Vivekananda Satabarshiki Mahavidyalaya

Paschim, India

Vivekananda Satabarshiki Mahavidyalaya

Paschim, India

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Pradhan P.,Vivekananda Satabarshiki Mahavidyalaya | Majumdar P.,Saha Institute of Nuclear Physics
Physics Letters, Section A: General, Atomic and Solid State Physics | Year: 2011

Circular null geodesic orbits, in extremal Reissner-Nordstrom spacetime, are examined with regard to their stability, and compared with similar orbits in the near-extremal situation. Extremization of the effective potential for null circular orbits shows the existence of a stable circular geodesic in the extremal spacetime, precisely on the event horizon which coincides with the null geodesic generator. Such a null orbit on the horizon is also indicated by the global minimum of the effective potential for circular timelike orbits. This type of geodesic is of course absent in the corresponding near-extremal spacetime, as we show here, testifying to differences between the extremal limit of a generic RN spacetime and the exactly extremal geometry. © 2010 Elsevier B.V. All rights reserved.


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.


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).


Pradhan P.,Vivekananda Satabarshiki Mahavidyalaya
Journal of Physics: Conference Series | Year: 2012

The effective potential in universal like coordinates(U, V, θ, φ), which are smooth across the event horizon is derived and investigated the ISCO(Innermost Stable Circular Orbits) explicitly in these coordinates for extremal Kerr spacetime. Extremization of the effective potential for timelike circular orbit shows that the existence of a stable circular geodesics in the extremal spacetime for direct orbit, precisely on the event horizon in terms of the radial coordinate which coincides with the principal null geodesic congruences of the event horizon. These null geodesic congruences mold themselves to the spacetime curvature in such a way that Weyl conformal tensor and its dual are vanished, that is why they are in-fact doubly degenerate principal null congruences.


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.


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.


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.


Pradhan P.,Vivekananda Satabarshiki Mahavidyalaya
Astrophysics and Space Science | Year: 2014

We show that an extremal Gibbons-Maeda-Garfinkle-Horowitz-Strominger black hole may act as a particle accelerator with arbitrarily high energy when two uncharged particles falling freely from rest to infinity on the near horizon. We show that the center of mass energy of collision is independent of the extreme fine tuning of the angular momentum of the colliding particles. We further show that the center of mass energy of collisions of particles at the ISCO (r ISCO) or at the photon orbit (r ph) or at the marginally bound circular orbit (r mb) i.e. at r≡r ISCO=r ph=r mb=2M could be arbitrarily large for the aforementioned space-time, which is quite different from the Schwarzschild and the Reissner-Nordstrøm space-time. For non-extremal GMGHS space-time the CM energy is finite and depends upon the asymptotic value of the dilation field (φ{symbol} 0). © 2014 Springer Science+Business Media Dordrecht.

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