6,207 research outputs found
OH Maser sources in W49N: probing differential anisotropic scattering with Zeeman pairs
Our analysis of a VLBA 12-hour synthesis observations of the OH masers in
W49N has provided detailed high angular-resolution images of the maser sources,
at 1612, 1665 and 1667 MHz. The images, of several dozens of spots, reveal
anisotropic scatter broadening; with typical sizes of a few tens of
milli-arc-seconds and axial ratios between 1.5 to 3. The image position angles
oriented perpendicular to the galactic plane are interpreted in terms of
elongation of electron-density irregularities parallel to the galactic plane,
due to a similarly aligned local magnetic field. However, we find the apparent
angular sizes on the average a factor of 2.5 less than those reported by Desai
et al., indicating significantly less scattering than inferred earlier. The
average position angle of the scattered broadened images is also seen to
deviate significantly (by about 10 degrees) from that implied by the magnetic
field in the Galactic plane. More intriguingly, for a few Zeeman pairs in our
set, we find significant differences in the scatter broadened images for the
two hands of polarization, even when apparent velocity separation is less than
0.1 km/s. Here we present the details of our observations and analysis, and
discuss the interesting implications of our results for the intervening
anisotropic magneto-ionic medium, as well as a comparison with the expectations
based on earlier work.Comment: 5 pages, 3 figures, submitted to the Proceedings of the IAU Symposium
287: "Cosmic masers - from OH to H0
Small-Scale Variations of HI Spectra from Interstellar Scintillatio
I suggest that radio-wave scattering by the interstellar plasma, in
combination with subsonic gradients in the Doppler velocity of interstellar HI,
is responsible for the observed small-scale variation in HI absorption spectra
of pulsars. Velocity gradients on the order of 0.05 to 0.3 km/s across 1 AU can
produce the observed variations. I suggest observational tests to distinguish
between this model and the traditional picture of small-scale opacity
variations from cloudlets.Comment: 24 pages, 2 figures, Latex, uses AASTe
Data Driven Surrogate Based Optimization in the Problem Solving Environment WBCSim
Large scale, multidisciplinary, engineering designs are always difficult due to the complexity and dimensionality of these problems. Direct coupling between the analysis codes and the optimization routines can be prohibitively time consuming due to the complexity of the underlying simulation codes. One way of tackling this problem is by constructing computationally cheap(er) approximations of the expensive simulations, that mimic the behavior of the simulation model as closely as possible. This paper presents a data driven, surrogate based optimization algorithm that uses a trust region based sequential approximate optimization (SAO) framework and a statistical sampling approach based on design of experiment (DOE) arrays. The algorithm is implemented using techniques from two packages—SURFPACK and SHEPPACK that provide a collection of approximation algorithms to build the surrogates and three different DOE techniques—full factorial (FF), Latin hypercube sampling (LHS), and central composite design (CCD)—are used to train the surrogates. The results are compared with the optimization results obtained by directly coupling an optimizer with the simulation code. The biggest concern in using the SAO framework based on statistical sampling is the generation of the required database. As the number of design variables grows, the computational cost of generating the required database grows rapidly. A data driven approach is proposed to tackle this situation, where the trick is to run the expensive simulation if and only if a nearby data point does not exist in the cumulatively growing database. Over time the database matures and is enriched as more and more optimizations are performed. Results show that the proposed methodology dramatically reduces the total number of calls to the expensive simulation runs during the optimization process
Mapping the Dirac point in gated bilayer graphene
We have performed low temperature scanning tunneling spectroscopy
measurements on exfoliated bilayer graphene on SiO2. By varying the back gate
voltage we observed a linear shift of the Dirac point and an opening of a band
gap due to the perpendicular electric field. In addition to observing a shift
in the Dirac point, we also measured its spatial dependence using spatially
resolved scanning tunneling spectroscopy. The spatial variation of the Dirac
point was not correlated with topographic features and therefore we attribute
its shift to random charged impurities.Comment: 3 pages, 3 figure
Spatially resolved spectroscopy of monolayer graphene on SiO2
We have carried out scanning tunneling spectroscopy measurements on
exfoliated monolayer graphene on SiO to probe the correlation between its
electronic and structural properties. Maps of the local density of states are
characterized by electron and hole puddles that arise due to long range
intravalley scattering from intrinsic ripples in graphene and random charged
impurities. At low energy, we observe short range intervalley scattering which
we attribute to lattice defects. Our results demonstrate that the electronic
properties of graphene are influenced by intrinsic ripples, defects and the
underlying SiO substrate.Comment: 6 pages, 7 figures, extended versio
Birth Kick Distributions and the Spin-Kick Correlation of Young Pulsars
Evidence from pulsar wind nebula symmetry axes and radio polarization
observations suggests that pulsar motions correlate with the spin directions.
We assemble this evidence for young isolated pulsars and show how it can be
used to quantitatively constrain birth kick scenarios. We illustrate by
computing several plausible, but idealized, models where the momentum thrust is
proportional to the neutrino cooling luminosity of the proto-neutron star. Our
kick simulations include the effects of pulsar acceleration and spin-up and our
maximum likelihood comparison with the data constrains the model parameters.
The fit to the pulsar spin and velocity measurements suggests that: i) the
anisotropic momentum required amounts to ~10% of the neutrino flux, ii) while a
pre-kick spin of the star is required, the preferred magnitude is small
10-20rad/s, so that for the best-fit models iii) the bulk of the spin is
kick-induced with ~120rad/s and iv) the models suggest that the
anisotropy emerges on a timescale ~1-3s.Comment: 37 pages, 13 figures, ApJ accepte
HI Density Distribution Driven by Supernovae: A Simulation Study
We model the complex distribution of atomic hydrogen (HI) in the interstellar
medium (ISM) assuming that it is driven entirely by supernovae (SN). We develop
and assess two different models. In the first approach, the simulated volume is
randomly populated with non-overlapping voids of a range of sizes. This may
relate to a snapshot distribution of supernova-remnant voids, although somewhat
artificially constrained by the non-overlap criterion. In the second approach,
a simplified time evolution (considering momentum conservation as the only
governing constraint during interactions) is followed as SN populate the space
with the associated input mass and energy.
We describe these simulations and present our results in the form of images
of the mass and velocity distributions and the associated power spectra. The
latter are compared with trends indicated by available observations. In both
approaches, we find remarkable correspondence with the observed statistical
description of well-studied components of the ISM, wherein the spatial spectra
have been found to show significant deviations from the Kolmogorov spectrum.
One of the key indications from this study, regardless of whether or not the
SN-induced turbulence is the dominant process in the ISM, is that the apparent
non-Kolmogorov spectral characteristics (of HI and/or electron column density
across thick or thin screens) needed to explain related observations may not at
all be in conflict with the underlying turbulence (i.e. the velocity structure)
being of Kolmogorov nature. We briefly discuss the limitations of our
simulations and the various implications of our results.Comment: To appear in Astrophysical Journal. 21 pages, 6 figure
Bow Shocks from Neutron Stars: Scaling Laws and HST Observations of the Guitar Nebula
The interaction of high-velocity neutron stars with the interstellar medium
produces bow shock nebulae, where the relativistic neutron star wind is
confined by ram pressure. We present multi-wavelength observations of the
Guitar Nebula, including narrow-band H-alpha imaging with HST/WFPC2, which
resolves the head of the bow shock. The HST observations are used to fit for
the inclination of the pulsar velocity vector to the line of sight, and to
determine the combination of spindown energy loss, velocity, and ambient
density that sets the scale of the bow shock. We find that the velocity vector
is most likely in the plane of the sky. We use the Guitar Nebula and other
observed neutron star bow shocks to test scaling laws for their size and
H-alpha emission, discuss their prevalence, and present criteria for their
detectability in targeted searches. The set of H-alpha bow shocks shows
remarkable consistency, in spite of the expected variation in ambient densities
and orientations. Together, they support the assumption that a pulsar's
spindown energy losses are carried away by a relativistic wind that is
indistinguishable from being isotropic. Comparison of H-alpha bow shocks with
X-ray and nonthermal, radio-synchrotron bow shocks produced by neutron stars
indicates that the overall shape and scaling is consistent with the same
physics. It also appears that nonthermal radio emission and H-alpha emission
are mutually exclusive in the known objects and perhaps in all objects.Comment: 12 pages, 7 figures (3 degraded), submitted to ApJ; minor revisions
and updates in response to referee report. (AASTeX, includes emulateapj5 and
onecolfloat5.
Electroweak Model Independent Tests for SU(3) Symmetry in Hadronic B Decays
We study effects of new physics beyond the Standard Model on SU(3) symmetry
in charmless hadronic two body B decays. It is found that several equalities
for some of the decay amplitudes, such as , , , predicted by SU(3) symmetry in the SM are not affected by new
physics. These relations provide important electroweak model independent tests
for SU(3) symmetry in B decays.Comment: 4 pages, revte
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