Narrow Results

The propagation of a boson particle in the Janis–Newman–Winicour–Wyman (JNWW) gravitational field is analyzed. We derive the wave function of the scalar particle, and the effective potential experienced by quantum particles. Besides, we show that the probability to find the scalar particle near the region where the naked singularity is finite. Finally, these results are proposed as a model to study the gamma ray bursts (GRBs) physics.

Neutrinos play the critical roles in nucleosyntheses of light-to-heavy mass elements in core-collapse supernovae (SNe). The light element synthesis is affected strongly by neutrino oscillations (MSW effect) through the ν-process in outer layers of supernova explosions. Specifically the ⁷Li and ¹¹B yields increase by factors of 1.9 and 1.3 respectively in the case of large mixing angle solution, normal mass hierarchy, and sin² 2θ₁₃ = 2 × 10⁻³ compared to those without the oscillations. In the case of inverted mass hierarchy or nonadiabatic 13-mixing resonance, the increment of their yields is much smaller. We thus propose that precise constraint on mass hierarchy and sin² 2θ₁₃ is given by future observations of Li/B ratio or Li abundance in stars and presolar grains which are made from supernova ejecta. Gamma ray burst (GRB) nucleosynthesis in contrast is not affected strongly by thermal neutrinos from the central core which culminates in black hole (BH), although the effect of neutrinos from proto-neutron star prior to black hole formation is still unknown. We calculate GRB nucleosynthesis by turning off the thermal neutrinos and find that the abundance pattern is totally different from ordinary SN nucleosynthesis which satisfies the universality to the solar abundance pattern.

While some space born observatories, such as SWIFT and FERMI, have been operating, early observation of optical after grow of GRBs is still remained as an unexplored region. The Ultra-Fast Flash Observatory (UFFO) project is a space observatory for optical follow-ups of GRBs, aiming to explore the first 60 seconds of GRBs optical emission. Using fast moving mirrors to redirect our optical path rather than slewing the entire spacecraft, UFFO is utilized to catch early optical emissions from GRB within 1 sec. We have developed the UFFO Pathfinder Telescope which is going to be on board of the Lomonosov satellite and launched in middle of 2012. We will discuss about scientific potentials of the UFFO project and present the payload development status, especially for Slewing Mirror Telescope which is the key instrument of the UFFO-pathfinder mission.

The propagation of boson particles in a gravitational field described by the Brans–Dicke (BD) theory of gravity is analyzed. We derive the wave function of the scalar particles, and the effective potential experienced by the quantum particles considering the role of the varying gravitational coupling. Besides, we calculate the probability to find the scalar particles near the region where a naked singularity is present. The extremely high energy radiated in such a situation could account for the huge emitted power observed in gamma ray bursts (GRBs).

We present a brief review of the present status of the standard model of core collapse supernovae and neutron star formation outlining the basic concepts and paying attention to the possibility of a transition to quark matter. We evaluate the consequences of this transition on the whole explosion mechanism, analyze the possible generation of beamed gamma ray bursts, and discuss the nature of the compact star born as a result of the supernova explosion.

In General Relativity, all forms of energy contribute to gravity and not only just ordinary matter as in Newtonian Physics. This fact can be seen in the modified hydrostatic equilibrium equation for relativistic stars pervaded by magnetic (B) fields. It has an additional term coupled to the matter part as well as an anisotropic term which is purely of magnetic origin. That additional term coming from the pressure changed by the radial component of the diagonal electromagnetic field tensor weakens the gravitational force when B is strong enough and can even produce an unexpected change in the attractive nature of the force by reversing its sign. In an extreme case, this new general relativistic (GR) effect can even trigger an instability in the star as a consequence of the sudden reversal of the hydrostatic pressure gradient. We suggest here that this GR effect may be the possible central engine driving the transient giant outbursts observed in Soft Gamma-ray Repeaters (SGRs). In small regions of the neutron star (NS), strong magnetic condensation can take place. Beyond a critical limit, these highly magnetized bubbles may explode releasing the trapped energy as a burst of γ-rays of ~10^36–40 erg.

In the case of high energy collimated outflows it is important to know the density and structure of material through which jets propagate. Here we present a simple study of the environment of GRBs where the relativistic ejecta travel during the event. We use observed spectral lines in the afterglow of gamma-ray bursts and by constructing the curve of growth for most redshifted lines, we extract the column densities and Doppler parameter. This can give us the overall picture of abundances in the vicinity of GRBs. Also, we compare this result with the spectral lines with lower redshift to examine the dependence of elemental abundances on the distance from GRB event.

In a few dozen seconds, gamma ray bursts (GRBs) emit up to ~10⁵⁴ erg in terms of an equivalent isotropically radiated energy E_iso, so they can be observed up to z ~ 10. Thus, these phenomena appear to be very promising tools to describe the expansion rate history of the universe. Here, we review the use of the E_p,i–E_iso correlation of GRBs to measure the cosmological density parameter Ω_M. We show that the present data set of GRBs, coupled with the assumption that we live in a flat universe, can provide independent evidence, from other probes, that Ω_M ~ 0.3. We show that current (e.g. Swift, Fermi/GBM, Konus-WIND) and forthcoming gamma ray burst (GRB) experiments (e.g. CALET/GBM, SVOM, Lomonosov/UFFO, LOFT/WFM) will allow us to constrain Ω_M with an accuracy comparable to that currently exhibited by Type Ia supernovae (SNe–Ia) and to study the properties of dark energy and their evolution with time.

The origin of gamma-ray burst (GRB) prompt emission, bursts of γ-rays lasting from shorter than one second to thousands of seconds, remains not fully understood after more than 40 years of observations. The uncertainties lie in several open questions in the GRB physics, including jet composition, energy dissipation mechanism, particle acceleration mechanism and radiation mechanism. Recent broad-band observations of prompt emission with Fermi sharpen the debates in these areas, which stimulated intense theoretical investigations invoking very different ideas. I will review these debates, and argue that the current data suggest the following picture: A quasi-thermal spectral component originating from the photosphere of the relativistic ejecta has been detected in some GRBs. Even though in some cases (e.g. GRB 090902B) this component dominates the spectrum, in most GRBs, this component either forms a sub-dominant "shoulder" spectral component in the low energy spectral regime of the more dominant "Band" component, or is not detectable at all. The main "Band" spectral component likely originates from the optically thin region due to synchrotron radiation. The diverse magnetization in the GRB central engine is likely the origin of the observed diverse prompt emission properties among bursts.

Recent observations, especially by the Fermi satellite, point out the importance of the thermal component in GRB spectra. This fact revives strong interest in photospheric emission from relativistic outflows. Early studies already suggested that the observed spectrum of photospheric emission from relativistically moving objects differs in shape from the Planck spectrum. However, this component appears to be subdominant in many GRBs and the origin of the dominant component is still unclear. One of the popular ideas is that energy dissipation near the photosphere may produce a nonthermal spectrum and account for such emission. Before considering such models, though, one has to determine precise spectral and timing characteristics of the photospheric emission in the simplest possible case. Hence this paper focuses on various physical effects which make the photospheric emission spectrum different from the black body spectrum and quantifies them.

While it is widely believed that the gravitational collapse of a sufficiently large mass will lead to a density singularity and an event horizon, we propose that this never happens when quantum effects are taken into account. In particular, we propose that when the conditions become ripe for the formation of a trapped surface, a quantum critical firewall sweeps over the collapsing body, transforming the nucleons in the collapsing matter into a lepton/photon gas together with droplets of a positive vacuum energy. This will happen regardless of the matter density at the time a trapped surface starts to form, and as a result, we predict that at least in all cases of gravitational collapse involving ordinary matter, a large fraction of the rest mass of the collapsing matter will be converted into a burst of neutrinos and γ-rays. We predict that the peak luminosity of these bursts is only weakly dependent on the mass of the collapsing object, and on the order of (ϵ_q/m_Pc²)^1/4c⁵/G where ϵ_q is the mean energy of a nucleon parton and m_P is the Planck mass. The duration of the bursts will depend on the mass of the collapsing object; in the case of stellar core collapse, we predict that the duration of both the neutrino and γ-ray bursts will be on the order of 10s.

Stars that are collapsing towards forming a black hole but appear frozen near their Schwarzschild horizon are termed “black stars”. The collision of two black stars leads to gravitational radiation during the merging phase followed by a delayed gamma ray burst during coalescence. The recent observation of gravitational waves by LIGO, followed by a possible gamma ray counterpart by Fermi, suggests that the source may have been a merger of two black stars with profound implications for quantum gravity and the nature of black holes.

An analysis of the effects of the passage of a gravitational wave (GW) on the quantum vacuum is made within the context of the Nexus paradigm of quantum gravity. Results indicate that if the quantum vacuum includes electrically charged virtual particle fields, then a GW will induce vacuum polarization. The equations of General Relativity (GR) are then reformulated to include electric charge displacements in the quantum vacuum imposed by an anisotropic stress — momentum tensor. It is then demonstrated that as a result of the spacetime piezoelectric effect, a gravitational wave is associated with a rotating electromagnetic wave and that the converse effect produced by strong electromagnetic fields is responsible for the generation of relativistic jets and gamma ray bursts. Objects with strong electromagnetic fields will apparently violate the strong equivalence principle.

Gamma ray bursts (GRBs) are the most explosive and brightest known photon sources in the universe. The initial bursts of GRBs found so far typically have a spectral peak between $\sim 100$keV and 1MeV. Most of their gamma ray prompt emissions last for $\sim 100$s or less. The jets ejected from ‘the central engine’ are thought to be responsible for these bursts. According to the currently accepted scenario, a black hole produced by the merger of compact objects (such as black holes and/or neutron stars) is responsible for shorter bursts while collapse of a massive star directly to a black hole produces longer bursts, although clear-cut distinction may not exist. In this paper, we consider the latter case, a collapsar model responsible for longer GRBs. They are more interesting in the sense that they are more energetic and hence their studies can be extended to first Population III GRBs, which may give some constraints on the understanding of the early universe. In this paper, we are interested mainly in the “central engine”, the region created by the collapse of a rotating massive star to a spinning black hole and its surrounding area created by the stellar remnant because the conditions of the jet propagation change once they are outside in the interstellar medium. We first follow the process of how a progenitor massive star collapses and forms a central disk-black hole system, how powerful jets are produced and ejected from that system, and how the jets propagate and penetrate the surrounding stellar medium. The continued journey of the jets outside the star and the mechanisms for GRB emissions are outside the scope of this short paper.

The connection between gamma ray bursts and supernovae is studied using a temperature-dependent vacuum model. A harmonically bound particle–antiparticle system is consistent with both Hawking radiation and Casimir effect, therefore, the Maxwell–Sellmeier model correlates the speed of light to temperature. According to quantum field theory, Lorentz invariance is violated only for temperatures larger than $4\times 10^9$K. Introducing in the temperature distribution of a 2D simulation for a type Ia supernova the speed of light temperature dependence proposed in this paper, results a speed of light distribution. A theoretical snapshot of this distribution at an arbitrary distance is consistent with photon photo finish resulted from experiments. Variable speed of light shows that supernova could be accompanied by gamma-ray bursts.

Gamma Ray Bursts (GRB) are the extremely energetic transient events, visible from the most distant parts of the Universe. They are most likely powered by accretion on the hyper-Eddington rates that proceeds onto a newly born stellar mass black hole. This central engine gives rise to the most powerful, high Lorentz factor jets that are responsible for energetic gamma ray emission. We investigate the accretion flow evolution in GRB central engine, using the 2D MHD simulations in General Relativity. We compute the structure and evolution of the extremely hot and dense torus accreting onto the fast spinning black hole, which launches the magnetized jets. We calculate the chemical structure of the disk and account for neutrino cooling. Our preliminary runs apply to the short GRB case (remnant torus accreted after NS-NS or NS-BH merger). We estimate the neutrino luminosity of such an event for chosen disk and central BH mass.

We present examples of rapid changes in spectral and timing properties in accretion flows around compact objects and discuss what could be going on in these systems. We find a new way of quantifying the variation of the flow geometry. We show the evolution of the variation of the Comptonization efficiency computed from the ratio of the Comptonized photons and the injected seed photons. The time evolution is a direct consequence of the variation of the accretion rates which changes the hydrodynamic and radiative properties of the flow and therefore the flow geometry.

We will discuss preliminary results of a systematic investigation devoted to study the time resolved broad-band spectra of the prompt emission of a sample of GRBs. These events were simultaneously detected with the BeppoSAX WFCs and the BATSE instrument aboard CGRO. We will discuss the fit of these spectra with the Band function, with a blackbody plus a power–law model, and with a Comptonization model developed by Titarchuk & Farinelli (2011, to be submitted).

The phenomenon that originates gamma ray bursts (GRBs) remains undefined. In this work the conversion of a hadronic star into a quark star is discussed as one of the possible causes of GRBs. Effective models are used to describe the compact stars and to obtain their equations of state. Macroscopic properties, such baryonic and gravitational masses, of both types of stars are then obtained from the solution of the hydrostatic equilibrium equations. The relation between this values allows to calculate the amount of energy possibly released in this process. The obtained results are then compared to actual GRB observational data, and are within the observational order of magnitude.

The actual knowledge of the structure and future evolution of our universe is based on the use of cosmological models, which can be tested through the so-called ‘probes’, namely astrophysical phenomena, objects or structures with peculiar properties that can help to discriminate among different cosmological models. Among all the existing probes, of particular importance are the Supernovae Ia (SNe Ia) and the Gamma Ray Bursts (GRBs): the former are considered among the best standard candles so far discovered but suffer from the fact that can be observed until redshift z = 2.26, while the latter are promising standardizable candles which have been observed up to z = 9.4, surpassing even the farthest quasar known to date, which is at z = 7.64. The standard candles can be used to test the cosmological models and to give the expected values of cosmological parameters, in particular the Hubble constant value. The Hubble constant is affected by the so-called “Hubble constant tension”, a discrepancy in more than 4 σ between its value measured with local probes and its value measured through the cosmological probes. The increase in the number of observed SNe Ia, as well as the future standardization of GRBs through their correlations, will surely be of help in alleviating the Hubble constant tension and in explaining the structure of the universe at higher redshifts. A promising class of GRBs for future standardization is represented by the GRBs associated with Supernovae Ib/c, since these present features similar to the SNe Ia class and obey a tight correlation between their luminosity at the end of the plateau emission in X-rays and the time at the end of the plateau in the rest-frame.