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.
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.
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).
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.
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.
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.
185692670
Study on the High Order Modes of the 3.5cell Cavity at Peking University
As part of the updated DC-SC injector, a 3.5cell cavity has been fabricated at Peking University, which includes two Coaxial High Order Mode (HOM) couplers. The effect of the HOM couplers has been studied by numerical simulation and measurement. The results are highly uniform and show that the two couplers do effectively damp the HOMs. INTRODUCTION The 3.5cell cavity is part of the DC-SC injector, the feasibility of which is proved in 2004 [1] with beam done test. The updated 3.5cell cavity was designed and fabricated from large grain niobium, and the vertical test result (Eacc=23.4MV/m@Q0>10) at JLAB is exciting. The main RF parameters are listed in Table1. Table 1: RF Parameters of 3.5 Cell Cavity Type of accelerating structure Standing wave Accelerating mode TM010, pi mode fundamental frequency 1300MHz Quality factor ≥5x10 Active length 41.7cm Cell-to-cell coupling 1.8% Effective shunt impedance 418Ω Transit-time factor 0.74 Total longitudinal loss factor* 3.95(V/pC) transverse loss factor* 2.15(V/pC/cm) *the standard deviation length of the bunch is 2mm Among those RF parameters, the total longitudinal loss factor and transverse loss factor are from ABCI simulation; all the others are from Superfish. NUMERICAL CALCULATIONS OF HOMS The electron bunches passing through the cavity excite the eigenmodes of higher frequency which are HOMs. Here Microwave Studio is used to simulation the HOMs. To check whether there are trapped modes, two different boundary conditions at the end of beam pipes are used in two separate runs [2]. Large frequency shift of one mode indicates the mode depends on the boundary condition sensitivity; otherwise the mode could be trapped. The simulation shows TM110(2) and TM020(1)could be trapped. But they will not be excited by the beam as the beam bunch repetition frequency is 81.25MHz. All of the interested modes with ideal electric boundary condition below 3GHz are listed in Table 2. Table 2: HOMs of 3.5 Cell Cavity by Simulation Mode Mode f(MHz) *R/Q Mode1 TM010(1) 1278.69 9.75 Mode2 TM010(2) 1286.80 13.09 Mode3 TM010(3) 1296.83 16.63 Mode4 TM010(4) 1301.07 417.26 Mode5 TE111(1) 1634.35 0.07 Mode6 TE111(2) 1681.53 2.49 Mode7 TE111(3) 1747.03 10.07 Mode8 TM110(1) 1798.05 3.43 Mode9 TM110(2) 1849.61 3.47 Mode10 TM110(3) 1872.20 1.42 Mode11 TM110(4) 1883.84 0.77 Mode12 TM011(1) 2391.49 0.62 Mode13 TE111 (4) 2391.79 0.18 Mode14 TM011(2) 2421.67 12.65 Mode15 TM011(3) 2449.83 84.33 Mode16 TM020(1) 2658.30 23.97 Mode17 TM020(2) 2690.74 8.93 Mode18 TM020(3) 2728.82 4.26 Mode19 TM020(4) 2762.00 2.08 Mode20 TM111(1) 2819.80 0.71 Mode21 TM111(2) 2880.96 0.85 *unit of R/Q isΩ/cm; for monopole mode, n=0; dipole mode,n=2. The number next to the mode indicates the passband of the mode. For some modes, the electric field along the beam centre axis is almost zero, so the voltage is zero. Here we integrate the electric field along the axis 10 mm away from the beam centre and get the accelerating THPD009 Proceedings of IPAC’10, Kyoto, Japan 4296 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators voltage Vc , and the effective shunt impedance is ( ) ( ) U V Q R C ω / * / 2 * 0 = , where ω is the angular frequency of the mode, U is the stored energy. The effects of HOM couplers are simulated on a 2cell cavity also by CST Microwave Studio, which deduces the simulation time obviously [3]. The simulation shows Qe of HOM coupler on 2-cell TESLA cavity is 10 for fundamental mode, and 10~10 for HOMs. MEASUREMENTS OF THE RESONATE MODES In order to confirm the type of the modes, the electric field profile of the modes is measured by bead pulling. Two beads are used, one is a teflon cylinder with diameter of 4 mm and length of 4mm, the other is a copper cylinder with diameter of 5mm and length of 4mm. By comparing the measured electric fields with the simulated ones, 16 of all the 21 interested modes are obtained. The block diagram of the test bench is shown is Figure 1. Figure 1: Block diagram of the test bench. RF MEASUREMENTS OF THE HOM DAMPING ON THE CAVITY Before the HOM couplers are welded to the cavity, they are tested on a self-designed coaxial transmission line [4], to ensure the tuning range of the resonate frequency. The antenna of the HOM coupler is match to the load and with a round tip to increase the coupling as shown in Figure 2. Figure 2: The shape of the antenna. The antenna is made of copper in a series of length. With these antennas amounted, the gap from the antenna to the HOM coupler is 0.5mm, 1mm, 1.5mm, 2mm, 2.3mm. During this change, the Qext does not change much. Finally 1mm is chosen and the result is shown in Table 3. Table 3: Qe of the HOMs by Measurements mode f (MHz) Qe TE111(1) 1652.406 9.55x10 TE111(2) 1693.484 3.82x10 TE111(3) 1757.300 peak overlaps TM110(1) 1797.265 1.66x10 6 TM110(2) 1864.493 1.89x10 7 TM110(3) 1876.990 1.06x10 7 TM110(4) 1882.496 3.45x10 6 TE111(4) 2374.582 4.69x10 6 TM011(1) 2412.774 7.46x10 5 TM011(2) 2429.367 2.27x10 5 TM011(3) 2462.537 7.26x10 5 TM020(1) 2678.580 1.57x10 5 TM020(2) 2703.662 1.60x10 4 TM020(3) 2732.066 1.50x10 4 TM020(4) 2769.828 8.93x10 4 During the measurements, one HOM coupler is used because the available vector network analyzer has only two ports. For TE111(3) pass band, the two polarization modes are too close to measure directly by the vector network analyzer. Table 3 only lays out one polarization mode of each passband, the Qe of the other one is much bigger. Obviously one coupler is not enough for all the modes. CONCLUSION The test results of the coaxial HOM coupler on the 3.5cell large grain niobium cavity at room temperature indicate that the coupler works efficiently after a careful adjustment. The effects at 2K with beam loading are looking forward. REFERENCES [1] J. Hao et al., NIM A 557 (2006) 138-141. [2] F. Marhauser, P. Huelsmann, H. Klein, Proceedings of the 1999 Particle Accelerator Conference, New York, 1999. 3405-3407. [3] X. Luo, CPC (HEP & NP), to be published. [4] E. Wang, CPC (HEP & NP), 2009, 33(5): 401-405 Proceedings of IPAC’10, Kyoto, Japan THPD009 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators 4297
55717747
The drive laser system for dc-sc injector
PKU-SCAF has developed a photoinjector which adopt a 1+1/2 cell super conducting cavity and DC electron gun. We also developed a low cost drive laser system for the photocathode DC gun to provide high average beam current. This laser system include a commercial high repetition rate, ps, all solid-state laser, the home made SHG and FHG, Fourier relay optics and the uniform illumination optics. The test results shows the output power at 266 nm of the laser system is more than 1.2W and got more than 500 µA beam current from CsTe cathode from the DC gun.
120045862
Inadequacy of the Coupled-Channels Born Approximation for Explaining Anomalous Rare-Earth (d,t) Transitions
119235128
Gamma-ray burst contributions to constraining the evolution of dark energy
Aims. We explore the gamma-ray bursts' (GRBs') contributions in constraining the dark energy equation of state (EOS) at high (1.8 < z < 7) and at middle redshifts (0.5 < z < 1.8) and estimate how many GRBs are needed to get substantial constraints at high redshifts.
118676242
Gravitational effects of condensate dark matter on compact stellar objects
We study the gravitational effect of non-self-annihilating dark matter on compact stellar objects. The self-interaction of condensate dark matter can give high accretion rate of dark matter onto stars. Phase transition to condensation state takes place when the dark matter density exceeds the critical value. A compact degenerate dark matter core is developed and alter the structure and stability of the stellar objects. Condensate dark matter admixed neutron stars is studied through the two-fluid TOV equation. The existence of condensate dark matter deforms the mass-radius relation of neutron stars and lower their maximum baryonic masses and radii. The possible effects on the Gamma-ray Burst rate in high redshift are discussed.