1,748 research outputs found
A timing formula for main-sequence star binary pulsars
In binary radio pulsars with a main-sequence star companion, the spin-induced
quadrupole moment of the companion gives rise to a precession of the binary
orbit. As a first approximation one can model the secular evolution caused by
this classical spin-orbit coupling by linear-in-time changes of the longitude
of periastron and the projected semi-major axis of the pulsar orbit. This
simple representation of the precession of the orbit neglects two important
aspects of the orbital dynamics of a binary pulsar with an oblate companion.
First, the quasiperiodic effects along the orbit, due to the anisotropic
nature of the quadrupole potential. Secondly, the long-term secular
evolution of the binary orbit which leads to an evolution of the longitude of
periastron and the projected semi-major axis which is non-linear in time.
In this paper a simple timing formula for binary radio pulsars with a
main-sequence star companion is presented which models the short-term secular
and most of the short-term periodic effects caused by the classical spin-orbit
coupling. I also give extensions of the timing formula which account for
long-term secular changes in the binary pulsar motion. It is shown that the
short-term periodic effects are important for the timing observations of the
binary pulsar PSR B1259--63. The long-term secular effects are likely to become
important in the next few years of timing observations of the binary pulsar PSR
J0045--7319. They could help to restrict or even determine the moments of
inertia of the companion star and thus probe its internal structure.
Finally, I reinvestigate the spin-orbit precession of the binary pulsar PSR
J0045--7319 since the analysis given in the literature is based on an incorrect
expression for the precession of the longitude of periastron.Comment: 12 pages (LaTeX), 20 Postscript figures; replaced by accepted versio
New limits on the violation of local position invariance of gravity
Within the parameterized post-Newtonian (PPN) formalism, there could be an
anisotropy of local gravity induced by an external matter distribution, even
for a fully conservative metric theory of gravity. It reflects the breakdown of
the local position invariance of gravity and, within the PPN formalism, is
characterized by the Whitehead parameter . We present three different
kinds of observation, from the Solar system and radio pulsars, to constrain it.
The most stringent limit comes from recent results on the extremely stable
pulse profiles of solitary millisecond pulsars, that gives (95% CL), where the hat denotes the strong-field generalization
of . This limit is six orders of magnitude more constraining than the
current best limit from superconducting gravimeter experiments. It can be
converted into an upper limit of on the spatial
anisotropy of the gravitational constant.Comment: 13 pages, 4 figures; accepted by Classical and Quantum Gravit
New tests of local Lorentz invariance of gravity with small-eccentricity binary pulsars
In the post-Newtonian parametrization of semi-conservative gravity theories,
local Lorentz invariance (LLI) violation is characterized by two parameters,
alpha_1 and alpha_2. In binary pulsars the isotropic violation of LLI in the
gravitational sector leads to characteristic preferred frame effects (PFEs) in
the orbital dynamics, if the barycenter of the binary is moving relative to the
preferred frame with a velocity w. For small-eccentricity binaries, the effects
induced by alpha_1 and alpha_2 decouple, and can therefore be tested
independently. We use recent timing results of two compact pulsar-white dwarf
binaries with known 3D velocity, PSRs J1012+5307 and J1738+0333, to constrain
PFEs for strongly self-gravitating bodies. We derive a limit |alpha_2| < 1.8e-4
(95% CL), which is the most constraining limit for strongly self-gravitating
systems up to now. Concerning alpha_1, we propose a new, robust method to
constrain this parameter. Our most conservative result, alpha_1 =
-0.4^{+3.7}_{-3.1} e-5 (95% CL) from PSR J1738+0333, constitutes a significant
improvement compared to current most stringent limits obtained both in Solar
system and binary pulsar tests. We also derive corresponding limits for alpha_1
and alpha_2 for a preferred frame that is at rest with respect to our Galaxy,
and preferred frames that locally co-move with the rotation of our Galaxy.
(Abridged)Comment: 34 pages, 8 figures, 2 tables; accepted by Classical and Quantum
Gravit
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