322 research outputs found
Hyoid bone morphology in patients with isolated robin sequence â A case-control study utilizing 3D morphable models
Background: Abnormalities of the hyoid bone are associated with impairment of oropharyngeal functions including feeding, swallowing, and breathing. Few studies have characterized anatomic abnormalities of the hyoid in patients with Robin sequence (RS), e.g. a less mineralized and voluminous hyoid. The purpose of this study was to compare normal hyoid bone morphology and hyoid bone morphology in children with isolated RS. Methods: Three-dimensional (3D) reconstructions of the hyoid bone were obtained from CT-imaging of children with RS and unaffected controls. A 3D morphable model was constructed using Principal Component Analysis (PCA). Partial least squares â Discriminant Analysis (PLS-DA) and multivariate analysis of variance (MANOVA) were used to characterize and compare hyoid shape differences between patients with RS and an age-matched control group. Results: The study included 23 subjects with RS (mean age 9.8 ± 10.3 months) and 46 age-matched control samples. A less voluminous hyoid was observed for the RS group with a larger lateral divergence of the greater horns compared to controls (MANOVA, p-value<0.001). The first shape variable from the PLS-DA model showed a significant correlation for the observed variance between the two groups (Spearman R = â0.56, p-value<0.001). The control samples and 151 CT-scans of subjects up to age 4 years were used to create a 3D morphable model of normal hyoid shape variation (n = 197, mean age 22.1 ± 13.1 months). For the normal 3D morphable model, a high degree of allometric shape variation was observed along the first principal component. Conclusions: The 3D morphable models provide a comprehensive and quantitative description of variation in normal hyoid bone morphology, and allow detection of distinct differences between patients with isolated RS and controls.</p
Hyoid bone morphology in patients with isolated robin sequence â A case-control study utilizing 3D morphable models
Background: Abnormalities of the hyoid bone are associated with impairment of oropharyngeal functions including feeding, swallowing, and breathing. Few studies have characterized anatomic abnormalities of the hyoid in patients with Robin sequence (RS), e.g. a less mineralized and voluminous hyoid. The purpose of this study was to compare normal hyoid bone morphology and hyoid bone morphology in children with isolated RS. Methods: Three-dimensional (3D) reconstructions of the hyoid bone were obtained from CT-imaging of children with RS and unaffected controls. A 3D morphable model was constructed using Principal Component Analysis (PCA). Partial least squares â Discriminant Analysis (PLS-DA) and multivariate analysis of variance (MANOVA) were used to characterize and compare hyoid shape differences between patients with RS and an age-matched control group. Results: The study included 23 subjects with RS (mean age 9.8 ± 10.3 months) and 46 age-matched control samples. A less voluminous hyoid was observed for the RS group with a larger lateral divergence of the greater horns compared to controls (MANOVA, p-value<0.001). The first shape variable from the PLS-DA model showed a significant correlation for the observed variance between the two groups (Spearman R = â0.56, p-value<0.001). The control samples and 151 CT-scans of subjects up to age 4 years were used to create a 3D morphable model of normal hyoid shape variation (n = 197, mean age 22.1 ± 13.1 months). For the normal 3D morphable model, a high degree of allometric shape variation was observed along the first principal component. Conclusions: The 3D morphable models provide a comprehensive and quantitative description of variation in normal hyoid bone morphology, and allow detection of distinct differences between patients with isolated RS and controls.</p
Detection of 16 Gamma-Ray Pulsars Through Blind Frequency Searches Using the Fermi LAT
Pulsars are rapidly-rotating, highly-magnetized neutron stars emitting
radiation across the electromagnetic spectrum. Although there are more than
1800 known radio pulsars, until recently, only seven were observed to pulse in
gamma rays and these were all discovered at other wavelengths. The Fermi Large
Area Telescope makes it possible to pinpoint neutron stars through their
gamma-ray pulsations. We report the detection of 16 gamma-ray pulsars in blind
frequency searches using the LAT. Most of these pulsars are coincident with
previously unidentified gamma-ray sources, and many are associated with
supernova remnants. Direct detection of gamma-ray pulsars enables studies of
emission mechanisms, population statistics and the energetics of pulsar wind
nebulae and supernova remnants.Comment: Corresponding authors: Michael Dormody, Paul S. Ray, Pablo M. Saz
Parkinson, Marcus Ziegle
Fermi Large Area Telescope Bright Gamma-ray Source List
Following its launch in June 2008, the Fermi Gamma-ray Space Telescope
(Fermi) began a sky survey in August. The Large Area Telescope (LAT) on Fermi
in 3 months produced a deeper and better-resolved map of the gamma-ray sky than
any previous space mission. We present here initial results for energies above
100 MeV for the 205 most significant (statistical significance greater than
~10-sigma) gamma-ray sources in these data. These are the best-characterized
and best-localized point-like (i.e., spatially unresolved) gamma-ray sources in
the early-mission data.Comment: Accepted by ApJS. Many helpful comments by referee incorporated 57
pages, 12 figure
Fermi Large Area Telescope observations of the Vela-X Pulsar Wind Nebula
We report on gamma-ray observations in the off-pulse window of the Vela
pulsar PSR B0833-45, using 11 months of survey data from the Fermi Large Area
Telescope (LAT). This pulsar is located in the 8 degree diameter Vela supernova
remnant, which contains several regions of non-thermal emission detected in the
radio, X-ray and gamma-ray bands. The gamma-ray emission detected by the LAT
lies within one of these regions, the 2*3 degrees area south of the pulsar
known as Vela-X. The LAT flux is signicantly spatially extended with a best-fit
radius of 0.88 +/- 0.12 degrees for an assumed radially symmetric uniform disk.
The 200 MeV to 20 GeV LAT spectrum of this source is well described by a
power-law with a spectral index of 2.41 +/- 0.09 +/- 0.15 and integral flux
above 100 MeV of (4.73 +/- 0.63 +/- 1.32) * 10^{-7} cm^{-2} s^{-1}. The first
errors represent the statistical error on the fit parameters, while the second
ones are the systematic uncertainties. Detailed morphological and spectral
analyses give strong constraints on the energetics and magnetic field of the
pulsar wind nebula (PWN) system and favor a scenario with two distinct electron
populations.Comment: 21 pages, 5 figures, accepted for publication in Astrophysical
Journa
PSR J1907+0602: A Radio-Faint Gamma-Ray Pulsar Powering a Bright TeV Pulsar Wind Nebula
We present multiwavelength studies of the 106.6 ms gamma-ray pulsar PSR
J1907+06 near the TeV source MGRO J1908+06. Timing observations with Fermi
result in a precise position determination for the pulsar of R.A. =
19h07m547(2), decl. = +06:02:16(2) placing the pulsar firmly within the TeV
source extent, suggesting the TeV source is the pulsar wind nebula of PSR
J1907+0602. Pulsed gamma-ray emission is clearly visible at energies from 100
MeV to above 10 GeV. The phase-averaged power-law index in the energy range E >
0.1 GeV is = 1.76 \pm 0.05 with an exponential cutoff energy E_{c} = 3.6 \pm
0.5 GeV. We present the energy-dependent gamma-ray pulsed light curve as well
as limits on off-pulse emission associated with the TeV source. We also report
the detection of very faint (flux density of ~3.4 microJy) radio pulsations
with the Arecibo telescope at 1.5 GHz having a dispersion measure DM = 82.1 \pm
1.1 cm^{-3}pc. This indicates a distance of 3.2 \pm 0.6 kpc and a
pseudo-luminosity of L_{1400} ~ 0.035 mJy kpc^2. A Chandra ACIS observation
revealed an absorbed, possibly extended, compact <(4 arcsec) X-ray source with
significant non-thermal emission at R.A. = 19h07m54.76, decl. = +06:02:14.6
with a flux of 2.3^{+0.6}_{-1.4} X 10^{-14} erg cm^{-2} s^{-1}. From archival
ASCA observations, we place upper limits on any arcminute scale 2--10 keV X-ray
emission of ~ 1 X 10^{-13} erg cm^{-2} s^{-1}. The implied distance to the
pulsar is compatible with that of the supernova remnant G40.5-0.5, located on
the far side of the TeV nebula from PSR J1907+0602, and the S74 molecular cloud
on the nearer side which we discuss as potential birth sites
Fermi Large Area Telescope Observations of the Crab Pulsar and Nebula
We report on gamma-ray observations of the Crab Pulsar and Nebula using 8
months of survey data with the Fermi Large Area Telescope (LAT). The high
quality light curve obtained using the ephemeris provided by the Nancay and
Jodrell Bank radio telescopes shows two main peaks stable in phase with energy.
The first gamma-ray peak leads the radio main pulse by (281 \pm 12 \pm 21) mus,
giving new constraints on the production site of non-thermal emission in pulsar
magnetospheres. The improved sensitivity and the unprecedented statistics
afforded by the LAT enable precise measurement of the Crab Pulsar spectral
parameters: cut-off energy at E_c = (5.8 \pm 0.5 \pm 1.2) GeV, spectral index
of Gamma = (1.97 \pm 0.02 \pm 0.06) and integral photon flux above 100 MeV of
(2.09 \pm 0.03 \pm 0.18) x 10^{-6} cm^{-2} s^{-1}. The first errors represent
the statistical error on the fit parameters, while the second ones are the
systematic uncertainties. Pulsed gamma-ray photons are observed up to ~ 20 GeV
which precludes emission near the stellar surface, below altitudes of around 4
to 5 stellar radii in phase intervals encompassing the two main peaks. The
spectrum of the nebula in the energy range 100 MeV - 300 GeV is well described
by the sum of two power-laws of indices Gamma_{sync} = (3.99 \pm 0.12 \pm 0.08)
and Gamma_{IC} = (1.64 \pm 0.05 \pm 0.07), corresponding to the falling edge of
the synchrotron and the rising edge of the inverse Compton components,
respectively. This latter, which links up naturally with the spectral data
points of Cherenkov experiments, is well reproduced via inverse Compton
scattering from standard Magnetohydrodynamics (MHD) nebula models, and does not
require any additional radiation mechanism.Comment: 17 pages, 9 figures, Accepted for publications in Astrophysical
Journa
Fermi-LAT Search for Pulsar Wind Nebulae around gamma-ray Pulsars
The high sensitivity of the Fermi-LAT (Large Area Telescope) offers the first
opportunity to study faint and extended GeV sources such as pulsar wind nebulae
(PWNe). After one year of observation the LAT detected and identified three
pulsar wind nebulae: the Crab Nebula, Vela-X and the PWN inside MSH 15-52. In
the meantime, the list of LAT detected pulsars increased steadily. These
pulsars are characterized by high energy loss rates from ~3 \times 10^{33} erg
s to 5 \times 10 erg s and are therefore likely to power a
PWN. This paper summarizes the search for PWNe in the off-pulse windows of 54
LAT-detected pulsars using 16 months of survey observations. Ten sources show
significant emission, seven of these likely being of magnetospheric origin. The
detection of significant emission in the off-pulse interval offers new
constraints on the gamma-ray emitting regions in pulsar magnetospheres. The
three other sources with significant emission are the Crab Nebula, Vela-X and a
new pulsar wind nebula candidate associated with the LAT pulsar PSR J1023-5746,
coincident with the TeV source HESS J1023-575. We further explore the
association between the H.E.S.S. and the Fermi source by modeling its spectral
energy distribution. Flux upper limits derived for the 44 remaining sources are
used to provide new constraints on famous PWNe that have been detected at keV
and/or TeV energies.Comment: Accepted for publication in Astrophysical Journal, 42 pages, 17
figure
Fermi LAT observations of the Geminga pulsar
We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the
second brightest non-variable GeV source in the -ray sky and the first
example of a radio-quiet -ray pulsar. The observations cover one year,
from the launch of the satellite through 2009 June 15. A data sample of
over 60,000 photons enabled us to build a timing solution based solely on
rays. Timing analysis shows two prominent peaks, separated by = 0.497 0.004 in phase, which narrow with increasing energy. Pulsed
rays are observed beyond 18 GeV, precluding emission below 2.7 stellar
radii because of magnetic absorption. The phase-averaged spectrum was fitted
with a power law with exponential cut-off of spectral index = (1.30
0.01 0.04), cut-off energy = (2.46 0.04 0.17)
GeV and an integral photon flux above 0.1 GeV of (4.14 0.02 0.32)
10 cm s. The first uncertainties are statistical
and the second are systematic. The phase-resolved spectroscopy shows a clear
evolution of the spectral parameters, with the spectral index reaching a
minimum value just before the leading peak and the cut-off energy having maxima
around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is
present at all rotational phases. The spectral shape, broad pulse profile, and
maximum photon energy favor the outer magnetospheric emission scenarios.Comment: 32 pages, 12 figures, 3 tables. Accepted for publication in The
Astrophysical Journal. Corresponding authors: Denis Dumora
([email protected]), Fabio Gargano ([email protected]),
Massimiliano Razzano ([email protected]
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