18,768 research outputs found
Rotation of the cosmic microwave background polarization from weak gravitational lensing
When a cosmic microwave background (CMB) photon travels from the surface of
last scatter through spacetime metric perturbations, the polarization vector
may rotate about its direction of propagation. This gravitational rotation is
distinct from, and occurs in addition to, the lensing deflection of the photon
trajectory. This rotation can be sourced by linear vector or tensor metric
perturbations and is fully coherent with the curl deflection field. Therefore,
lensing corrections to the CMB polarization power spectra as well as the
temperature-polarization cross-correlations due to non-scalar perturbations are
modified. The rotation does not affect lensing by linear scalar perturbations,
but needs to be included when calculations go to higher orders. We present
complete results for weak lensing of the full-sky CMB power spectra by general
linear metric perturbations, taking into account both deflection of the photon
trajectory and rotation of the polarization. For the case of lensing by
gravitational waves, we show that the B modes induced by the rotation largely
cancel those induced by the curl component of deflection.Comment: 5 pages, 3 figures, revised to match the version appeared in PR
Multimessenger Parameter Estimation of GW170817
We combine gravitational wave (GW) and electromagnetic (EM) data to perform a
Bayesian parameter estimation of the binary neutron star (NS) merger GW170817.
The EM likelihood is constructed from a fit to a large number of numerical
relativity simulations which we combine with a lower bound on the mass of the
remnant's accretion disk inferred from the modeling of the EM light curve. In
comparison with previous works, our analysis yields a more precise
determination of the tidal deformability of the binary, for which the EM data
provide a lower bound, and of the mass ratio of the binary, with the EM data
favoring a smaller mass asymmetry. The 90\% credible interval for the areal
radius of a NS is found to be (statistical and systematic uncertainties).Comment: 7 pages, 3 figures, accepted to the EPJA Topical Issue: The first
Neutron Star Merger Observation - Implications for Nuclear Physic
On the exponential convergence of the Kaczmarz algorithm
The Kaczmarz algorithm (KA) is a popular method for solving a system of
linear equations. In this note we derive a new exponential convergence result
for the KA. The key allowing us to establish the new result is to rewrite the
KA in such a way that its solution path can be interpreted as the output from a
particular dynamical system. The asymptotic stability results of the
corresponding dynamical system can then be leveraged to prove exponential
convergence of the KA. The new bound is also compared to existing bounds
Probing motion of fast radio burst sources by timing strongly lensed repeaters
Given the possible repetitive nature of fast radio bursts (FRBs), their
cosmological origin, and their high occurrence, detection of strongly lensed
sources due to intervening galaxy lenses is possible with forthcoming radio
surveys. We show that if multiple images of a repeating source are resolved
with VLBI, using a method independent of lens modeling, accurate timing could
reveal non-uniform motion, either physical or apparent, of the emission spot.
This can probe the physical nature of FRBs and their surrounding environments,
constraining scenarios including orbital motion around a stellar companion if
FRBs require a compact star in a special system, and jet-medium interactions
for which the location of the emission spot may randomly vary. The high timing
precision possible for FRBs () compared to the typical time
delays between images in galaxy lensing () enables the
measurement of tiny fractional changes in the delays (), and
hence the detection of time-delay variations induced by relative motions
between the source, the lens, and the Earth. We show that uniform cosmic
peculiar velocities only cause the delay time to drift linearly, and that the
effect from the Earth's orbital motion can be accurately subtracted, thus
enabling a search for non-trivial source motion. For a timing accuracy of ms and a repetition rate (of detected bursts) per day of a
single FRB source, non-uniform displacement AU of the
emission spot perpendicular to the line of sight is detectable if repetitions
are seen over a period of hundreds of days.Comment: 21 pages, 6 figures, 1 table. New version accepted to ApJ with
abstract revised, typo corrected, and references adde
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