Open Access
Access granted
Subscription Access
Vol 43, No 1 (2017)
- Year: 2017
- Articles: 4
- URL: https://journal-vniispk.ru/1063-7737/issue/view/11907
Article
Precursors of short gamma-ray bursts in the SPI-ACS/INTEGRAL experiment
Abstract
We have analyzed the light curves of 519 short gamma-ray bursts (GRBs) detected in the SPI-ACS/INTEGRAL experiment from December 2002 toMay 2014 to search for precursors (a possible activity of the GRB source before the beginning of themain episode). Both the light curves of 519 individual events and the averaged light curve of 372 brightest bursts have been analyzed. In several cases, we have found and thoroughly studied precursor candidates based on SPI-ACS/INTEGRAL, GBM/Fermi, and LAT/Fermi data. A statistical analysis of the averaged light curve for the entire sample of short bursts has revealed no regular precursor. Upper limits for the relative intensity of precursors have been estimated. We have compared our results with those of other authors; no convincing evidence for the existence of precursors of short GRBs has been found. We show that the fraction of short GRBs with precursors is less than 0.4% of all short bursts.
1-20
Sizes of the stellar subsystems of galaxies and intracluster stars in the Virgo cluster
Abstract
Stellar photometry for three fields in the Virgo cluster of galaxies has been performed on the basis of archival Hubble Space Telescope ACS/WFC images. The densely populated red giant branches belonging to cluster galaxies are seen on the constructed Hertzsprung–Russell diagrams. The distances to eight galaxies in the Virgo cluster have been determined by the TRGB method. The sizes of the stellar subsystems and the gradients in the number density of red giants along the galactic radius have been determined for the investigated galaxies. The stellar periphery of the galaxies has been found to extend to great distances and to cover the entire area of the images used. Therefore, the stars that have previously been thought to be intracluster stars actually belong to the periphery of neighboring galaxies.
21-35
Time-dependent ionization in the envelopes of type II supernovae at the photospheric phase
Abstract
The importance of allowance for the time-dependent effect in the kinetics at the photospheric phase during a supernova explosion has been confirmed by several independent research groups. The time-dependent effect provides a higher degree of hydrogen ionization in comparison with the steadystate solutions and strengthens the Hα line in the resulting simulated spectrum, with the intensity of the effect increasing with time. However, some researchers argue that the time-dependent ionization effect is unimportant. Its allowance leads to an insignificant strengthening of Hα in their modeling only in the first days after explosion. We have demonstrated the importance of the time-dependent effect with the models of SN 1999em as an example using the new original LEVELS software package. The role of a number of factors that can weaken the time-dependent effect has been checked. We have confirmed that the intensity of the effect is affected by the abundance of metal admixtures in the envelope, while the addition of extra levels to the model hydrogen atom weakens the time-dependent effect to a lesser degree and never removes it completely.
36-49
Variational principle for stars with a phase transition
Abstract
The variational principle for stars with a phase transition has been investigated. The term outside the integral in the expression for the second variation of the total energy of a star is shown to be obtained by passage to the limit from the integration over the region of mixed states in the star. The form of the trial functions ensuring this passage has been found. All of the results have been generalized to the case where general relativity is applicable. The known criteria for the dynamical stability of a star when a new phase appears at its center are shown to follow automatically from the variational principle. Numerical calculations of hydrostatically equilibrium models for hybrid stars with a phase transition have been performed. The form of the trial functions for the second variation of the total energy of a star that describes almost exactly the stability boundaries of such stellar models is proposed.
50-60