118397522
Gravitational field of a slowly rotating black hole with a phantom global monopole
We present a slowly rotating black hole solution with a phantom global monopole by solving Einstein's field equation and find that the presence of the global monopole changes the black hole structure. The metric coefficient gtϕ contains the hypergeometric function of the polar coordinate r, which is more complex than that in the usual slowly rotating black hole. The energy scale of symmetry breaking η affects the black hole horizon and the deficit solid angle. In particular, the solid angle is surplus rather than deficit for the black hole with the phantom global monopole. We also study the effects of the global monopole on the angular velocity of the horizon ΩH, Kepler's third law, the innermost stable circular orbit and the radiative efficiency ϵ in the thin accretion disc model. Our results also show that for a phantom black hole, the radiative efficiency ϵ is positive only in the case η ⩽ ηc. The threshold value ηc increases with the rotation parameter a.
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.
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.
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.
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.
14464884
Detector time offset and off-line calibration in EAS experiments
In extensive air shower (EAS) experiments, the primary direction is reconstructed by the space–time pattern of secondary particles. Thus the equalization of the transit time of signals coming from different parts of the detector is crucial in order to get the best angular resolution and pointing accuracy allowed by the detector. In this paper an off-line calibration method is proposed and studied by means of proper simulations. It allows to calibrate the array repeatedly just using the collected data without disturbing the standard acquisition. The calibration method is based on the definition of a Characteristic Plane introduced to analyze the effects of the time systematic offsets, such as the quasi-sinusoidal modulation on azimuth angle distribution. This calibration procedure works also when a pre-modulation on the primary azimuthal distribution is present.
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
15997425
A Multiwavelength Study of 1WGA J1346.5-6255: A New γ Cas Analog Unrelated to the Background Supernova Remnant G309.2-00.6
WGA J1346.5-6255 is a ROSAT X-ray source found within the radio lobes of the supernova remnant (SNR) G309.2-00.6. This source also appears to coincide with the bright and early-type star HD 119682, which is in the middle of the galactic open cluster NGC 5281. The radio morphology of the remnant, consisting of two brightened and distorted arcs of emission on opposite sides of the 1WGA J1346.5-6255 source and of a jet-like feature and break in the shell, led to the suggestion that 1WGA J1346.5-6255/G309.2-00.6 is a young analog of the microquasar SS 433 powering the W50 nebula. This motivated us to study this source at X-ray and optical wavelengths. We here present new Chandra observations of 1WGA J1346.5-6255, archival XMM- Newton observations of G309.2-00.6, and optical spectroscopic observations of HD 119682, in order to search for X-ray jets from 1WGA J1346.5-6255, study its association with the SNR, and test for whether HD 119682 represents its optical counterpart. We do not nd evidence for jets from 1WGA J1346.5-6255 down to an unabsorbed ux of 2.6 10 -13 ergs cm -2 s (0.5n7.5 keV), we rule out its association with G309.2-00.6, and we conrm that HD 119682 is its optical counterpart. We derive a distance of 1.2 0.3 kpc, which is consistent with the distance estimate to NGC 5281 (1.3 0.3 kpc), and much smaller than the distance derived to the SNR G309.2-00.6. We discuss the nature of the source, unveil that HD 119682 is a Be star and suggest it is a new member of the recently proposed group of -Cas analogs. The Chandra and XMM-Newton X-ray lightcurves show variability on timescales of hundreds of seconds, and the presence of a possible period of 1500 s that could be the rotational period of an accreting neutron star or white dwarf in this -Cas analog.
22810790
Detective quantum efficiency of electron area detectors in electron microscopy
Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology. We present MTF and DQE measurements for four types of detector: Kodak SO-163 film, TVIPS 224 charge coupled device (CCD) detector, the Medipix2 hybrid pixel detector, and an experimental direct electron monolithic active pixel sensor (MAPS) detector. Film and CCD performance was measured at 120 and 300 keV, while results are presented for the Medipix2 at 120 keV and for the MAPS detector at 300 keV. In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated. We also show that part of the response of the emulsion in film comes from light generated in the plastic support. Computer simulations of film and the MAPS detector have been carried out and show good agreement with experiment. The agreement enables us to conclude that the DQE of a backthinned direct electron MAPS detector is likely to be equal to, or better than, that of film at 300 keV.
195378147
Effects of cosmological constant on motion of UHECR particles
Recent astronomical observations manifest that about two-thirds of the whole energy in the Universe is contributed by a small positive cosmological constant Λ (>0). Then, an asymptotically de Sitter spacetime is premised naturally. However, physics in the de Sitter spacetime is very different from that in the Minkowski spacetime. As the first step, a covariant formalism of the kinematics in the de Sitter spacetime is presented here. By solving exactly the equations of motion for a field, we obtain the dispersion relation of a free particle. It is noticed that the dispersion relation is dependent on the degree of freedom of angular momentum of the particle. We show the threshold anomaly of the ultra high energy cosmic ray disappears naturally in the framework of the de Sitter kinematics.