MSU scientists have proposed a model explaining the features of the GRB 160625B gamma-ray flash

Lomonosov Moscow State University 31 May 2023 13:57
Astronomers from the Moscow State University GAISH proposed a three-stage model of the GRB 160625B gamma-ray burst, which suggests that the collapse of the stellar core is significantly affected by its rotation. The study of such processes makes it possible to better understand the mechanism of operation of the central engine of gamma-ray bursts and the equation of state of neutron stars. The results of the work are published in The Astrophysical Journal. 

On June 25, 2016, one of the brightest gamma-ray bursts in the history of observations occurred – GRB 160625B. It was observed by the FERMI-LAT, Konus-Wind, SWIFT telescopes, as well as the Lomonosov University spacecraft. In the optical range, observations were carried out, among other things, by the MASTER MSU robot telescope network, with the help of which, for the first time in history, the polarization of the gamma-ray burst's own radiation was measured. An article in Nature was devoted to this measurement. 

In addition to measuring polarization, the gamma-ray burst was interesting for its light curve – it contained three flashes, and instead of the usual afterglow attenuation, scientists saw oscillations. To explain how this could happen, astronomers have proposed a three-stage model of a gamma-ray burst. 

The gamma-ray burst lasted about 450 seconds, while the free fall time (i.e., the time for which the core collapses if the star instantly turns off) was about 10 microseconds. So, there had to be a process delaying the collapse time. 

"This star, apparently, has a successful core mass, which quite slightly exceeded the Oppenheimer-Volkov limit (the limit mass of a neutron star; more massive nuclei collapse into a black hole – ed.). It turns out that such a core should collapse into a black hole, but along the lower boundary. Therefore, even a small rotation was enough to significantly delay the collapse," commented Aristarkh Chasovnikov, a student of the Department of Experimental Astronomy of the Faculty of Physics of Moscow State University. 

Scientists have proposed the following model of the observed process. The collapse began –> the core fell on the centrifugal barrier –> centrifugal forces equalized gravitational forces due to rapid rotation -> the first flash occurred. Then the rotation slowed down, and the core continued to contract until the gravitational forces came into equilibrium with the nuclear ones – then there was a second flash. Finally, the rotation of the core slowed down enough for the core to collapse into a black hole and a third flare occurred. This model explains both the three-stage gamma-ray burst and the oscillations in the afterglow. 

The theory of gamma-ray bursts should include the operation of the central engine (how and at what moment energy is released) and how the released energy is radiated. In this work, the astronomers of the Moscow State University Traffic Police dealt with the first part of the question, because to describe the operation of the central engine, it is just necessary to study unusual bursts, where it is strongly manifested. 

"Studying such mechanisms can help to find out the kind of "equation of state" of a neutron star. This is in the interests of nuclear physics, since huge values of density, temperature and energy are achieved in neutron stars," said Vladimir Lipunov, head of the MASTER project, Honorary Professor of Moscow State University.

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