Magnetars are neutron stars, like pulsars, with the highest magnetic fields in the known universe. They are believed to be associated with supernova and hypernova record explosions. A new explanation for the enigmatic origin of their magnetic fields has just been found and goes in this direction.
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Influenced by the work of Fred Zwicky, Walter Baade and especially the brilliant physicist Russian Lev Landau, the physicist Robert Oppenheimer – the future father of the atomic bomb – had laid the foundation in 1939 on which the theories of neutron stars and that of thecollapse gravitational leading to the formation of a black hole will be built in the late 1950s and early 1960s. These were articles written in collaboration with his students at the time: ” On Massive Neutron Cores “, With Georges Volkoff, and” On Continued Gravitational Contraction With Hartland Snyder.
Recall that the neutron stars, themselves, are the residue of the gravitational collapse of part of the matter of a massive star exploding in supernova SN II type. This requires that its mass exceeds 8 to 10 solar masses. In broad outline, the collapse of the part of the star which is not blown by the explosion, mainly its heart of iron, leads them protons and the electrons to combine to transform into neutrons and this at the same time produces a flow of neutrinos very energetic. We then get stars which can contain the whole mass of the Sun in a sphere of only a few tens of kilometers in diameter.
Magnetars, special neutron stars
We are far from understanding everything about the process of their birth and we continue to wonder about thestate of matter nuclear in their depths, so that these stars still retain much of their mystery. We still know that pulsars are rotating neutron stars and we know thanks to the rise of gravitational astronomy, on the occasion of the spectacular announcement of detection by Ligo and Virgo from the source ofgravitational waves GW170817, that gamma-ray bursts shorts occur in collisions between two neutron stars associated in a binary system.
What is a neutron star? What is the difference between these stars and our Sun? Roland Lehoucq, astrophysicist at CEA, explains to us that neutron stars radiate very little in visible light, unlike our Sun. Also, neutron stars have much smaller sizes than that of the Sun: a neutron star has a diameter between 10 and 15 km, against 1.4 million km for the Sun. They are also compact objects that contain a large amount of material in a very small volume. Studying these stars allows nuclear physics theories to be tested on a different scale. © CEA Research
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However, in 1992, the bestiary of neutron stars expanded when astrophysicists Robert Duncan and Christopher Thompson postulated the existence of so-called magnetars to explain some sporadic, particularly intense sources of X-rays and of gamma rays, discovered since the end of the 1970s. Today, around thirty magnetars are known in the Milky Way and it seems that they are neutron stars with a particularly high magnetic field. So high even, that those that we measure with these unusual stars are the highest of theUniverse observable known and that they are sometimes up to 1,000 times more intense than those of conventional neutron stars which are already formidable. It is thus estimated that the magnetars have a dipolar magnetic field, of shape analogous to that revealed by iron filings around a magnet, around 1015 Gauss (G), while on Earth its intensity varies between 0.25 and 0.65 Gauss and that of the magnetic field of a magnet on a fridge is about 50 Gauss. We measure on average intensities of 1,500 Gauss for sunspots.
The analog of geodynamo in magnetars?
How to account for the monstrous intensity of magnetic fields magnetars? Several explanations are possible and one of them was presented by Futura in the previous article below. But today an article published in the journal Science Advances, by a Franco-German team led by Raphaël Raynaud from CEA-IRFU Astrophysics Department / AIM Laboratory and which can be read on arXiv, reports that these astrophysicists carried out the first numerical simulations, which describe how the genesis of the magnetars’ magnetic fields could occur in the first seconds after their formation.
This result was obtained by transposing and adapting the digital models of the Earth’s geodynamo in the case of neutron stars and by implementing the algorithms on the supercomputer Occigen from the National Computer Center for Higher Education (Cines). We know that in the case of our Blue Planet, the magnetic field indeed comes from a dynamo self-exciting originating in thealloy iron-nickel liquid in convection turbulent in the repository in rotation of the Earth and at its nucleus. The phenomenon is also reproduced in the laboratory, as theVKS experience. A similar dynamo but with the plasma from inside the Sun generates its global magnetic field.
Simulations show that at the very beginning of its birth, the interior of a neutron star is also convective, and that a magnetic field is generated in a similar way to that of Earth with an intensity of all the more important as the parent star of the neutron star was rapidly rotating. In these simulations there are instabilities which will lead in a few seconds to an exponential amplification of the magnetic field already possessed by the star before its collapse, up to values reaching 1016 Gauss. The magnetars would therefore be born from massive stars in rapid rotation.
Millisecond magnetars behind long gamma-ray bursts?
All this is very interesting, as explained by Raphaël Raynaud, Jérôme Guilet and Christian Gouiffès in a CEA press release. This result opens perspectives to understand among the releases ofenergies the most powerful known in the observable cosmos for stars, namely explosions ofhypernova in association as we think with bursts long gamma rays and, in particular, superluminous supernovae which emit a hundred times more light than a usual supernova but without being gamma-ray bursts.
Behind these events, explain the three researchers, would hide the formation of “millisecond magnetars”, therefore newly formed magnetars, with convective dynamo, and whose rotation periods would be of the order of a millisecond. The power of the explosion would come from the fact that with the magnetic fields of these extreme pulsars, there would be an efficient and rapid extraction of the energy of rotation of these millisecond magnetars. But until now, we were unable to generate the necessary magnetic field values, at least 1015 Gauss precisely, which is now done!
What you must remember
- Magnetars are neutron stars with magnetic fields having measured intensities on the order of 109 at 1011 teslas or 1014 at 1015 Gauss, that is to say hundreds of millions of times stronger than those of the fields of the most powerful magnets made by human hands. These magnetars must come from the collapse of massive stars.
- We knew about the existence of these stars for 70 years but, only today, computer simulations show that these fields can result from the fusion of two stars, formerly forming a binary system, or from convection movement generating an effect. dynamo in the first seconds of magnetar formation, resulting from the very fast rotating star collapse.
- In the latter case, we would get millisecond pulsars which can convert via their magnetic field their energy of rotation to feed particularly bright supernova explosions in particular, but also long gamma-ray bursts.
Magnetar: the riddle of the formation of the most powerful magnetic fields in the universe is solved
Article by Laurent Sacco published on 10/10/2019
70 year old, an enigma in astrophysics now seems to be resolved with simulations showing how massive stars can acquire an abnormally high magnetic field. These stars are destined to collapse by gravity, often giving magnetars, these neutron stars with the most powerful magnetic fields known in the cosmos.
The discovery of the characteristic effect of a magnetic field on the spectrum of atoms in which they are immersed won the Dutchman Pieter Zeeman the Nobel Prize for physical of 1902 with his compatriot, Hendrik Lorentz, the discoverer of the famous relativistic transformations bearing his name. TheZeeman effect would allow astrophysicists to measure the magnetic field on the surface of the Sun, especially at the level of its sunspots.
But it was not until 1947 that theastronomer US Horace Babcock (which is incidentally the first to consider what is now called adaptive optics in astronomy) was able to show thanks to this effect that magnetic fields also existed on the surface of other stars. The meteoric boom in astrophysics after the Second World War was to test and nourish the models of the structure and evolution of stars that we had started to develop from the 1920s. Astrophysicists were then came across a first enigma: some massive stars had an abnormally strong magnetic field whose origin was hard to understand.
Admittedly, one could, as in the case of the Sun and the Earth, involve the theory of self-exciting dynamo which allows to generate a magnetic field in a turbulent and conductive fluid but, in the case of massive stars, the major part of the envelope of a star is in a state called radiative, that is to say that the transfer of heat is done by radiation and not by convection, which is naturally turbulent.
The enigma of the intense magnetic fields of massive stars took on a new relief when the existence of magnetars was discovered, that is to say neutron stars having a magnetic field so intense that we do not know any. stronger in the observable universe.
Some explanations on the functioning of pulsars which are also valid when they are also magnetars. Translation by clicking on the white rectangle at the bottom right, then on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. © Nasa Goddard
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Magnetars, exotic pulsars
The existence of a large magnetic field for a neutron star, which can be detected in the form of a pulsar, is not a surprise either from a theoretical point of view or from an observational point of view. These fascinating objects – and so dense that the equivalent of the mass of the Sun is found there in a sphere of a few tens of kilometers in diameter – are the product of the collapse of stars at least 8 to 10 times more massive than the Sun when they become type II supernovae.
These stars already had a gold magnetic field, the laws ofelectromagnetism of Maxwell very clearly imply, via the law of conservation of the flux of the magnetic field, that this one must become all the more intense as the star becomes smaller and smaller while collapsing gravitationally. But the magnetic fields of the magnetars remain well above those measured in the case of many pulsars.
The intensities measured are of the order of 109 at 1011 teslas, which is hundreds of millions of times higher than those of the fields of the most powerful human-made magnets. These magnetic fields store a lot of energy and they can lead to broadcasts intense x-rays and even some kind of gamma-ray bursts called soft gamma-ray bursts Soft gamma repeater, SGR), i.e. gamma sources with violent and recurrent but irregular emission episodes, with photons less energetic than conventional gamma-ray bursts in general. The first SGR was detected by serendipity in 1979 by several space missions.
An extract from the simulation showing the current fusion of two stars with false colors (the color indicating the strength of the magnetic field, in yellow the strongest intensity, and in purple, one of the weakest), amplification of the resulting magnetic fields. © University of oxford, Ohlmann, Schneider, Röpke
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Star collisions causing blue streaks
To explain the vertiginous magnetic fields of magnetars, it must therefore again be assumed that they come from the collapse of massive stars with an abnormally high initial magnetic field. Fortunately, theorists had a solution for producing these stars. But, to accredit it, it was necessary to make learned and especially very powerful (because greedy in calculations) simulations of magnetohydrodynamics of plasmas constituting two stars in a binary system on the point of colliding to finally merge.
The available power of supercomputers eventually joined that necessary for a numerical test of this model of stellar collision as evidenced by an article published in Nature by an international team of astrophysicists from the universities of Heidelberg and Oxford, as well asHeidelberger Institut für Theoretische Studien (HITS) and the Max Institute data center Planck, in Garching (Munich). The calculations carried out are very encouraging because we can effectively explain in this way the existence of massive stars with an abnormal magnetic field by making them result from the fusion of two stars. These stars are intended to become SN II supernovae and therefore, often, to produce magnetars, and sometimes black holes if they are massive enough.
As one of the authors of the article explains Nature, Sebastian Ohlmann (Garching): “It was already over ten years ago that it was suggested that strong magnetic fields could be generated by the collision of two stars. But until now, we have not been able to test this hypothesis, because we did not have the necessary IT tools ”. But since then everything has changed, notably because the researchers realized that they could re-use the Arepo code, developed by Volker Springel and Rüdiger Pakmor of the Max Planck Institute, code originally developed to make simulations with galaxies in cosmology.
The results obtained are all the more convincing since we know that stellar fusions are frequent and it is thus estimated that around 10% of all massive stars in the Milky Way are the product of these stellar fusions, which corresponds well to the estimated formation rate of the magnetars. These stars may appear as blue stragglers, which can be translated into French by ” blue stragglers “
The puzzle of the formation of a magnetar solved by the VLT
Article by Laurent Sacco published on 05/29/2014
One of the magnetars spotted in the Milky Way was a problem for astrophysicists. Everything seemed to indicate that it came from a star initially containing at least 40 solar masses. However, at the end of life, such a star should have turned into a black hole and not a magnetar. The solution to the puzzle seems to have been found after detection by the VLT of a star which in the past would have formed a binary system with what would become this magnetar.
Since Fred Zwicky, Walter Baade, Lev Landau and Robert Oppenheimer understood during the 1930s that neutron stars must exist. Our theoretical understanding of these stars has made progress and they have been observed, in the form of pulsars, since the end of the 1960s. But these stars still keep much of their mystery. We continue to wonder about the state of nuclear matter in their depths but also about the astrophysical mechanisms that give rise to them. We still know that they are formed during certain star explosions, the SN II supernovae.
In 1992, the bestiary of neutron stars expanded when astrophysicists Robert Duncan and Christopher Thompson postulated the existence of so-called magnetars to explain some sporadic particularly intense sources of X-rays and gamma rays discovered since the late 1970s. today about twenty magnetars are known in the Milky Way and it seems that they are neutron stars with a particularly high magnetic field.
A star that should have turned into a black hole
One of these magnetars, present in scientific archives under the technical name of CXOU J164710.2-455216, has intrigued astrophysicists for some time. Its situation is particular, within a open cluster young stars discovered in 1961 by Swedish astronomer Bengt Westerlund. Located at 16,000 light years from Earth into the constellation Southern Altar (Ara), the Westerlund 1 cluster is considered to be the most massive and compact identified to date in the Galaxy, with approximately 100,000 solar masses contained in a volume whose size is only about 6 light years.
When it appeared that this cluster contained many stars of a few tens of solar masses, it attracted a lot of attention from astrophysicists who saw it as an excellent laboratory for studying the birth, evolution and death of massive stars. Indeed, the theory of structure and stellar evolution implies that such massive stars are destined to collapse by gravity. They will all explode in the form of a supernova, leaving behind stellar corpses, namely neutron stars and black holes.
As these stars burn their nuclear fuel very quickly, we deduce that Westerlund 1 is young. Four years ago, its age was estimated at 3.5 million years at most, thanks to a measurement of the masses of the components of a binary star. The presence of a magnetar immediately became problematic. The birth star of the magnetar must have contained at least 40 solar masses to have transformed into this compact star before its sisters born at the same time as it (since in an open cluster, all the stars were formed simultaneously). However, such a mass a priori conflicts with the theory of structure and stellar evolution which predicts that it should have turned into a black hole and not into a neutron star.
The theorists had found a way to resolve this conflict by postulating that the magnetar had also started its life in the form ofbinary star. But to prove it, you still had to find your companion star. None appeared near CXOU J164710.2-455216 but it could very well have been ejected once the binary star had been destabilized by the explosion of one of its components. AstronomersESO therefore went hunting for this possible companion star. They recently announced in an article posted on arxiv that they thought they had found it.
A training scenario for magnetars
Westerlund 1-5 is a star with a speed high which leads it to escape from the open cluster precisely as a star that has been part of a binary system would do before being ejected from it by the explosion in supernova of its companion star. Analysis of the composition of his atmosphere shows that this is probably what happened. As a light star, it contains far too much carbon. This is understandable if it has increased the gas enriched in these heavy nuclei by a companion star which exploded in supernova.
All these elements support the advanced scenario to account for the paradoxical existence of the magnetar. Westerlund 1-5 would have been initially the most massive star of this duo, with probably 41 solar masses, and it was to form with the future magnetar a system so tight that it would have been contained in theorbit of the Earth around the Sun. A first mass transfer would have taken place from the most massive star to the least massive star containing initially perhaps 35 solar masses and which would have gained in passing also from cinematic moment. Seeing its mass increase while that of Westerlund 1-5 decreased, it would have evolved more quickly by becoming a Wolf-Rayet star. Unstable and making part of its mass in the form of winds stellar to its companion star, it would have ended up exploding as a supernova SN Ibc, ejecting the heavy nuclei that it had synthesized, enriching the surface of Westerlund 1-5 in carbon. In the end, although having evolved faster than all the other stars due to its mass, the progenitor star of the magnetar must have been less massive than 40 solar masses at the time of its explosion. It was therefore light enough not to become a black hole.
We can think that it is the transfer of kinetic moment, having strongly increased the speed of rotation of the star, which played an essential role in the amplification of its magnetic field and which is therefore responsible for the existence of the magnetar . Perhaps this should be seen as the explanation for the formation of all magnetars.
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