3,834 research outputs found
Self lensing effects for compact stars and their mass-radius relation
During the last couple of years astronomers and astrophysicists have been
debating on the fact whether the so called `strange stars' - stars made up of
strange quark matter, have been discovered with the candidates like SAX
J1808.4-3658, 4U 1728-34, RX J1856.5-3754, etc. The main contention has been
the estimation of radius of the star for an assumed mass of ~ 1.4 M_sun and to
see whether the point overlaps with the graphs for the neutron star equation of
state or whether it goes to the region of stars made of strange matter equation
of state. Using the well established formulae from general relativity for the
gravitational redshift and the `lensing effect' due to bending of photon
trajectories, we, in this letter, relate the parameters M and R with the
observable parameters, the redshift z and the radiation radius R_\infty, thus
constraining both M and R for specific ranges, without any other arbitrariness.
With the required inputs from observations, one ought to incorporate the
effects of self lensing of the compact stars which has been otherwise ignored
in all of the estimations done so far. Nonetheless, these effect of self
lensing makes a marked difference and constraints on the M-R relation.Comment: 7 pages, 1 figure, accepted for publication in Mod. Phys. Lett.
Inertial Forces as Viewed from the ADM Slicing and their behaviour for Particles in Non-Circular Geodesics
Considering the definition of inertial forces acting on a test particle,
following non-circular geodesics, in static and stationary space times we show
that the centrifugal force reversal occurs only in the case of particles
following prograde orbits around black holes. We first rewrite the covariant
expressions for the acceleration components in terms of the lapse function,
shift vector and the 3-metric \gm_{ij}, using the ADM 3+1 splitting and use
these, for different cases as given by pure radial motion, pure azimuthal
motion and the general non-circular motion. It is found that the reversal
occurs only when the azimuthal angular velocity of the particle supersedes the
radial velocity, which indeed depends upon the physical parameters , and
the Kerr parameter .Comment: 16 Latex pages, 1 Table and 3 Figure
Gauge Invariant Wave Equations in Curved Space-Times and Primordial Magnetic Fields
The inflationary production of magnetic field seeds for galaxies is
discussed. The analysis is carried out by writing the wave equation of the
electromagnetic field in curved spacetimes. The conformal invariance is broken
by taking into account of the interaction of the electromagnetic field with the
curvature tensor of the form . Such a term induces an amplification of the
magnetic field during the reheating phase of the universe, but no growth of the
magnetic field occurs in the de Sitter epoch. The resulting primordial magnetic
field turns out to have strengths of astrophysical interest.Comment: 5 pages, no figures, 1 tabl
Centrifugal force in Kerr geometry
We have obtained the correct expression for the centrifugal force acting on a
particle at the equatorial circumference of a rotating body in the locally
non-rotating frame of the Kerr geometry. Using this expression for the
equilibrium of an element on the surface of a slowly rotating Maclaurin
spheroid, we obtain the expression for the ellipticity (as discussed earlier by
Abramowicz and Miller) and determine the radius at which the ellipticity is
maximum.Comment: 6 pages, LateX macro
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