A Model for Short Gamma-Ray Bursts: Heated Neutron Stars in Close Binary Systems

In this paper we present a model for the short (< second) population of gamma-ray bursts (GRBs). In this model heated neutron stars in a close binary system near their last stable orbit emit neutrinos at large luminosities (~ 10^53 ergs/sec). A fraction of these neutrinos will annihilate to form an electron-positron pair plasma wind which will, in turn, expand and recombine to photons which make the gamma-ray burst. We study neutrino annihilation and show that a substantial fraction (~ 50%) of energy deposited comes from inter-star neutrinos, where each member of the neutrino pair originates from each neutron star. Thus, in addition to the annihilation of neutrinos blowing off of a single star, we have a new source of baryon free energy that is deposited between the stars. To model the pair plasma wind between stars, we do three-dimensional relativistic numerical hydrodynamic calculations. Preliminary results are also presented of new, fully general relativistic calculations of gravitationally attracting stars falling from infinity with no angular momentum. These simulations exhibit a compression effect.Comment: 3 pages, 3 postscript figs (2 color), to appear in "Gamma-Ray Burst and Afterglow Astronomy 2001", Woods Hole; 5-9 Nov, 200

Numerical Modeling of the Radio Nebula from the 2004 December 27 Giant Flare of SGR 1806-20

We use the relativistic hydrodynamics code Cosmos++ to model the evolution of the radio nebula triggered by the Dec. 27, 2004 giant flare event of soft gamma repeater 1806-20. We primarily focus on the rebrightening and centroid motion occurring subsequent to day 20 following the flare event. We model this period as a mildly relativistic (gamma ~ 1.07 - 1.67) jetted outflow expanding into the interstellar medium (ISM). We demonstrate that a jet with total energy ~ 10^46 ergs confined to a half opening angle ~ 20 degrees fits the key observables of this event, e.g. the flux lightcurve, emission map centroid position, and aspect ratio. In particular, we find excellent agreement with observations if the rebrightening is due to the jet, moving at 0.5c and inclined ~ 0 - 40 degrees toward the observer, colliding with a density discontinuity in the ISM at a radius of several 10^16 cm. We also find that a jet with a higher velocity, >~ 0.7c, and larger inclination, >~ 70 degrees, moving into a uniform ISM can fit the observations in general, but tends to miss the details of rebrightening. The latter, uniform ISM model predicts an ISM density more than 100 times lower than that of the former model, and thus suggests an independent test which might discriminate between the two. One of the strongest constraints of both models is that the data seems to require a non-uniform jet in order to be well fit

Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks

In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing "plunging streams" and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured "cubed-sphere" grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.Comment: 15 pages, 11 figures, submitted to Ap

Lorentz Violation of Quantum Gravity

A quantum gravity theory which becomes renormalizable at short distances due to a spontaneous symmetry breaking of Lorentz invariance and diffeomorphism invariance is studied. A breaking of Lorentz invariance with the breaking patterns $SO(3,1)\to O(3)$ and $SO(3,1)\to O(2)$, describing 3+1 and 2+1 quantum gravity, respectively, is proposed. A complex time dependent Schr\"odinger equation (generalized Wheeler-DeWitt equation) for the wave function of the universe exists in the spontaneously broken symmetry phase at Planck energy and in the early universe, uniting quantum mechanics and general relativity. An explanation of the second law of thermodynamics and the spontaneous creation of matter in the early universe can be obtained in the symmetry broken phase of gravity.Comment: 10 pages, minor change and reference added. Typos corrected. To be published in Class. Quant. Grav

Gamma-Ray Bursts via the Neutrino Emission from Heated Neutron Stars

A model is proposed for gamma-ray bursts based upon a neutrino burst of about 10^52 ergs lasting a few seconds above a heated collapsing neutron star. This type of thermal neutrino burst is suggested by relativistic hydrodynamic studies of the compression, heating, and collapse of close binary neutron stars as they approach their last stable orbit, but may arise from other sources as well. We present a hydrodynamic simulation of the formation and evolution of the pair plasma associated with such a neutrino burst. This pair plasma leads to the production of ~10^51 - 10^52 ergs in gamma-rays with spectral and temporal properties consistent with observed gamma-ray bursts.Comment: Final version. 30 pages, 10 figures, 6 tables, accepted for publication in The Astrophysical Journa

On the jet structure and magnetic field configuration of GRB 020813

The polarization curve of GRB 020813 is discussed and compared to different models for the structure, evolution and magnetisation properties of the jet and the interstellar medium onto which the fireball impacts. GRB 020813 is best suited for this kind of analysis for the smoothness of its afterglow light curve, ensuring the applicability of current models. The polarization dataset allows us to rule out the standard GRB jet, in which the energy and Lorentz factor have a well defined value inside the jet opening angle and the magnetic field is generated at the shock front. We explore alternative models finding that a structured jet or a jet with a toroidal component of the magnetic field can fit equally well the polarization curve. Stronger conclusions cannot be drawn due to the incomplete sampling of the polarization curve. A more dense sampling, especially at early times, is required to pin down the structure of the jet and the geometry of its magnetic field.Comment: 7 pages, 5 postscript figures, minor revisions according to the referee comments. A&A in pres

GRB afterglow light curves from uniform and non-uniform jets

Here we calculate the GRB afterglow light curves from a relativistic jet as seen by observers at a wide range of viewing angles from the jet axis, and the jet is uniform or non-uniform. We find that, for uniform jet the afterglow light curves for different viewing angles are somewhat different: in general, there are two breaks in the light curve, corresponding to the time $\gamma\sim (\theta_j-\theta_v)^{-1}$ and $\gamma\sim (\theta_j+\theta_v)^{-1}$ respectively. However, for non-uniform jet, the things become more complicated. For the case $\theta_v=0$, we can obtain the analytical results, for $k<8/(p+4)$ there should be two breaks in the light curve correspond to $\gamma\sim\theta_c^{-1}$ and $\gamma\sim\theta_j^{-1}$ respectively, while for $k>8/(p+4)$ there should be only one break corresponds to $\gamma\sim\theta_c^{-1}$, and this provides a possible explanation for some rapidly fading afterglows whose light curves have no breaks since the time at which $\gamma\sim\theta_c^{-1}$ is much earlier than our first observation time. For the case $\theta_v\neq 0$, our numerical results show that, the afterglow light curves are strongly affected by the values of $\theta_v$, $\theta_c$ and $k$. If the values of $\theta_v/\theta_c$ and $k$ are larger, there will be a prominent flattening in the afterglow light curve, which is quite different from the uniform jet, and after the flattening a very sharp break will be occurred at the time $\gamma\sim (\theta_v + \theta_c)^{-1}.Comment: Latex, 5 pages, accepted for publication by A&

Spectral Lags and the Lag-Luminosity Relation: An Investigation with Swift BAT Gamma-ray Bursts

Spectral lag, the time difference between the arrival of high-energy and low-energy photons, is a common feature in Gamma-ray Bursts (GRBs). Norris et al. 2000 reported a correlation between the spectral lag and the isotropic peak luminosity of GRBs based on a limited sample. More recently, a number of authors have provided further support for this correlation using arbitrary energy bands of various instruments. In this paper we report on a systematic extraction of spectral lags based on the largest Swift sample to date of 31 GRBs with measured redshifts. We extracted the spectral lags for all combinations of the standard Swift hard x-ray energy bands: 15-25 keV, 25-50 keV, 50-100 keV and 100-200 keV and plotted the time dilation corrected lag as a function of isotropic peak luminosity. The mean value of the correlation coefficient for various channel combinations is -0.68 with a chance probability of ~ 0.7 x 10^{-3}. In addition, the mean value of the power-law index is 1.4 +/- 0.3. Hence, our study lends support for the existence of a lag-luminosity correlation, albeit with large scatter.Comment: 19 Pages, 11 Figures and 5 Tables; Accepted to The Astrophysical Journa

Epicyclic oscillations of non-slender fluid tori around Kerr black holes

Considering epicyclic oscillations of pressure-supported perfect fluid tori orbiting Kerr black holes we examine non-geodesic (pressure) effects on the epicyclic modes properties. Using a perturbation method we derive fully general relativistic formulas for eigenfunctions and eigenfrequencies of the radial and vertical epicyclic modes of a slightly non-slender, constant specific angular momentum torus up to second-order accuracy with respect to the torus thickness. The behaviour of the axisymmetric and lowest-order ($m=\pm 1$) non-axisymmetric epicyclic modes is investigated. For an arbitrary black hole spin we find that, in comparison with the (axisymmetric) epicyclic frequencies of free test particles, non-slender tori receive negative pressure corrections and exhibit thus lower frequencies. Our findings are in qualitative agreement with the results of a recent pseudo-Newtonian study of analogous problem defined within the Paczy{\'n}ski-Wiita potential. Implications of our results on the high-frequency QPO models dealing with epicyclic oscillations are addressed.Comment: 24 pages, 8 figure