431 research outputs found
Interpreting a conformally flat pure radiation space-time
A physical interpretation is presented of the general class of conformally
flat pure radiation metrics that has recently been identified by Edgar and
Ludwig. It is shown that, at least in the weak field limit, successive wave
surfaces can be represented as null (half) hyperplanes rolled around a
two-dimensional null cone. In the impulsive limit, the solution reduces to a
pp-wave whose direction of propagation depends on retarded time. In the general
case, there is a coordinate singularity which corresponds to an envelope of the
wave surfaces. The global structure is discussed and a possible vacuum
extension through the envelope is proposed.Comment: 9 pages, Plain TeX, 2 figures. To appear in Class. Quantum Grav.
Reference adde
On the Symmetries of the Edgar-Ludwig Metric
The conformal Killing equations for the most general (non-plane wave)
conformally flat pure radiation field are solved to find the conformal Killing
vectors. As expected fifteen independent conformal Killing vectors exist, but
in general the metric admits no Killing or homothetic vectors. However for
certain special cases a one-dimensional group of homotheties or motions may
exist and in one very special case, overlooked by previous investigators, a
two-dimensional homethety group exists. No higher dimensional groups of motions
or homotheties are admitted by these metrics.Comment: Plain TeX, 7 pages, No figure
Analysis of RR Lyrae Stars in the Northern Sky Variability Survey
We use data from the Northern Sky Variability Survey (NSVS), obtained from
the first generation Robotic Optical Transient Search Experiment (ROTSE-I), to
identify and study RR Lyrae variable stars in the solar neighborhood. We
initially identified 1197 RRab (RR0) candidate stars brighter than the ROTSE
median magnitude V = 14. Periods, amplitudes, and mean V magnitudes are
determined for a subset of 1188 RRab stars with well defined light curves.
Metallicities are determined for 589 stars by the Fourier parameter method and
by the relationship between period, amplitude, and [Fe/H]. We comment upon the
difficulties of clearly classifying RRc (RR1) variables in the NSVS dataset.
Distances to the RRab stars are calculated using an adopted
luminosity-metallicity relation with corrections for interstellar extinction.
The 589 RRab stars in our final sample are used to study the properties of the
RRab population within 5 kpc of the Sun. The Bailey diagram of period versus
amplitude shows that the largest component of this sample belongs to Oosterhoff
type I. Metal-rich ([Fe/H] > -1) RRab stars appear to be associated with the
Galactic disk. Our metal-rich RRab sample may include a thin disk as well as a
thick disk population, although the uncertainties are too large to establish
this. There is some evidence among the metal-rich RRab stars for a decline in
scale height with increasing [Fe/H], as was found by Layden (1995). The
distribution of RRab stars with -1 < [Fe/H] < -1.25 indicates that within this
metallicity range the RRab stars are a mixture of stars belonging to halo and
disk populations.Comment: 68 pages, 26 figures, 9 tables, accepted to A
Cataclysmic Variables from the Catalina Real-time Transient Survey
We present 855 cataclysmic variable candidates detected by the Catalina
Real-time Transient Survey (CRTS) of which at least 137 have been
spectroscopically confirmed and 705 are new discoveries. The sources were
identified from the analysis of five years of data, and come from an area
covering three quarters of the sky. We study the amplitude distribution of the
dwarf novae CVs discovered by CRTS during outburst, and find that in quiescence
they are typically two magnitudes fainter compared to the spectroscopic CV
sample identified by SDSS. However, almost all CRTS CVs in the SDSS footprint
have ugriz photometry. We analyse the spatial distribution of the CVs and find
evidence that many of the systems lie at scale heights beyond those expected
for a Galactic thin disc population. We compare the outburst rates of newly
discovered CRTS CVs with the previously known CV population, and find no
evidence for a difference between them. However, we find that significant
evidence for a systematic difference in orbital period distribution. We discuss
the CVs found below the orbital period minimum and argue that many more are yet
to be identified among the full CRTS CV sample. We cross-match the CVs with
archival X-ray catalogs and find that most of the systems are dwarf novae
rather than magnetic CVs.Comment: 15 pages, 17 figures, accepted MNRA
Explicit Kundt type II and N solutions as gravitational waves in various type D and O universes
A particular yet large class of non-diverging solutions which admits a
cosmological constant, electromagnetic field, pure radiation and/or general
non-null matter component is explicitly presented. These spacetimes represent
exact gravitational waves of arbitrary profiles which propagate in background
universes such as Minkowski, conformally flat (anti-)de Sitter, Edgar-Ludwig,
Bertotti-Robinson, and type D (anti-)Nariai or Plebanski-Hacyan spaces, and
their generalizations. All possibilities are discussed and are interpreted
using a unifying simple metric form. Sandwich and impulsive waves propagating
in the above background spaces with different geometries and matter content can
easily be constructed. New solutions are identified, e.g. type D pure radiation
or explicit type II electrovacuum waves in (anti-)Nariai universe. It is also
shown that, in general, there are no conformally flat Einstein-Maxwell fields
with a non-vanishing cosmological constant.Comment: 17 pages, LaTeX 2e. v2: added two references concerning generalized
Kerr-Schild transformations, minor changes in the tex
Identification of new cataclysmic variables in the 1RXS and USNO-B1.0 catalogs
As a result of applying the original optical variability search method on
publicly available data, we have found eight new cataclysmic variables and two
possible Optically Violent Variable quasars among the previously unidentified
X-ray sources in the ROSAT catalog. We describe the search method and present
the characteristics of the newly identified variable stars. The obtained
results demonstrate the large potential of the concept of Virtual Observatory
for identifying new objects of astrophysical interest.Comment: 16 pages, 11 figures, 2 tables; submitted to Astronomy Letter
Geodesic motion in the Kundt spacetimes and the character of envelope singularity
We investigate geodesics in specific Kundt type N (or conformally flat)
solutions to Einstein's equations. Components of the curvature tensor in
parallelly transported tetrads are then explicitly evaluated and analyzed. This
elucidates some interesting global properties of the spacetimes, such as an
inherent rotation of the wave-propagation direction, or the character of
singularities. In particular, we demonstrate that the characteristic envelope
singularity of the rotated wave-fronts is a (non-scalar) curvature singularity,
although all scalar invariants of the Riemann tensor vanish there.Comment: 21 pages, 3 figures. To appear in Class. Quantum Gra
All spacetimes with vanishing curvature invariants
All Lorentzian spacetimes with vanishing invariants constructed from the
Riemann tensor and its covariant derivatives are determined. A subclass of the
Kundt spacetimes results and we display the corresponding metrics in local
coordinates. Some potential applications of these spacetimes are discussed.Comment: 24 page
Static charged perfect fluid spheres in general relativity
Interior perfect fluid solutions for the Reissner-Nordstrom metric are
studied on the basis of a new classification scheme. General formulas are found
in many cases. Explicit new global solutions are given as illustrations. Known
solutions are briefly reviewed.Comment: 23 pages, Revtex (galley), journal version, to appear in Phys.Rev.
Homothetic perfect fluid space-times
A brief summary of results on homotheties in General Relativity is given,
including general information about space-times admitting an r-parameter group
of homothetic transformations for r>2, as well as some specific results on
perfect fluids. Attention is then focussed on inhomogeneous models, in
particular on those with a homothetic group (acting multiply
transitively) and . A classification of all possible Lie algebra
structures along with (local) coordinate expressions for the metric and
homothetic vectors is then provided (irrespectively of the matter content), and
some new perfect fluid solutions are given and briefly discussed.Comment: 27 pages, Latex file, Submitted to Class. Quantum Gra
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