Recently, it has been shown by Lobo, Parsaei and Riazi (LPR) that phantom energy with $\omega =p_{r}/\rho <-1$ could support phantom wormholes. Several classes of such solutions have been derived by them. While the inner spacetime is represented by asymptotically flat phantom wormhole that have repulsive gravity, it is most likely to be unstable to perturbations. Hence, we consider a situation, where a phantom wormhole is somehow trapped inside a Schwarzschild sphere across a thin shell. Applying the method developed by Garcia, Lobo and Visser (GLV), we shall exemplify that the shell can possess zones of stability depending on certain constraints. It turns out that zones corresponding to "force" constraint are more restrictive than those from the "mass" constraint. We shall also enumerate the interior energy content by using the gravitational energy integral proposed by Lynden-Bell, Katz and Bi% \v{c}\'ak. It turns out that, even though the interior mass is positive, the integral implies repulsive energy. This is consistent with the phantom nature of interior matter.Comment: 10 pages, 3 figures, Indian J Phys 201
It seems surprising that the emissivity properties of the accretion disk (à la Page and Thorne) surrounding the Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) black holes of heterotic string theory have not yet been studied. To fill this gap in the literature, we study the emissivity properties of the thin accretion disks around these black holes both in the Einstein and in the string frame using the Page-Thorne model. For illustration, we choose as a toy model a stellar-sized spherically symmetric black hole and find that, while the emissivity properties do not significantly differ from those of Reissner-Nordström and Schwarzschild black holes, they remarkably differ at GMGHS extreme limits corresponding to naked singularity and wormhole at higher frequencies. These differences provide a novel way to speculatively conclude about different types of objects from the observational point of view.
We extend a recent work on weak field first order light deflection in the MOdified Gravity (MOG) by comprehensively analyzing the actual observables in gravitational lensing both in the weak and strong field regime. The static spherically symmetric black hole (BH) obtained by Moffat is what we call here the Schwarzschild-MOG (abbreviated as SMOG) containing repulsive Yukawa-like force characterized by the MOG parameter α > 0 diminishing gravitational attraction. We point out a remarkable feature of SMOG, viz., it resembles a regular brane-world BH in the range −1 < α < 0 giving rise to a negative "tidal charge" Q (= 1 4 α 1+α ) interpreted as an imprint from the 5D bulk with an imaginary source charge q in the brane. The Yukawa-like force of MOG is attractive in the brane-world range enhancing gravitational attraction. For −∞ < α < −1, the SMOG represents a naked singularity. Specifically, we shall investigate the effect of α or Yukawa-type forces on the weak (up to third PPN order) and strong field lensing observables. For illustration, we consider the supermassive BH SgrA* with α = 0.055 for the weak field to quantify the deviation of observables from GR but in general we leave α unrestricted both in sign and magnitude so that future accurate lensing measurements, which are quite challenging, may constrain α. * Electronic address: izmailov.ramil@gmail.com † Electronic address: karimov ramis 92@mail.ru ‡ Electronic address: zhdanov@ufanet.ru § Electronic address: kamalnandi1952@yahoo.co.in Recent works [1,13,14,23] have motivated us to comprehensively look into the SMOG from another angle, viz., the weak and strong field lensing phenomena that are fundamentally different from each other. The purpose was
A new rotating generalization of the Damour-Solodukhin wormhole (RDSWH), called Kerr-like wormhole, has recently been proposed and investigated by Bueno et al. for echoes in the gravitational wave signal. We show a novel feature of the RDSWH, viz., that the kinematic properties such as the ISCO or marginally stable radius r ms , efficiency and the disk potential V eff are independent of λ (which means they are identical to their KBH counterparts for any given spin). Differences however appear in the emissivity properties for higher values 0.1 < λ ≤ 1 (say) and for the extreme spin a = 0.998. The kinematic and emissivity are generic properties as variations of the wormhole mass and the rate of accretion within the model preserve these properties. Specifically, the behavior of the luminosity peak is quite opposite to each other for the two objects, which could be useful from the viewpoint of observations. Apart from this, an estimate of the difference Δ λ in the maxima of flux of radiation F(r) shows non-zero values but is too tiny to be observable at present for λ < 10 −3 permitted by the strong lensing bound. The broad conclusion is that RDSWH are experimentally indistinguishable from KBH by accretion characteristics.
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