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.
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.
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).
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.
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]:
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.
109204818
Performance Limiting Effects in X‐Band Accelerators
Acceleration gradient is a critical parameter for the design of future TeV‐scale linear colliders. The major obstacle to higher gradient in room‐temperature accelerators is rf breakdown, which is still a very mysterious phenomenon that depends on the geometry and material of the accelerator as well as the input power and operating frequency. Pulsed heating has been associated with breakdown for many years however there have been no experiments that clearly separate field and heating effects on the breakdown rate. Recently, such experiments have been performed at SLAC with both standing‐wave and travelling‐wave structures. These experiments have demonstrated that pulsed heating is limiting the gradient. Also, a dual‐moded cavity has been designed to better distinguish the electric field, magnetic field and pulsed heating effects on breakdown.
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.
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.