Scientists call them “neutron stars” and they are one of the possible final stages of the star evolution. They are like giant atomic nucleuses kept together by a gravitational pull up to hundred billion more intense than the Earth gravitational pull. They are a collapsed and super-dense residue of a start with a great mass: they concentrate the equivalent of the Sun in a 10 km ray.
The explanation of these extraordinary extreme physics laboratories that can teach a lot on the fundamental forces of the Universe, represents today one of the greatest challenges for astrophysics.
When two neutron stars are kept together by a binary system they orbit one around the other, gradually reducing their distance, until crushing.
This rare, but crucial event, seems to be the cause of the origin of the “short gamma-ray burst”, very violent explosions, among the most luminous of the Universe (corresponding to the release of two million of trillion of trillion of megatons of explosive). They can be observed from far away galaxies; they last less than two seconds and still are among the most enigmatic events for science.
According to the current models, this radiation rays occur when the union of two neutron stars leads to a gravitational collapse and the creation of a black hole. The disc of residual matter which initially surrounds the black hole is “sucked in” in less than one second and a high quantity of magnetic and thermal energy creates a jet of energy and particles, resulting in the gamma radiation.
The “short gamma-ray bursts” are regularly detected by satellites like Swift. However, during the last years, a subsequent X ray emission has been recorded, which may last up to many hours: much longer than expected, considering the very short activity of the recently formed black hole.
Precisely this anomaly drew the attention of two UniTrento researchers (Department of Physics) and of the Max Planck Institute for Gravitational Physics, who hypothesised an explanation to this phenomenon in a recent study published on The Astrophysical Journal Letters. This hypothesis could open new possibilities in a research area - high energy astrophysics and gravitational waves - which has been experiencing unexpected progresses during the last few years.
Riccardo Ciolfi, research grant holder of the Department of Physics and coordinator of the work explained “We consider the possibility that the union of two neutron stars may lead to the temporary creation of a supermassive star, capable to resist against a gravitational collapse, for minutes up to hours, before creating the black hole. We show how its continuous energy emission until the moment of the collapse may explain the X rays observed for long time after the gamma-ray burst”.
Thanks to PC mathematical simulations it was possible to demonstrate that the supermassive star is surrounded by a dense cloud of matter and energy, due to the very strong magnetic bursts. A few moments after the black hole collapse, a strong jet is generated which travels through the cloud and produces the gamma radiation, while most of the energy emitted by the star before the collapse is still trapped in the cloud and will be released as X radiation using much more time. This delay causes the fact that a large part of the X rays is observed only after the gamma burst following the collapse, even if the causing energy is emitted by the star before the collapse. This phenomenon is known as “time reversal”. In comparison with the propagation time of light in the vacuum, the delay with which the last “photon” emitted by the star before the collapse was evaluated by the two researchers and it is compatible with the long duration of the X radiation observed after the explosion of the gamma rays.
“Up to now we had hypothesised that the signal peak in gravitational waves, generated by the union of two neutron stars and the release of the gamma rays were almost simultaneous events. On the contrary, our model highlights that these two events differ in time, are separated by the “life” of the supermassive star, generated before the inevitable black hole collapse. The gravitational wave detectors like Virgo (in Italy) and LIGO (in the US), which will be active since this year, will be able to alert the gamma satellites and X satellites of the imminent event, giving them the opportunity to follow it and grasp its secrets”.
Lorenzo Pavesi, Director of the Department of Physics added “The research on gravitational waves and high energy astrophysics is extremely satisfactory for our researchers and our University. The research activity and consequent awards to Professors Roberto Battiston and Stefano Vitale demonstrate who the University of Trento and the Department of Physics have become very authoritative in this field. The involvement of young researchers like Riccardo Ciolfi and the collaboration with advanced research institutions such as the Albert Einstein Institute of the Max Planck Society strengthen the position of our University”.
The study by Riccardo Ciolfi and his contributor Daniel Siegel, PhD student at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) was published on The Astrophysical Journal Letters.