Pulsed gamma-ray emission from neutron and collapsing stars and supernovae

Abstract

Three mechanisms generating pulsed γ-ray emission at late stages of stellar evolution are investigated: (1) γ-ray burst produced by the absorption of neutrino emission of a collapsing star in its envelope; (2) γ-ray burst of thermal emission when the outer layers of a compact star (R=0.01–0.1R⊙) are heated up by a powerful shock wave; and (3) γ-ray emission due to ejection of matter from neutron stars at an active stage of their existence. In case (1) the expected flux of γ-quanta with the energy 0.1 MeV amounts to about 10−4 cm−2 which is considerably less than the observed flux. However, observation of such γ-ray pulses would be an important supplement to the direct observations of neutrino emission of collapsing stars. In case (2) the outer layers of the star are heated up to the temperature of about 108 K. This results in a short burst with the emission energy ∼ 1042–43 erg whose main part is concentrated in the X-ray range (∼ 25 keV). In this case approximately 10% of the energy is emitted in the γ-ray range (≳ 0.1 MeV). Generally speaking, this mechanism is sufficient as to the energy for accounting for the observed bursts; however, probably under the condition that supernovae are exploding in our Galaxy. This involves difficulties concerning the frequency of bursts and the spectrum of emission. In case (3) ejection of chemically non-equilibrium matter from the neutron star leads to an intensive emission which is produced due to fission of superheavy nuclei, β-decay of radioactive elements and radiative capture of free neutrons. Ejection of matter from the neutron stars may be related to the observed jumps of periods of pulsars. From the observed gain of kinetic energy of the filaments of the Crab Nebula (∼ 2×1041 erg) the mass of the ejected material may be estimated (∼ 1021 g). This leads to energies of the γ-ray bursts of the order of 1038–1039 erg, which agrees fully with observations at the mean distance up to the sources 0.25 kpc. As distinct from the outbursts of supernovae it would seem that no difficulties concerning the burst frequency and the spectrum of emission are encountered. From the mechanisms of γ-ray emission examined here the mechanism connected with the ejection of matter from neutron stars seems to be the most promising as far as the interpretation of observations is concerned.