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).
11226958
Constraining the cosmological parameters and transition redshift with gamma-ray bursts and supernovae
A new method of measuring cosmology with gamma-ray bursts (GRBs) has been proposed by Liang and Zhang recently. In this method, only observable quantities including the rest-frame peak energy of the νF ν spectrum (E' p ), the isotropic energy of GRB (E γ,iso ) and the break time of the optical afterglow light curves in the rest frame (t' b ) are used. By considering this method, we constrain the cosmological parameters and the redshift at which the Universe expanded from the deceleration to acceleration phase. We add five recently detected GRBs to the sample and derive E γ,iso /10 52 erg = (0.93 ± 0.25) (E' p /100 keV) 1.91±0.32 (t'b/1d) -0.93±0.38 for a flat universe with Ω M = 0.28 and H 0 = 71.0km s -1 Mpc -1 . This relation is independent of the medium density around bursts and the efficiency of conversion of the explosion energy to gamma-ray energy. We consider the E γ,iso (E' p , t' b ) relationship as a standard candle and find 0.05 < Ω M < 0.48 and Ω Λ < 1.15 (at the la confidence level). In a flat universe with the cosmological constant, we obtain 0.25 < Ω M < 0.46 and 0.54 < Ω Λ < 0.78 at the la confidence level. The transition redshift is z T = 0.69 +0.11 -0.12 . Combining 20 GRBs with 157 Type Ia supernovae (SNe Ia), we find Ω M = 0.29 ± 0.03 for a flat universe and the transition redshift is z T = 0.61 +0.06 -0.05 , which is slightly larger than the value found by considering SNe Ia alone. In particular, we also discuss several dark-energy models in which the equation of state w(z) is parametrized, and investigate constraints on the cosmological parameters in detail.
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.
18159661
Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae
The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernovae (SNe Ia) and anisotropy of cosmic microwave background (CMB). The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``Gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure Ω M = 0.28 −0.05 +0.04 , which is consistent with Riess et al. The transition redshift is z T = 0.60 −0.08 +0.06 . We also discuss several dark energy models that define w ( z ) of the parameterized equation of state of dark energy including one parameter and two parameters ( w ( z ) being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from z T = 0.29 −0.06 +0.07 to z T = 0.60 −0.08 +0.06 across these models. We also calculate the minimum redshift z c at which the current observations need the universe to accelerate.
73619195
Probing the cosmographic parameters to distinguish between dark energy and modified gravity models
Aims. In this paper we investigate the deceleration, jerk and snap parameters to distinguish between the dark energy and modified gravity models using high redshift gamma-ray bursts (GRBs) and supernovae (SNe). Methods. We first derive the expressions of deceleration, jerk and snap parameters in dark energy and modified gravity models. In order to constrain the cosmographic parameters, we calibrate the GRB luminosity relations without assuming any cosmological models using SNe Ia. Then we constrain the model parameters (including dark energy and modified gravity models) using type Ia supernovae and gamma-ray bursts. Finally we calculate the cosmographic parameters. GRBs can extend the redshift-distance relation up to high redshifts, because they can be detected to high redshifts. Results. We find that the statefinder pair (r, s) could not be used to distinguish between some dark energy and modified gravity models, but these models could be differentiated by the snap parameter. Using the model-independent constraints on cosmographic parameters, we conclude that the ΛCDM model is consistent with current data.
110770779
Planned waveguide electric field breakdown studies
This paper presents an experimental setup for X-band rf breakdown studies. The setup is composed of a section of WR90 waveguide with a tapered pin located at the middle of the waveguide E-plane. Another pin is used to rf match the waveguide so it operates in a travelling wave mode. By adjusting the penetration depth of the tapered pin, different surface electric field enhancements can be obtained. The setup will be used to study the rf breakdown rate dependence on power flow in the waveguide for a constant maximum surface electric field on the pin. Two groups of pins have been designed. The Q of one group is different and very low. The other has a similar Q. With the test of the two groups of pins, we should be able to discern how the net power flow and Q affect the breakdown. Furthermore, we will apply an electron beam treatment to the pins to study its effect on breakdown. Overall, these experiments should be very helpful in understanding rf breakdown phenomena and could significantly benefit the design of high gradient accelerator structures.
119293060
Gamma-ray spectra of hexane in gas phase and liquid phase
Theoretical gamma-ray spectra of molecule hexane have been calculated and compared with the experimental results in both gas (Surko et al, 1997) and liquid (Kerr et al, 1965) phases. The present study reveals that in gas phase not all valence electrons of hexane exhibit the same probability to annihilate a positron. Only the positrophilic electrons in the valence space dominate the gamma-ray spectra, which are in good agreement with the gas phase measurement. When hexane is confined in liquid phase, however, the intermolecular interactions ultimately eliminate the free molecular orientation and selectivity for the positrophilic electrons in the gas phase. As a result, the gamma-ray spectra of hexane become an averaged contribution from all valence electrons, which is again in agreement with liquid phase measurement. The roles of the positrophilic electrons in annihilation process for gas and liquid phases of hexane have been recognized for the first time in the present study.
17589853
AMiBA Observations, Data Analysis and Results for Sunyaev-Zel'dovich Effects
We present observations, analysis and results for the first-year operation of AMiBA, an interferometric experiment designed to study cosmology via the measurement of Cosmic Microwave Background (CMB). AMiBA is the first CMB interferometer operating at 3 mm to have reported successful results, currently with seven close-packed antennas of 60-cm diameter giving a synthesized resolution of around 6'. During 2007 AMiBA detected the Sunyaev-Zel'dovich effects (SZE) of six galaxy clusters at redshift 0.091 <= z <= 0.322. An observing strategy with on-off-source switching is used to minimize the effects from electronic offset and ground pickup. Planets were used to test the observational capability of AMiBA and to calibrate the conversion from correlator time-lag data to visibilities. The detailed formalism for data analysis is given. We summarize our early tests including observations of planets and quasars, and present images, visibility profiles, the estimated central coordinates, sizes, and SZE amplitudes of the galaxy clusters. Science results are summarized. We also discuss possible systematic effects in the results.
122814293
Constraints on generalized Chaplygin gas model including gamma-ray bursts
Generalized Chaplygin gas (whose equation of state is p GCG = − A/ρ α G CG ) was proposed as a candidate for uni fi cation of dark energy and dark matter. We investigate constraints on this model with the latest observed data. We test the model with type-Ia supernovae (SNe Ia), cosmic microwave background (CMB) anisotropy, X-ray gas mass fractions in clusters, and gamma-ray bursts (GRBs). We calibrate the GRB luminosity relations without assuming any cosmological models using SNe Ia. We show that GRBs can extend the Hubble diagram to higher redshifts ( z > 6 ). The GRB Hubble diagram is well behaved and delineates the shape of the Hubble diagram well. We measure A s ≡ A /ρ α +1 GCG , 0 = 0 . 68 +0 . 04 − 0 . 08 (where ρ GCG , 0 is the energy density today) and α = − 0 . 22 +0 . 15 − 0 . 13 at the 1 σ con fi dence level using all the datasets. Our results rule out the standard Chaplygin gas model ( α = 1 ) at the 3 σ con fi dence level. The Λ CDM is allowed at the 2 σ con fi dence level. We fi nd that acceleration could have started at a redshift of z ∼ 0 . 70 . The concordance of the generalized Chaplygin gas model with the age estimate of an old high redshift quasar is found. In addition, we show that GRBs can break the degeneracy between the generalized Chaplygin gas model and the XCDM model.
124213992
High performance computation on beam dynamics problems in high intensity compact cyclotrons
This paper presents the research progress in the beam dynamics problems for future high intensity compact cyclotrons by utilizing the state-of-the-art high performance computation technology. A “Start-to-Stop” model, which includes both the interaction of the internal particles of a single bunch and the mutual interaction of neighboring multiple bunches in the radial direction, is established for compact cyclotrons with multi-turn extraction. This model is then implemented in OPAL-CYCL, which is a 3D object-oriented parallel code for large scale particle simulations in cyclotrons. In addition, to meet the running requirement of parallel computation, we have constructed a small scale HPC cluster system and tested its performance. Finally, the high intensity beam dynamics problems in the 100 MeV compact cyclotron, which is being constructed at CIAE, are studied using this code and some conclusions are drawn.