviving star inside the nebula or the emission of energy caused by the interaction between the ejected material and the surrounding environment. Finally, the authors have established that the ionizing source must come directly from the compact object born in the explosion: a black hole or a neutron star.
To discriminate what type of object is hidden inside the cloud of waste, an indirect clue has been used: the amount of chemical elements belonging to the iron family existing in the material expelled by the explosion.
This element, iron, accumulates in the deepest part of the star just before its death, and after the supernova explosion it begins to occupy the innermost region of the ejected nebula, along with other atoms such as nickel or cobalt. . Estimating the amount of these elements allows us to deduce the mass that the core of the star had before its death, and in the case of SN1987A the result suggests a core clearly below the 2.2 solar masses necessary to form a hole. black.
Therefore, the conclusion of the study is that the only possible candidate as an ionizing source is a neutron star inside the supernova remnant. However, and even in the case of this type of object, two different possibilities open up regarding the specific mechanism that generates the radiation.
Neutron stars are objects that typically, in a diameter of about twenty kilometers, pack masses equivalent to one and a half Suns. It is believed that its composition contains an abundance of neutrons (which result from the fusion between the protons and electrons of atoms due to the enormous weight they must support) as well as exotic particles.
According to the authors, a neutron star with a surface temperature greater than one million degrees (an expected value for this type of object) would be enough to ionize, with its powerful radiation, the cloud of material from the supernova remnant and would fit with the detections made by the Webb telescope.
But there is also another possibility. It is known that some neutron stars (called pulsars) are capable of rotating rapidly and generating intense magnetic fields around them. As a consequence, nearby particles are accelerated to speeds close to those of light and can impact the surrounding material, ionizing it. Thus, the presence of a pulsar in the center of the SN1987A remnant could also explain the observations.
In the study, the researchers have tried to check which of these options best fits the data captured. To do this, they have considered factors such as the total energy generated by the supernova remnant, its possible attenuation by the dust in the cloud or the intensity of the emission lines of the light spectrum.
Although the analysis is not conclusive, and both options are possible, the authors suggest that it is more likely that the ionization of the nebula is a consequence of the radiation caused by a neutron star and not so much by the wind generated by the spin of a press.
On February 23, 1987, three instruments located in Japan, the United States and Russia suddenly detected the passage of 24 neutrinos within a period of 13 seconds. These subatomic particles barely interact with matter and are capable of passing through it, including detectors, without colliding with atoms. Therefore, a positive in 24 neutrinos indicated that, in reality, many more had reached our planet.
Scientists estimated that the original source must have produced an extraordinary number of neutrinos, estimated at 10,000 trillion sextillions (a figure consistent with the fact that most of the energy of a supernova explosion is released in the form of these particles).
About two hours later, light from the explosion of a giant star in the Large Magellanic Cloud reached Earth (the light, unlike neutrinos, had been temporarily trapped interacting with the dense envelope of material ejected by the explosion). The powerful point of light that the telescopes showed, coming from the place previously occupied by the blue supergiant star called Sanduleak -69° 202a, confirmed the death of the star.
The observations that have been carried out systematically over the years have allowed us to contemplate the expansion of the nebula created by the explosion, as well as the effects caused in the layers of material that the star had expelled years before the explosion when the wave of shock gave them reach.
