10,757 research outputs found
Linear Einstein equations and Kerr-Schild maps
We prove that given a solution of the Einstein equations for the
matter field , an autoparallel null vector field and a solution
of the linearized Einstein equation on the
given background, the Kerr-Schild metric ( arbitrary constant) is an exact solution of the Einstein equation for the
energy-momentum tensor . The mixed form of the Einstein equation for
Kerr-Schild metrics with autoparallel null congruence is also linear. Some more
technical conditions hold when the null congruence is not autoparallel. These
results generalize previous theorems for vacuum due to Xanthopoulos and for
flat seed space-time due to G\"{u}rses and G\"{u}rsey.Comment: 9 pages, accepted by Class. Quant. Gra
Kerr-Schild Symmetries
We study continuous groups of generalized Kerr-Schild transformations and the
vector fields that generate them in any n-dimensional manifold with a
Lorentzian metric. We prove that all these vector fields can be intrinsically
characterized and that they constitute a Lie algebra if the null deformation
direction is fixed. The properties of these Lie algebras are briefly analyzed
and we show that they are generically finite-dimensional but that they may have
infinite dimension in some relevant situations. The most general vector fields
of the above type are explicitly constructed for the following cases: any
two-dimensional metric, the general spherically symmetric metric and
deformation direction, and the flat metric with parallel or cylindrical
deformation directions.Comment: 15 pages, no figures, LaTe
Locating the intense interstellar scattering towards the inner Galaxy
We use VLBA+VLA observations to measure the sizes of the scatter-broadened
images of 6 of the most heavily scattered known pulsars: 3 within the Galactic
Centre (GC) and 3 elsewhere in the inner Galactic plane. By combining the
measured sizes with temporal pulse broadening data from the literature and
using the thin-screen approximation, we locate the scattering medium along the
line of sight to these 6 pulsars. At least two scattering screens are needed to
explain the observations of the GC sample. We show that the screen inferred by
previous observations of SGR J1745-2900 and Sgr A*, which must be located far
from the GC, falls off in strength on scales < 0.2 degree. A second scattering
component closer to (< 2 kpc) or even (tentatively) within (< 700 pc) the GC
produces most or all of the temporal broadening observed in the other GC
pulsars. Outside the GC, the scattering locations for all three pulsars are ~2
kpc from Earth, consistent with the distance of the Carina-Sagittarius or
Scutum spiral arm. For each object the 3D scattering origin coincides with a
known HII region (and in one case also a supernova remnant), suggesting that
such objects preferentially cause the intense interstellar scattering seen
towards the Galactic plane. We show that the HII regions should contribute >
25% of the total dispersion measure (DM) towards these pulsars, and calculate
reduced DM distances. Those distances for other pulsars lying behind HII
regions may be similarly overestimated.Comment: 16 pages, 10 figures, MNRAS, in pres
Curved Noncommutative Tori as Leibniz Quantum Compact Metric Spaces
We prove that curved noncommutative tori, introduced by Dabrowski and Sitarz,
are Leibniz quantum compact metric spaces and that they form a continuous
family over the group of invertible matrices with entries in the commutant of
the quantum tori in the regular representation, when this group is endowed with
a natural length function.Comment: 16 Pages, v3: accepted in Journal of Math. Physic
Radio Detection of the Fermi LAT Blind Search Millisecond Pulsar J1311-3430
We report the detection of radio emission from PSR J1311-3430, the first
millisecond pulsar discovered in a blind search of Fermi Large Area Telescope
(LAT) gamma-ray data. We detected radio pulsations at 2 GHz, visible for <10%
of ~4.5-hrs of observations using the Green Bank Telescope (GBT). Observations
at 5 GHz with the GBT and at several lower frequencies with Parkes, Nancay, and
the Giant Metrewave Radio Telescope resulted in non-detections. We also report
the faint detection of a steep spectrum continuum radio source (0.1 mJy at 5
GHz) in interferometric imaging observations with the Jansky Very Large Array.
These detections demonstrate that PSR J1311-3430, is not radio quiet and
provides additional evidence that the radio beaming fraction of millisecond
pulsars is very large. The radio detection yields a distance estimate of 1.4
kpc for the system, yielding a gamma-ray efficiency of 30%, typical of
LAT-detected MSPs. We see apparent excess delay in the radio pulsar as the
pulsar appears from eclipse and we speculate on possible mechanisms for the
non-detections of the pulse at other orbital phases and observing frequencies.Comment: 6 pages, 4 figures. ApJ Letters, in pres
Advancing the link between ocean connectivity, ecological function and management challenges
âOcean connectivityâ is a dynamic and rapidly evolving field of research in marine science, partly because there is an increasing demand for information on connectivity that informs effective assessment and management of marine resources. Achieving this will require a better alignment between ocean connectivity tools and developments and the needs and challenges of assessments and conservation. For these reasons, the ICES Journal of Marine Science solicited contributions to the article theme set (TS), âBeyond ocean connectivity.â We briefly summarize the nine articles that appear herein, grouping them into four general topics: methodological advances, population dynamics and assessment implications of connectivity, spatial and management implications, and connectivity in ecosystem processes. We also discuss the challenges facing ocean connectivity research if it is to effectively support advancing fisheries assessment frameworks and integrated ecosystem approaches. We hope that the contributions included in this TS serve to convince managers and fisheries scientists of the need to incorporate results from research on connectivity
Entropy and the variational principle for actions of sofic groups
Recently Lewis Bowen introduced a notion of entropy for measure-preserving
actions of a countable sofic group on a standard probability space admitting a
generating partition with finite entropy. By applying an operator algebra
perspective we develop a more general approach to sofic entropy which produces
both measure and topological dynamical invariants, and we establish the
variational principle in this context. In the case of residually finite groups
we use the variational principle to compute the topological entropy of
principal algebraic actions whose defining group ring element is invertible in
the full group C*-algebra.Comment: 44 pages; minor changes; to appear in Invent. Mat
Time-reversible Dynamical Systems for Turbulence
Dynamical Ensemble Equivalence between hydrodynamic dissipative equations and
suitable time-reversible dynamical systems has been investigated in a class of
dynamical systems for turbulence. The reversible dynamics is obtained from the
original dissipative equations by imposing a global constraint. We find that,
by increasing the input energy, the system changes from an equilibrium state to
a non-equilibrium stationary state in which an energy cascade, with the same
statistical properties of the original system, is clearly detected.Comment: 16 pages Latex, 4 PS figures, on press on J. Phy
Is the black-widow pulsar PSR J1555-2908 in a hierarchical triple system?
The 559 Hz black-widow pulsar PSR J1555-2908, originally discovered in radio, is also a bright gamma-ray pulsar. Timing its pulsations using 12 yr of Fermi-LAT gamma-ray data reveals long-term variations in its spin frequency that are much larger than is observed from other millisecond pulsars. While this variability in the pulsar rotation rate could be intrinsic "timing noise", here we consider an alternative explanation: the variations arise from the presence of a very-low-mass third object in a wide multi-year orbit around the neutron star and its low-mass companion. With current data, this hierarchical-triple-system model describes the pulsar's rotation slightly more accurately than the best-fitting timing-noise model. Future observations will show if this alternative explanation is correct
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