Cosmological models and latest observational data

In this note, we consider the observational constraints on some cosmological models by using the 307 Union type Ia supernovae (SNIa), the 32 calibrated gamma-ray bursts (GRBs) at z>1.4, the updated shift parameter R from WMAP 5-year data (WMAP5), and the distance parameter A of the measurement of the baryon acoustic oscillation (BAO) peak in the distribution of SDSS luminous red galaxies with the updated scalar spectral index n _s from WMAP5. The tighter constraints obtained here update the ones obtained previously in the literature.     

Studying the mechanism of gamma quantum radiation in cosmic gamma-ray bursts using data from the Swift,CGRO, and FERMI spacecraft

Light curves of 115 bright gamma-ray bursts with known cosmological distances z from sources detected by the BAT telescope on board the Swift spacecraft in four energy channels are analyzed. An averaged curve describing the shift of the light curves is obtained that depends on the energy of the channels in the coordinate system associated with the source. The results are compared to similar detailed curves for individual gamma-ray bursts detected in a wider range of energies by the BATSE (CGRO) and GBM (FERMI) telescopes. The divergence of curves in the region of low energies for some gamma ray bursts indicates the existence of an additional radiation source.     

Casimir Energy in Astrophysics: Gamma-Ray Bursts from QED Vacuum Transitions

Motivated by analogous applications to sonoluminescence, neutron stars mergers are examined in the context of Schwinger's dynamical Casimir effect. When the dielectric properties of the QED vacuum are altered through the introduction of dense matter, energy shifts in the zero-point fluctuations can appear as photon bursts at gamma-ray frequencies. The amount of radiation depends upon the properties and amount of matter in motion and the suddenness of the transition. It is shown that the dynamical Casimir effect can convert sufficient energy of neutron star mergers into gamma rays. Using information extracted from simulations of matter flow in neutron star mergers by Ruffert and Janka, we estimate that the total Casimir energy released can exceed 10 ⁵³ ergs in gamma-ray frequencies. The Casimir energy approach is capable of explaining the most energetic gamma-ray bursts.     

Cosmological γ-ray bursts from a neutron star collapse induced by a primordial black hole

A detailed analysis is presented for a novel scenario in which gamma-ray bursts are of intergalactic origin and arise from the induced collapse of an isolated neutron star triggered by a primordial black hole. The energy released from the phase transition of accreted nucleon matter into a quark-gluon plasma is transferred by degenerate neutrinos to the star’s surface, where neutrinos annihilate into an electron-positron plasma and produce an inverted temperature layer that preserves a fire-ball from undue baryonic pollution. Possible observational tests include the absence of apparent cosmological time dilation, the location of γ-ray bursts primarily outside of galaxies, a specific shape of the log N-log S curve, with a large peak near red shift z∼10, the emission of ∼10⁻³ of the total energy in the form of 100-GeV photons, a bimodal distribution of durations, a very weak accompanying pulse of gravitational radiation, etc. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 10, 642–647 (25 November 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Search for 10TeV Gamma-Ray Bursts

A search for gamma-ray bursts at 1OTeV energy region was made using data of Yangbajing air shower experiment in Tibet.About 4×10⁸ events were analysed to search for shower clusters appearing in a given time interval and a given small sky bin. An equal-zenith angle method is adopted to estimate the background. Some clusters show excesses over the background but with less significance as the evidence of gamma-ray bursts.The much higher sensitivity of the Yangbajing second phase array to the detection of 1OTeV gamma-ray bursts is discussed.     

Bursts of gravitational radiation from superconducting cosmic strings and the neutrino mass spectrum

Berezinsky, Hnatyk and Vilenkin showed that superconducting cosmic strings could be central engines for cosmological gamma-ray bursts and for producing the neutrino component of ultra-high energy cosmic rays. A consequence of this mechanism would be that a detectable cusp-triggered gravitational wave burst should be released simultaneously with the γ-ray surge. If contemporary measurements of both γ and ν radiation could be made for any particular source, then the cosmological time-delay between them might be useful for putting unprecedentedly tight bounds on the neutrino mass spectrum. Such measurements could consistently verify or rule out the model, since strictly correlated behaviour is expected for the duration of the event and for the time variability of the spectra.     

Supernova bangs as a tool to study big bang

Supernovae and gamma-ray bursts are the most powerful explosions in observed Universe. This educational review tells about supernovae and their applications in cosmology. It is explained how to understand the production of light in the most luminous events with minimum required energy of explosion. These most luminous phenomena can serve as primary cosmological distance indicators. Comparing the observed distance dependence on red shift with theoretical models one can extract information on evolution of the Universe from Big Bang until our epoch.     

A Cyclic Model of Universe with Energy Exchange between Radiation,Matter and Vacuum Energy

We further extend the cosmological scenario with energy exchange by Barrow and Clifton and our previous work to the more complex case with energy exchange between three fluids: radiation, matter and vacuum energy. By prescribing the form of energy exchange function, we construct an infinitely cyclic cosmological model, in which the universe undergoes an endless sequence of cosmic epoch and each consisting of expansion and contraction, and the cosmological parameters, such as the Hubble parameter H, deceleration parameter q, transition red-shift Z _T, and densities ρ _r ,ρ _m , and ρ _Λ are consistent with the present observed values.     

γ射线暴的研究概况

γ射线暴是宇宙中自从大爆炸以来最猛烈的爆发现象，它在几十秒钟的时间内所释放的能量相当于太阳一生（约一百亿年）所释放能量的几百倍！文章简要介绍了γ射线暴的新近研究进展，其中包括：简要说明了观测事实，并在此基础上建立标准火球模型，阐述了γ射线暴及其余辉的运动和演化规律，讨论了偏离标准模型的种种观测现象以及这些后标准效应所包含的重要天体物理意义。进而讨论了至今仍不清楚的能源机制问题，也指出了这个领域的研究前景。     

Experimental search for gamma-ray bursts from evaporating primordial black holes

The energy spectra and temporal characteristics of high-energy gamma-ray bursts from evaporating primordial black holes have been calculated using various evaporation models. The currently existing theoretical uncertainties in the shape of the evaporated photon spectrum are discussed. The data from the Andyrchy and Carpet-2 arrays of the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) obtained in the mode of detection of a single cosmic-ray component are used to search for cosmic gamma-ray bursts with a primary photon energy of about 8 GeV. New upper limits have been obtained for the number density of evaporating black holes in a local region of space with a characteristic size of ～10^?3 pc for various evaporation models.     

Tachyon field in cosmology

We study cosmological effects of homogeneous tachyon field as dark energy. We concentrate on two different scalar field potentials, the inverse square potential and the exponential potential. These models have a unique feature that the matter density parameter and the density parameter for tachyons remain comparable for a large range in red-shift. It is shown that there exists a range of parameters for which the universe undergoes an accelerated expansion and the evolution is consistent with structure formation requirements. For a viable model we require fine tuning of parameters comparable to that in ACDM or in quintessence models. For the exponential potential, the accelerated phase is followed by a phase witha(t) α t ^2/3 thus eliminating a future horizon.     

Spectra and luminosities in gamma-ray bursts

Summary A spatial-distribution analysis of 124 gamma-ray bursts recorded by the soviet experiment Konus has been done in a previous paper. The physical parameters used in this analysis were the flux intensity and the spectral parameterKT, derived fitting the spectra with a thermal bremsstrahlung law. In this paper an analogous analysis is performed with the spectral parameterv _c (the Larmor frequency times the square of the electron temperature) derived fitting the spectra with a thermal synchrotron of semi-relativistic electron model. In this second case, differently from the first one, we must consider also the spectrum dependence on the angle between the observer direction and the neutron star magnetic-field direction. We analyse the possible effects of this dependence on the logN-logP and on the event distributionvs. v _c . The peak luminosity of the bursts as a function ofv _c is also derived. Paper presented at the 2^o Convegno Nazionale di Fisica Cosmica, held at L'Aquila 29 May–2 June 1984.     

Search for low-energy neutrino radiation accompanying gamma-ray bursts on the Baksan subterranean scintillation telescope

An analysis aimed at finding possible neutrino radiation accompanying gamma-ray bursts in a 24-h period about them is performed on the basis of the data in the 4B BATSE Gamma-Ray Burst Catalog and data from the Baksan scintillation telescope according to a program for finding neutrinos from collapsing stars. Values significantly exceeding the background are not discovered. A lower bound for the distance to the source is established under the assumption that the anticipated radiation has characteristics similar to the characteristics of collapse neutrinos. It attests to the cosmological origin of gamma-ray bursts with a high degree of probability. Zh. éksp. Teor. Fiz. 114, 1921–1929 (December 1998)     

Gamma-ray bursts and related phenomena

Gamma-ray bursts (GRBs) have puzzled astronomers since their accidental discovery in the sixties. The BATSE detector on the COMPTON-GRO satellite has been detecting one burst per day for the last six years. Its findings have revolutionized our ideas about the nature of these objects. They have shown that GRBs are at cosmological distances. This idea was accepted with difficulties at first. However, the recent discovery of an X-ray afterglow by the Italian/Dutch satellite BeppoSAX led to a detection of high red-shift absorption lines in the optical afterglow of GRB970508 and to a confirmation of its cosmological origin. The simplest and practically inevitable interpretation of these observations is that GRBs result from the conversion of the kinetic energy of ultra-relativistic particles flux to radiation in an optically thin region. The “inner engine” that accelerates the particles is hidden from direct observations. Recent studies suggest the “internal-external” model: internal shocks that take place within the relativistic flow produce the GRB while the subsequent interaction of the flow with the external medium produce the afterglow. The “inner engine” that produces the flow is, however, hidden from direct observations. We review this model with a specific emphasis on its implications to underground physics.     

γ射线暴的最新研究进展:火球模型、余辉及前身星

γ射线暴（称简γ暴）的研究在最近几年里有了巨大的突破。观测上，人们发现了γ暴的低能余辉以及与γ射线发同时的光学爆发，还发现了它位于宇学距离的寄主星系。越来越多的观测证据还表明长时标γ暴与恒星形成区、甚至可能与超新星成协。在γ暴的相对论火球模型框架下，人们对γ暴以及余辉的产生机制的认识也有了进展。进而人们对γ暴的前身星以及环境效应等有了新的认识。本文旨在对这些进展和认识给一个扼要的评述。     

Efficiency of electron-positron pair production by neutrino flux from accretion disk of a Kerr black hole

Dominant processes of neutrino production and neutrino-induced e ⁺ e ^?-pair production are examined in the model of a disk hyperaccreting onto a Kerr black hole. The efficiency of plasma production is analytically estimated for both the presence and the absence of a strong magnetic field. It is shown that the efficiency of plasma production by a neutrino flux from the disk in this model is no more than several tenths of percent and, therefore, cannot account for the origin of cosmological gamma-ray bursts.     

A matter-dominated cosmological model with variable G and Λ and its confrontation with observational data

In the framework of renormalization-group improved cosmologies, we analyze both theoretically and observationally the exact and general solution of the matter-dominated cosmological equations, by using the expression of the cosmological term as a function of the Newton parameter already determined by the integration method employed in a previous paper. A rough comparison between such a model and the concordance ΛCDM model from the point of view of the magnitude-redshift relationship has been already considered, without showing any appreciable differences. Here we test our model by using astrophysical data (the Union2 type Ia supernovae (SNIa) dataset, the Hubble diagram constructed from some gamma ray bursts luminosity distance indicator), to constrain its parameters. We also apply a cosmographic approach to our cosmological model. In order to estimate the cosmographic parameters we fit a large dataset, including not only the Hubble diagram, as traced by SNIa and gamma ray bursts, but also the H(z) measurements from passively evolving galaxies, baryon acoustic oscillations and the distance priors from the cosmic microwave background radiation anisotropy spectrum. We show that this matter-dominated cosmological model with variable Newton parameter and variable cosmological term is indeed compatible with the observations above. The cosmographic approach adopted confirms such conclusions. Last, it seems possible to include radiation into the model, since numerical integration of the equations derived by the presence of both radiation and matter shows that, after inflation, the total density parameter is initially dominated by the radiation contribution and later by the matter one.     

Repeated gamma-ray bursts from the June 15, 1991 solar flare

The results of the detection of repeated bursts of gamma rays with energies exceeding 30 MeV from the June 15, 1991 solar flare, observed with the Gamma-1 telescope, are examined. It is shown that they occur on the declining part of the temporal profile of the gamma radiation and last for ∼10 min (they occur 27 min after the optical part of the flare starts). No appreciable radiation bursts were observed during the subsequent quasiconstant part (∼20 min). It is concluded that repeated events in which particles are accelerated occur in the active stage of the powerful June 15, 1991 solar flare. It is noted that while the June 15, 1991 and March 26, 1991 gamma-ray bursts share a common character, their spectral and temporal characteristics are substantially different. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 889–893 (25 June 1996)     

Temporal structure of the gamma-ray bursts GRB881024 and GRB880810

Summary In this paper we analyse two particularly significant cosmic gamma-ray bursts recorded by the French-Soviet experiment APEX on board the space mission PHOBOS. The October 24th 1988 event, lasting more than 16 seconds, is among the strongest events observed up to now. The very high temporal resolution available for its light curve, 1/8 second, allows us to study the power spectrum density (PSD) up to a frequency range more extended than in the past. The August 10th 1988 event, lasting about 1 second, recorded with a temporal resolution 1/128 second, higher than in the precedent one, permits us to reach still higher frequencies. We discuss the results of our analysis, comparing them with the results obtained for other events recorded by precedent space missions, and giving interpretative suggestions. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.