Vi siete chiesti cosa accade quando una stella di grande massa subisce un collasso gravitazionale? Uno dei “prodotti finali” è la formazione di una supernova. Le osservazioni di questi eventi spettacolari ci permettono di avere una serie di informazioni della superficie stellare quando la stella esplode mentre invece risulta alquanto complicato capire quali sono i meccanismi responsabili che avvengono nelle regioni più centrali e profonde all’interno della stella. Per studiare in dettaglio questi processi di alta energia, gli astrofisici devono eseguire diverse simulazioni numeriche che si basano sui vari tipi di stelle e sulle proprietà fondamentali delle interazioni massa-energia.
Research from an international team of scientists led by the University of Leicester has discovered for the first time that one of the most powerful events in our Universe, Gamma-Ray Bursts (GRB), behave differently than previously thought. The study, published in the prestigious scientific journal Nature, uses evidence from observation of a GRB to rule out most of the existing theoretical predictions concerning the afterglow of the explosions.
Una serie di simulazioni numeriche mostra per la prima volta che la presenza di instabilità nel nucleo delle stelle di neutroni può determinare la formazione di campi magnetici giganteschi che possono a loro volta causare violente e drammatiche esplosioni stellari mai osservate nell’Universo.
An ultra-dense (“hypermassive”) neutron star is formed when two neutron stars in a binary system finally merge. Its short life ends with the catastrophic collapse to a black hole, possibly powering a short gamma-ray burst, one of the brightest explosions observed in the Universe. Short gamma-ray bursts as observed with satellites like XMM Newton, Fermi or Swift release within a second the same amount of energy as our Galaxy in one year. It has been speculated for a long time that enormous magnetic field strengths, possibly higher than what has been observed in any known astrophysical system, are a key ingredient in explaining such emission.
Scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) have now succeeded in simulating a mechanism which could produce such strong magnetic fields prior to the collapse to a black hole.
How can such ultra-high magnetic fields, stronger than ten or hundred million billion times the Earth’s magnetic field, be generated from the much lower initial neutron star magnetic fields? This could be explained by a phenomenon that can be triggered in a differentially rotating plasma in the presence of magnetic fields: neighbouring plasma layers, which rotate at different speeds, “rub against each other”, eventually setting the plasma into turbulent motion. In this process called magnetorotational instability magnetic fields can be strongly amplified. This mechanism is known to play an important role in many astrophysical systems such as accretion disks and core-collapse supernovae. It had been speculated for a long time that magnetohydrodynamic instabilities in the interior of hypermassive neutron stars could bring about the necessary magnetic field amplification. The actual demonstration that this is possible has only now been achieved with the present numerical simulations. The scientists of the Gravitational Wave Modelling Group at the AEI simulated a hypermassive neutron star with an initially ordered (“poloidal”) magnetic field, whose structure is subsequently made more complex by the star’s rotation. Since the star is dynamically unstable, it eventually collapses to a black hole surrounded by a cloud of matter, until the latter is swallowed by the black hole.
These simulations have unambiguously shown the presence of an exponentially rapid amplification mechanism in the stellar interior, the magnetorotational instability. This mechanism has so far remained essentially unexplored under the extreme conditions of ultra-strong gravity as found in the interior of hypermassive neutron stars.
This is because the physical conditions in the interior of these stars are extremely challenging. The discovery is interesting for at least two reasons. First, it shows for the first time unambiguously the development of the magnetorotational instability in the framework of Einstein’s theory of general relativity, in which there exist no analytical criteria to date to predict the instability. Second, this discovery can have a profound astrophysical impact, supporting the idea that ultra strong magnetic fields can be the key ingredient in explaining the huge amount of energy released by short gamma-ray bursts.
As reported in this press release, a new study has used Chandra X-ray Observatory to identify what triggered this explosion. It had already been shown that the type of explosion was a so-called Type Ia supernova, the thermonuclear explosion of a white dwarf star. These supernovas are important cosmic distance markers for tracking the accelerated expansion of the Universe. However, there is an ongoing controversy about Type Ia supernovas. Are they caused by a white dwarf pulling so much material from a companion star that it becomes unstable and explodes? Or do they result from the merger of two white dwarfs? The new Chandra analysis shows that the Kepler supernova was triggered by an interaction between a white dwarf and a red giant star. The crucial evidence from Chandra was a disk-shaped structure near the center of the remnant. The researchers interpret this X-ray emission to be caused by the collision between supernova debris and disk-shaped material that the giant star expelled before the explosion. Another possibility was that the structure is just debris from the explosion. The disk structure seen by Chandra in X-rays is very similar in both shape and location to one observed in the infrared by the Spitzer Space Telescope. This composite image shows Spitzer data in pink and Chandra data from iron emission in blue. The disk structure is identified with a label. The authors also produced a video showing a simulation of the supernova explosion as it interacts with material expelled by the giant star companion. It was assumed that the bulk of this material was expelled in a disk-like structure, with a gas density that is ten times higher at the equator, running from left to right, than at the poles. This simulation was performed in two dimensions and then projected into three dimensions to give an image that can be compared with observations. The good agreement with observations supports their interpretation of the data.
Chandra X-ray Observatory: Kepler's Supernova Remnant: Famous Supernova Reveals Clues About Crucial Cosmic Distance Markers