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Pradhan P.,Vivekananda Satabarshiki Mahavidyalaya
Astroparticle Physics | Year: 2015

We examine the possibility of arbitrarily high energy in the center-of-mass (CM) frame of colliding neutral particles in the vicinity of the horizon of a charged dilation black hole (BH). We show that it is possible to achieve the infinite energy in the background of the dilation black hole without fine-tuning of the angular momentum parameter. It is found that the CM energy (Ecm) of collisions of particles near the infinite red-shift surface of the extreme dilation BHs are arbitrarily large while the non-extreme charged dilation BHs have the finite energy. We have also compared the Ecm at the horizon with the ISCO (Innermost Stable Circular Orbit) and MBCO (Marginally Bound Circular Orbit) for extremal Reissner-Nordstrøm (RN) BH and Schwarzschild BH. We find that for extreme RN BH the inequality becomes Ecm|r+>Ecm|rmb>Ecm|rISCO i.e. Ecm|r+=M:Ecm rmb=3+52M:Ecm|rISCO=4M=∞:3.23:2.6. While for Schwarzschild BH the ratio of CM energy is Ecm|r+=2M:Ecm|rmb=4M:Ecm|rISCO=6M=5:2:133. Also for Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) BHs the ratio is being Ecm|r+=2M:Ecm|rmb=2M:Ecm|rISCO=2M=∞:∞:∞. © 2014 Elsevier B.V. Source


Pradhan P.P.,Vivekananda Satabarshiki Mahavidyalaya | Majumdar P.,Ramakrishna Mission Vivekananda University
European Physical Journal C | Year: 2013

The fact that one must evaluate the near-extremal and near-horizon limits of Kerr spacetime in a specific order, is shown to lead to discontinuity in the extremal limit, such that this limiting spacetime differs nontrivially from the precisely extremal spacetime. This is established by first showing a discontinuity in the extremal limit of the maximal analytic extension of the Kerr geometry, given by Carter. Next, we examine the ISCO of the exactly extremal Kerr geometry and show that on the event horizon of the extremal Kerr black hole, it coincides with the principal null geodesic generator of the horizon, having vanishing energy and angular momentum. We find that there is no such ISCO in the near-extremal geometry, thus garnering additional support for our primary contention. We relate this disparity between the two geometries to the lack of a trapping horizon in the extremal situation. © 2013 Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica. Source


Pradhan P.P.,Vivekananda Satabarshiki Mahavidyalaya
European Physical Journal C | Year: 2013

We compute the proper time Lyapunov exponent for the charged Myers-Perry black hole spacetime and investigate the instability of the equatorial circular geodesics (both time-like and null) via this exponent. We also show that for more than four spacetime dimensions (N≥3), there are no Innermost Stable Circular Orbits (ISCOs) in the charged Myers-Perry black hole spacetime. We further show that among all possible circular orbits, time-like circular orbits have longer orbital periods than null circular orbits (photon spheres) as measured by asymptotic observers. Thus, time-like circular orbits provide the slowest way to orbit around the charged Myers-Perry black hole. © 2013 Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica. Source


Chakraborty C.,Saha Institute of Nuclear Physics | Pradhan P.P.,Vivekananda Satabarshiki Mahavidyalaya
European Physical Journal C | Year: 2013

An exact expression of Lense-Thirring precession rate is derived for non-extremal and extremal Plebański-Demiański spacetimes. This formula is used to find the exact Lense-Thirring precession rate in various axisymmetric spacetimes i.e., Kerr-Newman, Kerr-de Sitter etc. We also show that if the Kerr parameter vanishes in the Plebański-Demiański spacetime, the Lense-Thirring precession does not vanish due to the existence of NUT charge. To derive the Lense-Thirring precession rate in the extremal Plebański-Demiański spacetime, we first derive the general extremal condition for Plebański-Demiański spacetimes. This general result could be applied to obtain the extremal limit in any stationary and axisymmetric spacetimes. © 2013 Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica. Source


Pradhan P.,Vivekananda Satabarshiki Mahavidyalaya
European Physical Journal C | Year: 2014

We argue by explicit computations that, although the area product, horizon radii product, entropy product, and irreducible mass product of the event horizon and Cauchy horizon are universal, the surface gravity product, the surface temperature product and the Komar energy product of the said horizons do not seem to be universal for Kerr-Newman black hole spacetimes. We show the black hole mass formula on the Cauchy horizon following the seminal work by Smarr [Phys Rev Lett 30:71 (1973), Phys Rev D 7:289 (1973)] for the outer horizon. We also prescribe the four laws of black hole mechanics for the inner horizon. A new definition of the extremal limit of a black hole is discussed. © 2014 The Author(s). Source

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