118842469
Strong gravitational lensing in a squashed Kaluza-Klein black hole spacetime
We investigate the strong gravitational lensing in a Kaluza-Klein black hole with squashed horizons. We find the size of the extra dimension imprints in the radius of the photon sphere, the deflection angle, the angular position, and magnification of the relativistic images. Supposing that the gravitational field of the supermassive central object of the Galaxy can be described by this metric, we estimated the numerical values of the coefficients and observables for gravitational lensing in the strong field limit.
118654249
Particle acceleration on the background of the Kerr―Taub―NUT spacetime
Abstract We study the collision of two particles with the different rest masses moving in the equatorial plane of a Kerr–Taub–NUT spacetime and get the center-of-mass (CM) energy for the particles. We find that the CM energy depends not only on the rotation parameter, a , but also on the NUT charge of the Kerr–Taub–NUT spacetime, n . Especially, for the extremal Kerr–Taub–NUT spacetime, an unlimited CM energy can be approached if the parameter a is in the range [ 1 , 2 ] , which is different from that of the Kerr and Kerr–Newman black holes.
121869215
Dirac quasinormal modes of the Garfinkle-Horowitz-Strominger dilaton black-hole spacetime
Adopting the P?schl?Teller potential approximation, the quasinormal frequencies of the massless Dirac fields are evaluated in the Garfinkle?Horowitz?Strominger dilaton black-hole spacetime. We find that the real parts of the quasinormal frequencies increase as the orbital angular momentum k or a increases, where a is a parameter related to the dilaton field. The magnitude of imaginary parts decreases with k for the fixed a and overtone number n, while for fixed k and n, it is not a monotonic function of a and has a maximum at a point near a = 1.4?1.6, where the waves decay most rapidly. Another interesting feature is that the imaginary parts tend to zero and the real parts to k/2 as a approaches 2. We also find that the variation of the imaginary parts with the real parts in the Garfinkle?Horowitz?Strominger dilaton hole spacetimes is nonlinear, which becomes more complex than that in the Reissner?Nordstr?m black-hole spacetime.
119285485
$P-V$ criticality of AdS black hole in $f(R)$ gravity
We study thermodynamics of a charged AdS black hole in the special $f(R)$ correction with the constant Ricci scalar curvature. Our results show that the the $f(R)$ correction influences the Gibbs free energy and the phase transition of system. The ratio $\rho_c$ occurred at the critical point increases monotonically with the derivative term $f'(R_0)$. We also disclose that the critical exponents are the same as those of the liquid-gas phase transition in the Van der Waals model, which does not depend on the $f(R)$ correction considered here.
119127372
Rotating charged black hole with Weyl corrections
We present firstly a four-dimensional spherical symmetric black hole with small Weyl corrections and find that with increasing Weyl corrections the region of the event horizon existence for the black hole in the parameter space increases for the negative Weyl coupling parameter and decreases for the positive one. Moreover, we also obtain a rotating charged black hole with weak Weyl corrections by the method of complex coordinate transformation. Our results show that the sign of Weyl coupling parameter $\alpha$ yields the different spatial topology of the event horizons for the black hole with its parameters lied in some special regions in the parameter space. We also analyze the dependence of the ergosphere on the Weyl coupling parameter $\alpha$ and find that with the increase of the Weyl corrections the ergosphere in the equatorial plane becomes thick for a black hole with $\alpha>0$, but becomes thin in the case with $\alpha<0$, which means that the energy extraction become easier in the background of a black hole with the positive Weyl coupling parameter, but more difficult in the background of a black hole with the negative one.
118567951
Phantom scalar emission in the Kerr black hole spacetime
We study the absorption probability and Hawking radiation spectra of a phantom scalar field in the Kerr black hole spacetime. We find that the presence of negative kinetic energy terms modifies the standard results in the graybody factor, super-radiance and Hawking radiation. Comparing with the usual scalar particle, the phantom scalar emission is enhanced in the black hole spacetime.
118376307
Search for GeV Gamma‐Ray Bursts with the ARGO‐YBJ Detector in Scaler Mode
The ARGO‐YBJ experiment is a full coverage array made of Resistive Plate Chambers (RPCs) operated in streamer mode to detect the extensive array showers (EASs). With large field of view (>2sr) and high duty cycle (>90%), the ARGO‐YBJ detector is the right ground‐based apparatus to detect VHE GRBs, while the high altitude (4300 m a.s.l) and scaler working mode enable it to detect GRBs in the GeV region as high energy follow‐up observation of satellites such as SWIFT. If a GRB signal is detected, the four scaler channels, which correspond to different energy thresholds, can provide further information on the energy spectrum and its cut‐off. More than 40 SWIFT GRBs were in ARGO‐YBJ field of view, no significant excess was detected till now.
11090952
Cosmic ray threshold anomaly and kinematics in the dS spacetime
We present a covariant framework of kinematics in the dS spacetime, which is a natural postulate of recent astronomical observations ($\Lambda>0)$. One-particle states are presented explicitly. It is noticed that the dispersion relation of free particles is dependent on the degrees of freedom of angular momentum and spin. This fact can be referred to as the effects of the cosmological constant on kinematics of particles. The kinematics in dS spacetime is used to investigate the phenomenon of ultra high energy cosmic rays. We emphasize the possibility of solving the threshold anomalies of the interactions between ultra high energy cosmic rays and soft photons in the covariant framework of kinematics in the dS spacetime.
59928370
Matter bounce cosmology with the f(T) gravity
We show that the f(T) gravitational paradigm, in which gravity is described by an arbitrary function of the torsion scalar, can provide a mechanism for realizing bouncing cosmologies, thereby avoiding the Big Bang singularity. After constructing the simplest version of an f(T) matter bounce, we investigate the scalar and tensor modes of cosmological perturbations. Our results show that metric perturbations in the scalar sector lead to a background-dependent sound speed, which is a distinguishable feature from Einstein gravity. Additionally, we obtain a scale-invariant primordial power spectrum, which is consistent with cosmological observations, but suffers from the problem of a large tensor-to-scalar ratio. However, this can be avoided by introducing extra fields, such as a matter bounce curvaton.Communicated by P R L V Moniz
124712220
Injection locking of a long-pulse relativistic magnetron
The authors report the injection locking results of a long-pulse (200-400 ns) high power (20-30 MW) relativistic magnetron at 3.3 GHz. Phase-locking with reproducible locked angle was achieved with an injection power ratio as low as 1:200. The locked states were phase stable to within +or-30 degrees during the pulse. Phase locking physics was studied and important effects due to frequency pushing were identified. The locking bandwidth and the dependence of the final locked phase on injection parameters were measured and found to agree well with theory. >
110035692
DESIGN OF LOW IMPEDANCE ELECTROSTATIC SEPARATORS FOR BEPC II
The multi bunch train collision is adopted in BEPC II. There are 4 horizontal electrostatic separators in the main ring to generate close-orbit displacement as "pretzels" which allow the electrons and positrons to be stored in the same vacuum chamber simultaneously without unwanted destructive collisions. In the separator the beam current (576mA) and the gap voltage (220 kV) are both very high. So there are five key problems that must be handled well: To reduce the beam impedance and loss factor; To absorb the high order modes (HOMs); To reduce the leakage of RF EM field; To extract the heats due to the HOMs losses and the SR power; To reduce the spark rate. Their analysis and the solutions are presented in this paper.