arXiv subjects

Joseph Sultana

Publications and source records attributed to Joseph Sultana.

27 records · Page 2Linked to original sources

arXiv2016

Thermodynamic Geodesics of a Reissner Nordström Black Hole

Starting from a Geometrothermodynamics metric for the space of thermodynamic equilibrium states in the mass representation, we use numerical techniques to analyse the thermodynamic geodesics of a supermassive Reissner Nordström black hole in isolation. Appropriate constraints are obtained by taking into account the processes of Hawking radiation and Schwinger pair--production. We model the black hole in line with the work of Hiscock and Weems. It can be deduced that the relation which the geodesics establish between the entropy $S$ and electric charge $Q$ of the black hole extremises changes in the black hole's mass. Indeed, the expression for the entropy of an extremal black hole is an exact solution to the geodesic equation. We also find that in certain cases, the geodesics describe the evolution brought about by the constant emission of Hawking radiation and charged-particle pairs.

gr-qc↗

arXiv2016

The $R_{\mathrm{h}}=ct$ universe and quintessence

Over the last few years the $R_{\mathrm{h}}=ct$ universe has received a lot of attention, particularly when observational evidence seems to favor this over the standard $Λ$ cold dark matter ($ΛCDM$) universe. Like the $ΛCDM$, the $R_{\mathrm{h}}=ct$ universe is based on a Friedmann-Robertson-Walker (FRW) cosmology where the total energy density $ρ$ and pressure $p$ of the cosmic fluid contain a dark energy component besides the usual (dark and baryonic) matter and radiation components. However unlike the $ΛCDM$ this model has the simple equation of state (EOS) $ρ+ 3p = 0$, i.e., its total active gravitational mass vanishes, which would therefore exclude a cosmological constant as the source of its dark energy component. Faced with this issue, in this paper we examine various possible sources for the dark energy component of the $R_{\mathrm{h}}=ct$ universe and show that quintessence which has been used in other various dynamical dark energy models could also be a possible source in this case.

astro-ph.CO↗

arXiv2015

The statistics of BAT-to-XRT flux ratio in GRB: Evidence for a characteristic value and its implications

We present the statistics of the ratio, ${\cal R}$, between the prompt and afterglow "plateau" fluxes of GRB. This we define as the ratio between the mean prompt energy flux in {\em Swift} BAT and the {\em Swift} XRT one, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the "plateau". Like the distribution of many other GRB observables, the histogram of ${\cal R}$~ is log-normal with maximum at a value ${\cal R}_m \simeq 2,000$, FWHM of about 2 decades and with the entire distribution spanning about 5 decades in the value of ${\cal R}$. We note that the peak of the distribution is close to the proton-to-electron mass ratio $({\cal R}_m \simeq m_p/m_e = 1836)$, as proposed to be the case in an earlier publication, on the basis of a specific model of the GRB dissipation process. It therefore appears that, in addition to the values of the energy of peak luminosity ${E_{\rm pk}\sim m_{e} c^2}$, GRB present us with one more quantity with an apparent characteristic value. The fact that the values of both these quantities (\Ep~and \R) are consistent with the same specific model invoked to account for the efficient conversion of their relativistic proton energies to electrons, argues favorably for its underlying assumptions.

astro-ph.HE↗

arXiv2014

Gravitomagnetic effects in conformal gravity

Gravitomagnetic effects are characterized by two phenomena: first, the geodetic effect which describes the precession of the spin of a gyroscope in a free orbit around a massive object, second, the Lense-Thirring effect which describes the precession of the orbital plane about a rotating source mass. We calculate both these effects in the fourth-order theory of conformal Weyl gravity for the test case of circular orbits. We show that for the geodetic effect a linear term arises which may be interesting for high radial orbits, whereas for the Lense-Thirring effect the additional term has a diminishing effect for most orbits. Circular orbits are also considered in general leading up to a generalization of Kepler's third law.

gr-qc↗

arXiv2013

The supercritical pile gamma-ray burst model: The GRB afterglow steep-decline-and-plateau phase

We present a process that accounts for the steep-decline-and-plateau phase of the Swift-XRT light curves, vexing features of GRB phenomenology. This process is an integral part of the "supercritical pile" GRB model, proposed a few years ago to provide an account for the conversion of the GRB kinetic energy into radiation with a spectral peak at $E_{\rm pk} \sim m_ec^2$. We compute the evolution of the relativistic blast wave (RBW) Lorentz factor $Γ$ to show that the radiation--reaction force due to the GRB emission can produce an abrupt, small ($\sim 25%$) decrease in $Γ$ at a radius which is smaller (depending on conditions) than the deceleration radius $R_D$. Because of this reduction, the kinematic criticality criterion of the "supercritical pile" is no longer fulfilled. Transfer of the proton energy into electrons ceases, and the GRB enters abruptly the afterglow phase at a luminosity smaller by $\sim m_p/m_e$ than that of the prompt emission. If the radius at which this slow-down occurs is significantly smaller than $R_D$, the RBW internal energy continues to drive the RBW expansion at a constant (new) $Γ$, and its X-ray luminosity remains constant until $R_D$ is reached, at which point it resumes its more conventional decay, thereby completing the "unexpected" XRT light curve phase. If this transition occurs at $R \simeq R_D$, the steep decline is followed by a flux decrease instead of a "plateau", consistent with the conventional afterglow declines. Besides providing an account of these peculiarities, the model suggests that the afterglow phase may in fact begin before the RBW reaches $R \simeq R_D$, thus introducing novel insights into the GRB phenomenology.

astro-ph.HE↗

arXiv2013

Charged Cylindrical Black Holes in Conformal Gravity

Considering cylindrical topology we present the static solution for a charged black hole in conformal gravity. We show that unlike the general relativistic case there are two different solutions, both including a factor that when set to zero recovers the familiar static charged black string solution in Einstein's theory. This factor gives rise to a linear term in the potential that also features in the neutral case and may have significant ramifications for particle trajectories.

gr-qc↗

arXiv2012

Luminosity correlations for gamma-ray bursts and implications for their prompt and afterglow emission mechanisms

We present the relation between the ($z-$ and $k-$corrected) spectral lags, $τ$, for the standard Swift energy bands 50-100 keV and 100-200 keV and the peak isotropic luminosity, $L_{\mathrm{iso}}$ (a relation reported first by Norris et al.), for a subset of 12 long Swift GRBs taken from a recent study of this relation by Ukwatta et al. The chosen GRBs are also a subset of the Dainotti et al. sample, a set of Swift GRBs of known redshift, employed in establishing a relation between the (GRB frame) luminosity, $L_X$, of the shallow (or constant) flux portion of the typical XRT GRB-afterglow light curve and the (GRB frame) time of transition to the normal decay rate, $T_{\mathrm{brk}}$. We also present the $L_X-T_{\mathrm{brk}}$ relation using only the bursts common in the two samples. The two relations exhibit a significant degree of correlation ($ρ= -0.65$ for the $L_{\mathrm{iso}}-τ$ and $ρ= -0.88$ for the $L_{X} - T_{\mathrm{brk}}$ relation) and have surprisingly similar best-fit power law indices ($-1.19 \pm 0.17$ for $L_{\mathrm{iso}}-τ$ and $-1.10 \pm 0.03$ for $L_{X} - T_{\mathrm{brk}}$). Even more surprisingly, we noted that although $τ$ and $T_{\mathrm{brk}}$ represent different GRB time variables, it appears that the first relation ($L_{\mathrm{iso}}-τ$) extrapolates into the second one for timescales $τ\simeq T_{\mathrm{brk}}$. This fact suggests that these two relations have a common origin, which we conjecture to be kinematic. This relation adds to the recently discovered relations between properties of the prompt and afterglow GRB phases, indicating a much more intimate relation between these two phases than hitherto considered.

astro-ph.HE↗

arXiv2012

Exact Static Cylindrical Solution to Conformal Weyl Gravity

We present the exact exterior solution for a static and neutral cylindrically symmetric source in locally conformal invariant Weyl gravity. As a special case the general relativity analogue still can be attained, however only as a sub-family of solutions. Our solution contains a linear term that would thus result in a potential that grows linearly over large distances. This may have implications for exotic astrophysical structures as well as matter fields on the extremely small scale.

gr-qc↗

arXiv2011

The Problem of Inertia in Friedmann Universes

In this paper we study the origin of inertia in a curved spacetime, particularly the spatially flat, open and closed Friedmann universes. This is done using Sciama's law of inertial induction, which is based on Mach's principle, and expresses the analogy between the retarded far fields of electrodynamics and those of gravitation. After obtaining covariant expressions for electromagnetic fields due to an accelerating point charge in Friedmann models, we adopt Sciama's law to obtain the inertial force on an accelerating mass $m$ by integrating over the contributions from all the matter in the universe. The resulting inertial force has the form $F = -kma$, where $k < 1 $ depends on the choice of the cosmological parameters such as $Ω_{M}$, $Ω_Λ$, and $Ω_{R}$ and is also red-shift dependent.

astro-ph.CO↗