13294483
Gamma‐ray burst precursors as the remnant of the thermal radiation initially trapped in the fireball
In the standard fireball model of gamma-ray bursts (GRBs), the fireball starts with an optically thick phase. As it expands, the fireball becomes optically thin at some stage. The thermal radiation trapped in the originally opaque fireball then leaks out, producing a transient event. The appearance of the event is investigated in the framework of a homogeneous, spherically symmetric and freely expanding fireball produced instantly by an explosive process without continuous injection of mass and energy. We find that, generally, the event has a time duration shorter than that of the main burst, which is presumably produced by the internal shock after the fireball becomes optically thin. The event is separated from the main burst by a quiescent time interval, and is weaker than the main burst at least in a high-energy band. Hence, the event corresponds to a GRB precursor. The precursor event predicted by our model has a smooth and Fast Rise and Exponential Decay (FRED) shaped light curve, and a quasi-thermal spectrum. Typically, the characteristic blackbody photon energy is in the X-ray band. However, if the distortion of the blackbody spectrum by electron scattering is considered, the characteristic photon energy could be boosted to the gamma-ray band. Our model may explain a class of observed GRB precursors - those having smooth and FRED-shaped light curves and quasi-thermal spectra.
44433480
Two open universes connected by a wormhole: Exact solutions
Abstract In this paper, I present a space–time of two open universes connected by a Lorentzian wormhole. The space–time has the following features: (1) it can exactly solve the Einstein equations; (2) the weak energy condition is satisfied everywhere; (3) it has a topology of R 2 × T g ( g ≥2); (4) it has no event horizons.
118518653
Constraining fast radio burst progenitors with gravitational lensing
Fast Radio Bursts (FRBs) are new transient radio sources discovered recently. Because of the angular resolution restriction in radio surveys, no optical counter part has been identified yet so it is hard to determine the progenitor of FRBs. In this paper we propose to use radio lensing survey to constrain FRB progenitors. We show that, different types of progenitors lead to different probabilities for a FRB to be gravitationally lensed by dark matter halos in foreground galaxies, since different type progenitors result in different redshift distributions of FRBs. For example, the redshift distribution of FRBs arising from double stars shifts toward lower redshift than of the FRBs arising from single stars, because double stars and single stars have different evolution timescales. With detailed calculations, we predict that the FRB sample size for producing one lensing event varies significantly for different FRB progenitor models. We argue that this fact can be used to distinguish different FRB models and also discuss the practical possibility of using lensing observation in radio surveys to constrain FRB progenitors.
11692409
Electromagnetic energy for a charged Kerr black hole in a uniform magnetic field
With the Komar mass formula we calculate the electromagnetic energy for a charged Kerr black hole in a uniform magnetic field. We find that the total electromagnetic energy takes the minimum when the Kerr black hole possesses a nonzero net charge $Q=2\ensuremath{\xi}{B}_{0}{J}_{H}$ where ${B}_{0}$ is the strength of the magnetic field, ${J}_{H}$ is the angular momentum of the black hole, and $\ensuremath{\xi}$ is a dimensionless parameter determined by the spin of the black hole. However, the Wald state with ${Q=2B}_{0}{J}_{H}$ does not minimize the electromagnetic energy.
10326293
New light on time machines: Against the chronology protection conjecture.
I construct a spacetime with closed timelike curves and without closed causal geodesics. There is a noncompactly generated Cauchy horizon separating the region with closed timelike curves from that without closed causal curves. This spacetime might be stable against the vacuum fluctuation of matter fields. Whether such a spacetime is stable against the vacuum fluctuation of a gravitational field is an open question.
39373169
Can the universe create itself
The question of first-cause has troubled philosophers and cosmologists alike. Now that it is apparent that our universe began in a Big Bang explosion, the question of what happened before the Big Bang arises. Inflation seems like a very promising answer, but as Borde and Vilenkin have shown, the inflationary state preceding the Big Bang must have had a beginning also. Ultimately, the difficult question seems to be how to make something out of nothing. This paper explores the idea that this is the wrong question --- that that is not how the Universe got here. Instead, we explore the idea of whether there is anything in the laws of physics that would prevent the Universe from creating itself. Because spacetimes can be curved and multiply connected, general relativity allows for the possibility of closed timelike curves (CTCs). Thus, tracing backwards in time through the original inflationary state we may eventually encounter a region of CTCs giving no first-cause. This region of CTCs, may well be over by now (being bounded toward the future by a Cauchy horizon). We illustrate that such models --- with CTCs --- are not necessarily inconsistent by demonstrating self-consistent vacuums for Misner space and a multiply connected de Sitter space in which the renormalized energy-momentum tensor does not diverge as one approaches the Cauchy horizon and solves Einstein's equations. We show such a Universe can be classically stable and self-consistent if and only if the potentials are retarded, giving a natural explanation of the arrow of time. Some specific scenarios (out of many possible ones) for this type of model are described. For example: an inflationary universe gives rise to baby universes, one of which turns out to be itself. Interestingly, the laws of physics may allow the Universe to be its own mother.
17105580
Inclusion of binaries in evolutionary population synthesis
Using evolutionary population synthesis we present integrated colours, integrated spectral energy distributions and absorption-line indices defined by the Lick Observatory image dissector scanner (referred to as the Lick/IDS) system, for an extensive set of instantaneous-burst binary stellar populations with and without binary interactions. The ages of the populations are in the range 1-15 Gyr and the metallicities are in the range 0.0001-0.03. By comparing the results for populations with and without binary interactions we show that the inclusion of binary interactions makes the integrated U-B, B-V, V-R and R-I colours and all Lick/IDS spectral absorption indices (except for H) substantially smaller. In other words, binary evolution makes a population appear bluer. This effect raises the derived age and metallicity of the population. We calculate several sets of additional solar-metallicity binary stellar populations to explore the influence of the binary evolution algorithm input parameters (the common-envelope ejection efficiency and the stellar wind mass-loss rate) on the resulting integrated colours. We also look at the dependence on the choice of distribution functions used to generate the initial binary population. The results show that variations in the choice of input model parameters and distributions can significantly affect the results. However, comparing the discrepancies that exist between the colours of various models, we find that the differences are less than those produced between the models with and those without binary interactions. Therefore it is very necessary to consider binary interactions in order to draw accurate conclusions from evolutionary population synthesis work.
122632442
In-medium kaon-nucleon vector interaction based on chiral effective Lagrangian
We investigate the implications of the four-nucleon terms in the chiral effective Lagrangian for the s-wave K-nucleon effective interaction. We nd that such terms play a nontrivial role in the description of symmetric nuclear matter. Our results for the kaon effective energy and in-medium K+- nucleon scattering are in agreement with the previous prediction obtained by in-medium scaling qualitatively.
118348723
Some aspects of QGP phase in a hQCD model
A bstractWe continue to study the holographic QCD (hQCD) model, proposed in a previous paper, in an Einstein-Maxwell-Dilaton (EMD) system. In this paper we discuss some aspects of quark gluon plasma (QGP) in the hQCD model, such as drag force, jet quenching parameter and screening length. The results turn out to be consistent with those as expected in QCD qualitatively. By calculating free energy of the background black hole solution, we find that there exists a phase transition between small black hole and large black hole when chemical potential μ is less than the critical one μc, and the phase transition is absent when chemical potential is beyond the critical one.
89612925
A COMPREHENSIVE STUDY OF GAMMA-RAY BURST OPTICAL EMISSION. III. BRIGHTNESS DISTRIBUTIONS AND LUMINOSITY FUNCTIONS OF OPTICAL AFTERGLOWS
We continue our systematic statistical study on optical afterglow data of gamma-ray bursts (GRBs). We present the apparent magnitude distributions of early optical afterglows at different epochs (t = 102 s, 103 s, and 1 hr) for the optical light curves of a sample of 93 GRBs (the global sample) and for sub-samples with an afterglow onset bump or a shallow decay segment. For the onset sample and shallow decay sample we also present the brightness distribution at the peak time t p and break time t b, respectively. All the distributions can be fit with Gaussian functions. We further perform Monte Carlo simulations to infer the luminosity function of GRB optical emission at the rest-frame time 103 s, t p, and t b. Our results show that a single power-law luminosity function is adequate to model the data with indices –1.40 ± 0.10, –1.06 ± 0.16, and –1.54 ± 0.22. Based on the derived rest-frame 103 s luminosity function, we generate the intrinsic distribution of the R-band apparent magnitude M R at the observed time 103 s post-trigger, which peaks at M R = 22.5 mag. The fraction of GRBs whose R-band magnitude is fainter than 22 mag and 25 mag and at the observer time 103 s are ~63% and ~25%, respectively. The detection probabilities of the optical afterglows with ground-based robotic telescopes and the UV-Optical Telescope on board Swift are roughly consistent with that inferred from this intrinsic M R distribution, indicating that the variations of the dark GRB fraction among the samples with different telescopes may be due to the observational selection effect, although the existence of an intrinsically dark GRB population cannot be ruled out.
54197110
A COMPREHENSIVE STUDY OF GAMMA-RAY BURST OPTICAL EMISSION. II. AFTERGLOW ONSET AND LATE RE-BRIGHTENING COMPONENTS
We continue our systematic statistical study of various components of gamma-ray burst (GRB) optical light curves. We decompose the early onset bump and the late re-brightening bump with empirical fits and analyze their statistical properties. Among the 146 GRBs that have well-sampled optical light curves, the onset and re-brightening bumps are observed in 38 and 26 GRBs, respectively. It is found that the typical rising and decaying slopes for both the onset and re-brightening bumps are similar to 1.5 and similar to-1.15, respectively. No early onset bumps in the X-ray band are detected to be associated with the optical onset bumps, while an X-ray re-brightening bump is detected for half of the re-brightening optical bumps. The peak luminosity is anti-correlated with the peak time L-p proportional to t(p)(-1.81 +/- 0.32) for the onset bumps and L-p proportional to t(p)(-0.83 +/- 0.17) for the re-brightening bumps. Both L-p and the isotropic energy release of the onset bumps are correlated with E-gamma,E- iso, whereas no similar correlation is found for the re-brightening bumps. These results suggest that the afterglow onset bumps are likely due to the deceleration of the GRB fireballs. Taking the onset bumps as probes for the properties of the fireballs and their ambient medium, we find that the typical power-law index of the relativistic electrons is 2.5 and the medium density profile behaves as n proportional to r(-1) within the framework of the synchrotron external shock models. With the medium density profile obtained from our analysis, we also confirm the correlation between the initial Lorentz factor (Gamma(0)) and E-iso,E-gamma in our previous work. The jet component that produces the re-brightening bump seems to be on-axis and independent of the prompt emission jet component. Its typical kinetic energy budget would be about one order of magnitude larger than the prompt emission component, but with a lower Gamma(0), typically several tens.