124365356
Current constraints on early dark energy and growth index using latest observations
Aims. In this paper we study observational constraints on early dark energy model proposed by Doran & Robbers and growth index using the latest Union2 type Ia supernovae (SNe Ia), the baryon acoustic oscillations (BAO) in the large-scale correlation function of the Sloan Digital Sky Survey and Two-degree Field Galaxy Redshift Survey, cosmic microwave background (CMB) from the Wilkinson Microwave Anisotropy Probe seven-year data, the linear growth factors data and gamma-ray bursts (GRBs). Methods. By using the χ 2 statics method, we constrain the early dark energy model and growth factor from the above datasets. When including the GRB data, we use the cosmographic parameters to calibrate the GRB luminosity relations using SNe Ia. Results. At 2σ confidence level, we find that the fractional dark energy density at early times is Ω e < 0.05 using SNe Ia, CMB and BAO. When we include high-redshift probes, such as measurements of the linear growth factors and GRBs, the constraint is tightened considerably and becomes Ω e < 0.03 at 2σ confidence level. We also discuss the growth rate index γ .W e f indγ = 0.661 +0.302 −0.203 (1σ) using the SNe Ia, CMB, BAO and linear growth factor data. After including high-redshift GRB data, the growth index is γ = 0.653 +0.372 −0.363 .
121474555
Constraints on the Cardassian expansion models from the latest observational data
We study observational constraints on Cardassian expansion models that account for the accelerated expansion of the universe via modification of the Friedmann equation. We find significant constraints on the model parameters using the latest type Ia supernovae, the baryon acoustic peak in the large scale correlation function of the Sloan Digital Sky Survey of luminous red galaxies, the cosmic microwave background radiation shift parameter, the x-ray gas mass fractions in clusters of galaxies and the observational H(z) data. In the original Cardassian model, we find Ωm = 0.27 ± 0.03, n = 0.01 ± 0.09 at the 1σ confidence level. This result is consistent with the flat ΛCDM cosmology. In the modified polytropic Cardassian model, we obtain β = 0.27−0.25+3.65 and n = −1.86−4.20+2.26(1σ). We also find that the acceleration happens for zT = 0.70 ± 0.05(1σ) in the original Cardassian model. In the modified polytropic Cardassian model, the transition redshift is zT = 0.58−0.18+0.17(1σ).
119245048
An Evolving Stellar Initial Mass Function and the Gamma-ray Burst Redshift Distribution
Recent studies suggest that Swift gamma-ray bursts (GRBs) may not trace an ordinary star formation history (SFH). Here, we show that the GRB rate turns out to be consistent with the SFH with an evolving stellar initial mass function (IMF). We first show that the latest Swift sample of GRBs reveals an increasing evolution in the GRB rate relative to the ordinary star formation rate at high redshifts. We then assume only massive stars with masses greater than the critical value to produce GRBs and use an evolving stellar IMF suggested by Dave to fit the latest GRB redshift distribution. This evolving IMF would increase the relative number of massive stars, which could lead to more GRB explosions at high redshifts. We find that the evolving IMF can well reproduce the observed redshift distribution of Swift GRBs.
44994799
Internal shock model for the X-ray flares of Swift J1644+57
Swift J1644+57 is an unusual transient event, likely powered by the tidal disruption of a star by a massive black hole. Multiple short time-scale X-ray flares were seen over a span of several days. We propose that these flares could be produced by internal shocks. In the internal shock model, the forward and reverse shocks are produced by collisions between relativistic shells ejected from a central engine. The synchrotron emission from the forward and reverse shocks could dominate in two quite different energy bands; under some conditions, the relativistic reverse shock dominates the X-ray emission and the Newtonian forward shock dominates the infrared and optical emission. We show that the spectral energy distribution of Swift J1644+57 could be explained by an internal shock model.
14538301
Measuring dark energy with gamma-ray bursts and other cosmological probes
It has been widely shown that cosmological parameters and dark energy can be constrained by using data from Type Ia supernovae (SNe Ia), the cosmic microwave background (CMB) anisotropy, the baryon acoustic oscillation (BAO) peak from Sloan Digital Sky Survey (SDSS), the X-ray gas mass fraction in clusters, and the linear growth rate of perturbations at z = 0.15 as obtained from the 2dF Galaxy Redshift Survey. Recently, gamma-ray bursts (GRBs) have also been argued to be promising standard candles for cosmography. In this paper, we present constraints on the cosmological parameters and dark energy by combining a recent GRB sample including 69 events with the other cosmological probes. First, we find that for the ΛCDM cosmology this combination makes the constraints stringent, and the best fit is close to the flat universe. Second, we fit the flat Cardassian expansion model and find that this model is consistent with the ΛCDM cosmology. Third, we present constraints on several two-parameter dark energy models and find that these models are also consistent with the ΛCDM cosmology. Finally, we reconstruct the dark energy equation of state parameter w(z) and the deceleration parameter q(z). We see that the acceleration could have started at a redshift from zt = 0.40 to 0.65. This difference in the transition redshift is due to different dark energy models that we adopt. The most stringent constraint on w(z) lies in the redshift range z ~ 0.3-0.6.
118854953
High-redshift star formation rate up to z~8.3 derived from gamma-ray bursts and influence of background cosmology
The high-redshift star formation rate (SFR) is difficult to measure directly even by modern approaches. Long-duration gamma-ray bursts (GRBs) can be detected to the edge of the visible universe because of their high luminosities. The collapsar model of long GRBs indicates that they may trace the star formation history. So, long GRBs may be a useful tool of measuring the high-redshift SFR. Observations show that long GRBs prefer to form in a low-metallicity environment. We study the high-redshift SFR up to z∼ 8.3 considering the Swift GRBs tracing the star formation history and the cosmic metallicity evolution in different background cosmological models including Λ cold dark matter (ΛCDM), quintessence, quintessence with a time-varying equation of state and brane-world model. We use latest Swift GRBs including two highest-z GRBs, GRB 080913 at z= 6.7 and GRB 090423 at z= 8.3. We find that the SFR at z > 4 shows a steep decay with a slope of ∼−5.0 in ΛCDM. In the other three models, the high-redshift SFR is slightly different from ΛCDM model and also shows a steep decay.
124800347
U(1/1)q q-Coherent States and Path Integrals for the q-Deformed Jaynes-Cummings Model*
The q-coherent states for the quantum superalgebra U(1/1)q, are introduced. Path integrals in the q-coherent state representations of the U(1/1)q, are constructed and applied to the q-deformed Jaynes-Cummings model whose dynamical superalgebra is the quantum super-algebra U(1/1)q.
119254658
Deviation from the Cosmological Constant or Systematic Errors
In the work of Qi, Wang & Lu, gamma-ray bursts (GRBs) are used together with an earlier Type Ia supernova (SN Ia) data set to constrain the dark energy equation of state (EOS) in a nearly model-independent way. The improvements made by including GRBs show a slight shift of the dark energy EOS toward w > -1 at redshifts z greater than or similar to 0.5. It is interesting that, when we have more SNe Ia, SNe Ia themselves also show the same trend. Motivated by the fact that both SNe Ia and GRBs seem to prefer a dark energy EOS greater than -1 at redshifts z greater than or similar to 0.5, we perform a careful investigation of this situation, including more careful treatments of measurement errors of GRBs and cross-checking the results by using different ways of including GRBs. We find that the deviation of dark energy from the cosmological constant at redshifts z greater than or similar to 0.5 is large enough that we should pay close attention to it with future observational data. Such a deviation may arise from some biasing systematic errors in the handling of SNe Ia and/or GRBs, or more interestingly from the nature of the dark energy itself.
14421267
Constraining the evolution of dark energy with type Ia supernovae and gamma-ray bursts
Aims. The behavior of the dark energy equation of state (EOS) is crucial in distinguishing different cosmological models. With a model independent approach, we constrain the possible evolution of the dark energy EOS. Methods. Gamma-ray bursts (GRBs) of redshifts up to z > 6 are used, in addition to type Ia supernovae (SNe Ia). We separate the redshifts into 4 bins and assume a constant EOS parameter for dark energy in each bin. The EOS parameters are decorrelated by diagonalizing the covariance matrix. And the evolution of dark energy is estimated out of the uncorrelated EOS parameters. Results. By including GRB luminosity data, we significantly reduce the confidence interval of the uncorrelated EOS parameter whose contribution mostly comes from the redshift bin of 0.5 < z < 1.8. At high redshift where we only have GRBs, the constraints on the dark energy EOS are still very weak. However, we can see an obvious cut at about zero in the probability plot of the EOS parameter, from which we can infer that the ratio of dark energy to matter most probably continues to decrease beyond redshift 1.8. We carried out analyses with and without including the latest BAO measurements, which themselves favor a dark energy EOS of w< −1. If they are included, the results show some evidence of an evolving dark energy EOS. If not included, however, the results are consistent with the cosmological constant within 1σ for redshift 0 < z 0. 5a nd 2σ for 0.5 z < 1.8.
93891215
Gamma-ray spectra of methane in the positron–electron annihilationprocess
Abstract Bound electron contribution to the Doppler-shift of gamma-ray spectra in the positron–electron annihilation process of molecular methane has been studied in gas phase. Two accurate ab initio quantum mechanical schemes, i.e. the delocalised molecular orbital (MO) and the localised natural bond orbital (NBO) schemes, are applied to study the multi-centred methane molecule. The present ab initio calculations of methane indicate that the C–H bonds are polarised with the partial negative charge of −0.36 a.u on the carbon atom and the partial positive charge of +0.09 a.u. on each of the hydrogen atoms. The positively charged hydrogen atoms produce repulsive Coulomb potentials to a positron. Both the MO and NBO schemes further reveal that the 2a 1 electrons of methane, that is, the 2a 1 electron component of the CH bonds rather than the whole CH bonds of methane, predominates the positron–electron annihilation gamma-ray spectra of the molecule. Electrons of a molecule which are dominant the positron–electron annihilation processes are called “positrophilic” electrons in the present study. It is further shown that the negative electrostatic potential (ESP) of methane facilitates with the density of the “positrophilic” 2a 1 electrons of methane. Other valence electrons (e.g. 1t 2 ) in the CH bonds play a minor “spectator” role in the annihilation process of methane.
36422466
Comment on "generating high-current monoenergetic proton beams by a circularly polarized laser pulse in the phase-stable acceleration regime".
In a recent Letter X.Q. Yan et al. [1] propose a method named phase-stable acceleration to accelerate protons or ions via a circularly polarized laser pulse interacting with an ultrathin target. The key point of this method is that the target should be as thin as possible so that the foil can be more easily accelerated due to the small mass. Based on this, an expression is obtained as follows [1,2]: