1,250 research outputs found
Extracting the three- and four-graviton vertices from binary pulsars and coalescing binaries
Using a formulation of the post-Newtonian expansion in terms of Feynman
graphs, we discuss how various tests of General Relativity (GR) can be
translated into measurement of the three- and four-graviton vertices. In
problems involving only the conservative dynamics of a system, a deviation of
the three-graviton vertex from the GR prediction is equivalent, to lowest
order, to the introduction of the parameter beta_{PPN} in the parametrized
post-Newtonian formalism, and its strongest bound comes from lunar laser
ranging, which measures it at the 0.02% level. Deviation of the three-graviton
vertex from the GR prediction, however, also affects the radiative sector of
the theory. We show that the timing of the Hulse-Taylor binary pulsar provides
a bound on the deviation of the three-graviton vertex from the GR prediction at
the 0.1% level. For coalescing binaries at interferometers we find that,
because of degeneracies with other parameters in the template such as mass and
spin, the effects of modified three- and four-graviton vertices is just to
induce an error in the determination of these parameters and, at least in the
restricted PN approximation, it is not possible to use coalescing binaries for
constraining deviations of the vertices from the GR prediction.Comment: 10 pages, 5 figures; v2: an error corrected; references adde
A post-Keplerian parameter to test gravito-magnetic effects in binary pulsar systems
We study the pulsar timing, focusing on the time delay induced by the
gravitational field of the binary systems. In particular, we study the
gravito-magnetic correction to the Shapiro time delay in terms of Keplerian and
post-Keplerian parameters, and we introduce a new post-Keplerian parameter
which is related to the intrinsic angular momentum of the stars. Furthermore,
we evaluate the magnitude of these effects for the binary pulsar systems known
so far. The expected magnitude is indeed small, but the effect is important per
se.Comment: 6 pages, RevTeX, 1 eps figure, accepted for publication in Physical
Review D; references adde
Detection of OH absorption against PSR B1849+00
We have searched for OH absorption against seven pulsars using the Arecibo
telescope. In both OH mainlines (at 1665 and 1667 MHz), deep and narrow
absorption features were detected toward PSR B1849+00. In addition, we have
detected several absorption and emission features against B33.6+0.1, a nearby
supernova remnant (SNR). The most interesting result of this study is that a
pencil-sharp absorption sample against the PSR differs greatly from the
large-angle absorption sample observed against the SNR. If both the PSR and the
SNR probe the same molecular cloud then this finding has important implications
for absorption studies of the molecular medium, as it shows that the statistics
of absorbing OH depends on the size of the background source. We also show that
the OH absorption against the PSR most likely originates from a small (<30
arcsec) and dense (>10^5 cm^-3) molecular clump.Comment: 12 pages, 8 figures. Accepted for publication in Ap
A Modified Scalar-Tensor-Vector Gravity Theory and the Constraint on its Parameters
A gravity theory called scalar-tensor-vector gravity (STVG) has been recently
developed and succeeded in solar system, astrophysical and cosmological scales
without dark matter [J. W. Moffat, J. Cosmol. Astropart. Phys. 03, 004 (2006)].
However, two assumptions have been used: (i) , where and
are and in the Schwarzschild coordinates (static and
spherically symmetric); (ii) scalar field in the solar system. These
two assumptions actually imply that the standard parametrized post-Newtonian
parameter . In this paper, we relax these two assumptions and study
STVG further by using the post-Newtonian (PN) approximation approach. With
abandoning the assumptions, we find in general cases of STVG.
Then, a version of modified STVG (MSTVG) is proposed through introducing a
coupling function of scalar field G: . We have derived the metric
and equations of motion (EOM) in 1PN for general matter without specific
equation of state and point masses firstly. Subsequently, the secular
periastron precession of binary pulsars in harmonic coordinates
is given. After discussing two PPN parameters ( and ) and two
Yukawa parameters ( and ), we use of four
binary pulsars data (PSR B1913+16, PSR B1534+12, PSR J0737-3039 and PSR
B2127+11C) to constrain the Yukawa parameters for MSTVG:
m and if
we fix .Comment: 39 pages, 4 figures, accepted by PR
Vainshtein Mechanism in Binary Pulsars
We compute the scalar gravitational radiation from a binary pulsar system in
the simplest model that exhibits the Vainshtein mechanism. The mechanism is
successful in screening the effect from scalar fields conformally coupled to
matter, although gravitational radiation is less suppressed relative to its
general relativity predictions than static fifth forces effects within the
pulsar system. This is due to a combination of two effects: firstly the
existence of monopole and dipole radiation; secondly the Vainshtein suppression
comes from the hierarchy of scales between the inverse frequency scale and the
Vainshtein radius, rather than the orbital radius of the pulsar system.
Extensions of these results will have direct relevance to infrared
modifications of gravity, such as massive gravity theories, which are known to
exhibit a Vainshtein mechanism. Generalization to Galileon models with higher
order interactions are likely to provide stronger constraints.Comment: minor revisions to match published version in pr
Management of imatinib-resistant CML patients
Imatinib has had marked impact on outcomes in chronic myelogenous leukemia (CML) patients for all stages of the disease and is endorsed by international treatment guidelines as the first line option. Although imatinib is highly effective and well tolerated, the development of resistance represents a clinical challenge. Since the most frequently identified mechanism of acquired imatinib resistance is bcr-abl kinase domain point mutations, periodic hematologic, cytogenetic, and molecular monitoring is critical throughout imatinib therapy. Once cytogenetic remission is achieved, residual disease can be monitored by bcr-abl transcript levels as assayed by reverse transcription polymerase chain reaction (RT-PCR). Detection of bcr-abl mutants prior to and during imatinib therapy can aid in risk stratification as well as in determining therapeutic strategies. Thus, mutation screening is indicated in patients lacking or losing hematologic response. Moreover, search for mutations should also be performed when a 3-log reduction of bcr-abl transcripts is not achieved or there is a reproducible increase of transcript levels. In patients harboring mutations which confer imatinib resistance, novel second line tyrosine kinase inhibitors have demonstrated encouraging efficacy with low toxicity. Only the T315I bcr-abl mutant has proved totally resistant to all clinically available bcr-abl inhibitors. Strategies to further increase the rates of complete molecular remissions represent the next frontier in the targeted therapy of CML patients
The Strange Prospects for Astrophysics
The implications of the formation of strange quark matter in neutron stars
and in core-collapse supernovae is discussed with special emphasis on the
possibility of having a strong first order QCD phase transition at high baryon
densities. If strange quark matter is formed in core-collapse supernovae
shortly after the bounce, it causes the launch of a second outgoing shock which
is energetic enough to lead to a explosion. A signal for the formation of
strange quark matter can be read off from the neutrino spectrum, as a second
peak in antineutrinos is released when the second shock runs over the
neutrinosphere.Comment: 10 pages, 8 figures, invited talk given at the international
conference on strangeness in quark matter (SQM2008), Beijing, October 6-10,
Beijing, China, version to appear in J. Phys.
The Evolution of PSR J0737-3039B and a Model for Relativistic Spin Precession
We present the evolution of the radio emission from the 2.8-s pulsar of the
double pulsar system PSR J0737-3039A/B. We provide an update on the Burgay et
al. (2005) analysis by describing the changes in the pulse profile and flux
density over five years of observations, culminating in the B pulsar's radio
disappearance in 2008 March. Over this time, the flux density decreases by
0.177 mJy/yr at the brightest orbital phases and the pulse profile evolves from
a single to a double peak, with a separation rate of 2.6 deg/yr. The pulse
profile changes are most likely caused by relativistic spin precession, but can
not be easily explained with a circular hollow-cone beam as in the model of
Clifton & Weisberg (2008). Relativistic spin precession, coupled with an
elliptical beam, can model the pulse profile evolution well. This particular
beam shape predicts geometrical parameters for the two bright orbital phases
which are consistent and similar to those derived by Breton et al. (2008).
However, the observed decrease in flux over time and B's eventual disappearance
cannot be easily explained by the model and may be due to the changing
influence of A on B.Comment: 20 pages, 18 figures, Accepted by ApJ on 2 August 201
The Wisconsin Plasma Astrophysics Laboratory
The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a flexible user
facility designed to study a range of astrophysically relevant plasma processes
as well as novel geometries that mimic astrophysical systems. A multi-cusp
magnetic bucket constructed from strong samarium cobalt permanent magnets now
confines a 10 m, fully ionized, magnetic-field free plasma in a spherical
geometry. Plasma parameters of to eV and
to cm provide an ideal testbed
for a range of astrophysical experiments including self-exciting dynamos,
collisionless magnetic reconnection, jet stability, stellar winds, and more.
This article describes the capabilities of WiPAL along with several
experiments, in both operating and planning stages, that illustrate the range
of possibilities for future users.Comment: 21 pages, 12 figures, 2 table
Circular Polarization in Pulsar Integrated Profiles: Updates
We update the systematic studies of circular polarization in integrated pulse
profiles by Han et al (1998). Data of circular polarization profiles are
compiled. Sense reversals can occur in core or cone components, or near the
intersection between components. The correlation between the sense of circular
polarization and the sense of position angle variation for conal-double pulsars
is confirmed with a much large database. Circular polarization of some pulsars
has clear changes with frequency. Circular polarization of millisecond pulsars
is marginally different from that of normal pulsars.Comment: 10 pages, 6 figures, accepted and will be published soon by Chinese
Journal of Astronomy and Astrophysics (ChJAA
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