7. Summary

Despite the great progress in the interpretation of the prompt GRB radiation, crucial issues still remain open and especially on how the mildly relativistic shocks accelerate particles. As a result, today no model that describes self consistently the whole process exists and most of the approach still uses the fractions εB, ε<sup>e</sup> of the internal energy that is dissipated on an enhanced magnetic field of the shocked gas and on some fraction of electrons accelerated to a non-thermal energetic distribution. Sequentially to the approximation of the shocked regime, the radiation models can be applied and used to examine all the intervening radiative and kinetic processes. An interesting point to notice is that the cell high magnetization, σ ≫ 1, leads to inefficient collisions preventing the dissipation of the energy [94, 95]. In such a case, the acceleration of the electron can be obtained through the reconnection process. Such a process can occur before or after the photospheric radius [96–98] and despite the extensive outgoing study of the process there are still even bigger ambiguities than the other two just mentioned [99]. For a review on the issue, the reader can refer Kagan

6. Multimessanger discoveries of electromagnetic and gravitational wave

The assembly of black hole binaries detected in gravitational waves by the LIGO interferometer was established since the discovery of GW150914 [101]. These systems contain very massive black holes, whose origin poses a puzzle for the stellar evolution models [102]. One of the possible scenarios for the formation of such a black hole is a process of direct collapse of massive stars. Here, no spectacular hypernova explosion is proposed, and hence no gamma ray burst should have occured during the formation of a very massive black hole neither for the first nor for the second component in the binary. An additional issue is the feedback from a rotationally supported innermost parts of the star during the collapse. It is rather natural that the star at its final stages of evolution should posses some non-negligible angular momentum in the envelope. This angular momentum may, however, help unbind the outer layers and halt accretion (Ramirez-Ruiz 2017, private communication). This will have a consequence for both the ultimate mass of the black hole, and its resultant spin, to be independently verified by the values obtained for these parameters from gravitational wave-

One of the possibilities when the gravitational wave signal would be found in relation to the rotating massive star collapse, and coincident with a gamma ray burst, was proposed by Janiuk et al. [103]. In this scenatio, the collapse of a massive rotating star in a close binary system with a companion black hole. The primary BH which forms during the core collapse is first being spun up and increases its mass during the fall back of the stellar envelope. As the companion BH enters the outer envelope, it provides an additional angular momentum to the gas. After the infall and spiral-in toward the primary, the two BHs merge inside the circumbinary disk. The second episode of mass accretion and high final spin of the postmerger

et al. [100].

28 Cosmic Rays

counterparts

form constraints.

BH feeds the gamma ray burst.

Gamma ray bursts are known since almost 50 years now and are still an exciting field of research for both observers and theoretitians. Their energetic requirements proved the fundamental role of the stellar mass black hole formation and mass accretion in the production of ultrarelativistic jets.

The details of this process are, however, far from being fully understood. In short GRBs, the process of black hole birth after the neutron star merger may proceed through different channels, with the possible presence of a transient hypermassive neutron star, depending on the EOS and rotation of the progenitors. In long GRBs, the properties of progenitor star, its envelope rotation, metallicity, etc., as well as the binarity of the whole system, may affect the core collapse in an even greater way. The question of binarity is of a great interest in the context of the fate of high mass X-ray binaries, such as Cygnus X-3, which in addition to the prehypernova star contains a companion which is most probably a black hole.

Such fundamental questions are now being attacked with the modern tools of numerical astrophysics, which involve relativistic magnetohydrodynamics and nuclear physics. With the discovery of gravitational waves, a new window has also opened from the observational point of view, especially since the gamma ray signal has been identified in connection with the compact object merger. The identification of the additional electromagnetic signal from the radioactive decay of the GRB ejecta provided a completely new way to probe the whole process and hopefully build a comprehensive picture in the near future.
