Publication date: Jun 2015
Abstract:
We study deflection of light rays and gravitational lensing in the
regular Bardeen no-horizon spacetimes. Flatness of these spacetimes in
the central region implies existence of interesting optical effects
related to photons crossing the gravitational field of the no-horizon
spacetimes with low impact parameters. These effects occur due to
existence of a critical impact parameter giving maximal deflection of
light rays in the Bardeen no-horizon spacetimes. We give the critical
impact parameter in dependence on the specific charge of the spacetimes,
and discuss "ghost" direct and indirect images of Keplerian discs,
generated by photons with low impact parameters. The ghost direct images
can occur only for large inclination angles of distant observers, while
ghost indirect images can occur also for small inclination angles. We
determine the range of the frequency shift of photons generating the
ghost images and determine distribution of the frequency shift across
these images. We compare them to those of the standard direct images of
the Keplerian discs. The difference of the ranges of the frequency shift
on the ghost and direct images could serve as a quantitative measure of
the Bardeen no-horizon spacetimes. The regions of the Keplerian discs
giving the ghost images are determined in dependence on the specific
charge of the no-horizon spacetimes. For comparison we construct direct
and indirect (ordinary and ghost) images of Keplerian discs around
Reissner-Nördström naked singularities demonstrating a clear
qualitative difference to the ghost direct images in the regular Bardeen
no-horizon spacetimes. The optical effects related to the low impact
parameter photons thus give clear signature of the regular Bardeen
no-horizon spacetimes, as no similar phenomena could occur in the black
hole or naked singularity spacetimes. Similar direct ghost images have
to occur in any regular no-horizon spacetimes having nearly flat central
region.
Authors:
Schee, Jan; Stuchlík, Zdeněk;