1,070 research outputs found
A Clumpy Stellar Wind and Luminosity-Dependent Cyclotron Line Revealed by The First Suzaku Observation of the High-Mass X-ray Binary 4U 1538-522
We present results from the first Suzaku observation of the high-mass X-ray
binary 4U 1538-522. The broad-band spectral coverage of Suzaku allows for a
detailed spectral analysis, characterizing the cyclotron resonance scattering
feature at keV and the iron K line at
keV, as well as placing limits on the strengths of the iron K line and
the iron K edge. We track the evolution of the spectral parameters both in time
and in luminosity, notably finding a significant positive correlation between
cyclotron line energy and luminosity. A dip and spike in the lightcurve is
shown to be associated with an order-of-magnitude increase in column density
along the line of sight, as well as significant variation in the underlying
continuum, implying the accretion of a overdense region of a clumpy stellar
wind. We also present a phase-resolved analysis, with most spectral parameters
of interest showing significant variation with phase. Notably, both the
cyclotron line energy and the iron K line intensity vary significantly
with phase, with the iron line intensity significantly out-of-phase with the
pulse profile. We discuss the implications of these findings in the context of
recent work in the areas of accretion column physics and cyclotron resonance
scattering feature formation.Comment: 15 pages, 8 figures. Accepted to ApJ on 2 July 201
A Study of the 20 Day Superorbital Modulation in the High-Mass X-ray Binary IGR J16493-4348
We report on Nuclear Spectroscopic Telescope Array (NuSTAR), Neil Gehrels
Swift Observatory (Swift) X-ray Telescope (XRT) and Swift Burst Alert Telescope
(BAT) observations of IGR J16493-4348, a wind-fed Supergiant X-ray Binary
(SGXB) showing significant superorbital variability. From a discrete Fourier
transform of the BAT light curve, we refine its superorbital period to be
20.058 0.007 days. The BAT dynamic power spectrum and a fractional root
mean square analysis both show strong variations in the amplitude of the
superorbital modulation, but no observed changes in the period were found. The
superorbital modulation is significantly weaker between MJD 55,700 and MJD
56,300. The joint NuSTAR and XRT observations, which were performed near the
minimum and maximum of one cycle of the 20 day superorbital modulation, show
that the flux increases by more than a factor of two between superorbital
minimum and maximum. We find no significant changes in the 3-50 keV pulse
profiles between superorbital minimum and maximum, which suggests a similar
accretion regime. Modeling the pulse-phase averaged spectra we find a possible
Fe K emission line at 6.4 keV at superorbital maximum. The feature is
not significant at superorbital minimum. While we do not observe any
significant differences between the pulse-phase averaged spectral continua
apart from the overall flux change, we find that the hardness ratio near the
broad main peak of the pulse profile increases from superorbital minimum to
maximum. This suggests the spectral shape hardens with increasing luminosity.
We discuss different mechanisms that might drive the observed superorbital
modulation.Comment: 17 pages, 14 figures, 3 tables, accepted for publication in The
Astrophysical Journal on 2019 May 1
UV Exposed Optical Fibers with Frequency Domain Reflectometry for Device Tracking in Intra-Arterial Procedures
Shape tracking of medical devices using strain sensing properties in optical
fibers has seen increased attention in recent years. In this paper, we propose
a novel guidance system for intra-arterial procedures using a distributed
strain sensing device based on optical frequency domain reflectometry (OFDR) to
track the shape of a catheter. Tracking enhancement is provided by exposing a
fiber triplet to a focused ultraviolet beam, producing high scattering
properties. Contrary to typical quasi-distributed strain sensors, we propose a
truly distributed strain sensing approach, which allows to reconstruct a fiber
triplet in real-time. A 3D roadmap of the hepatic anatomy integrated with a 4D
MR imaging sequence allows to navigate the catheter within the
pre-interventional anatomy, and map the blood flow velocities in the arterial
tree. We employed Riemannian anisotropic heat kernels to map the sensed data to
the pre-interventional model. Experiments in synthetic phantoms and an in vivo
model are presented. Results show that the tracking accuracy is suitable for
interventional tracking applications, with a mean 3D shape reconstruction
errors of 1.6 +/- 0.3 mm. This study demonstrates the promising potential of
MR-compatible UV-exposed OFDR optical fibers for non-ionizing device guidance
in intra-arterial procedures
Dilatation of Lateral Ventricles with Brain Volumes in Infants with 3D Transfontanelle US
Ultrasound (US) can be used to assess brain development in newborns, as MRI
is challenging due to immobilization issues, and may require sedation.
Dilatation of the lateral ventricles in the brain is a risk factor for poorer
neurodevelopment outcomes in infants. Hence, 3D US has the ability to assess
the volume of the lateral ventricles similar to clinically standard MRI, but
manual segmentation is time consuming. The objective of this study is to
develop an approach quantifying the ratio of lateral ventricular dilatation
with respect to total brain volume using 3D US, which can assess the severity
of macrocephaly. Automatic segmentation of the lateral ventricles is achieved
with a multi-atlas deformable registration approach using locally linear
correlation metrics for US-MRI fusion, followed by a refinement step using
deformable mesh models. Total brain volume is estimated using a 3D ellipsoid
modeling approach. Validation was performed on a cohort of 12 infants, ranging
from 2 to 8.5 months old, where 3D US and MRI were used to compare brain
volumes and segmented lateral ventricles. Automatically extracted volumes from
3D US show a high correlation and no statistically significant difference when
compared to ground truth measurements. Differences in volume ratios was 6.0 +/-
4.8% compared to MRI, while lateral ventricular segmentation yielded a mean
Dice coefficient of 70.8 +/- 3.6% and a mean absolute distance (MAD) of 0.88
+/- 0.2mm, demonstrating the clinical benefit of this tool in paediatric
ultrasound
Multiple cyclotron line-forming regions in GX 301-2
We present two observations of the high-mass X-ray binary GX 301-2 with
NuSTAR, taken at different orbital phases and different luminosities. We find
that the continuum is well described by typical phenomenological models, like a
very strongly absorbed NPEX model. However, for a statistically acceptable
description of the hard X-ray spectrum we require two cyclotron resonant
scattering features (CRSF), one at ~35 keV and the other at ~50 keV. Even
though both features strongly overlap, the good resolution and sensitivity of
NuSTAR allows us to disentangle them at >=99.9% significance. This is the first
time that two CRSFs are seen in GX 301-2. We find that the CRSFs are very
likely independently formed, as their energies are not harmonically related
and, if it were a single line, the deviation from a Gaussian shape would be
very large. We compare our results to archival Suzaku data and find that our
model also provides a good fit to those data. We study the behavior of the
continuum as well as the CRSF parameters as function of pulse phase in seven
phase bins. We find that the energy of the 35 keV CRSF varies smoothly as
function of phase, between 30-38 keV. To explain this variation, we apply a
simple model of the accretion column, taking the altitude of the line-forming
region, the velocity of the in-falling material, and the resulting relativistic
effects into account. We find that in this model the observed energy variation
can be explained simply due to a variation of the projected velocity and
beaming factor of the line forming region towards us.Comment: 18 pages, 10 figures, accepted for publication in A&
A 1.4 GHz radio continuum and polarization survey at medium Galactic latitudes: I. Observation and reduction technique
A radio continuum survey at medium Galactic latitudes with the Effelsberg
100-m telescope is being carried out at a centre frequency of 1.4 GHz in total
power and linear polarization. Areas up to +/- 20 degree of Galactic latitude
are now being observed at a sensitivity of 15 mK TB total intensity and 8 mK TB
in linear polarization with an angular resolution of 9'35. This paper describes
the observing and reduction technique applied which results in absolutely
calibrated maps. The methods are illustrated by examples of images from the
survey.Comment: 12 pages, 9 figures, Accepted for publication in Astronomy and
Astrophysics Supp. Se
Investigating the superorbital modulations in 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 with Swift XRT, BAT and NuSTAR observations
A puzzling variety of superorbital modulations have been discovered in
several supergiant High-Mass X-ray binaries (sgHMXBs). To investigate the
mechanisms driving these superorbital modulations, we have analyzed long-term
Neil Gehrels Swift Observatory (Swift) Burst Alert Telescope (BAT) observations
of three sgHMXBs: 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 and
constructed their dynamic power spectra and superorbital intensity profiles.
These Swift BAT observations are complemented by pointed Swift X-ray Telescope
(XRT) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations performed
near the predicted maximum and minimum phase of a single superorbital cycle for
each of these sources. The BAT dynamic power spectra show changes in the
strength of the superorbital modulation on timescales of years, with either the
peak at the fundamental frequency and/or the second harmonic present at
different times for all three sources. The pointed Swift XRT and NuSTAR
observations show no significant differences between the pulse profiles and
spectral parameters at the superorbital maximum and minimum phase. This is
likely due to the fact the superorbital modulation had weakened significantly
during the times when the NuSTAR observations were carried out for all three
sources. The results from the Swift XRT, BAT and NuSTAR analysis indicate the
possible presence of multiple co-rotating interaction regions (CIRs) in the
stellar winds of the supergiant stars, although a structured stellar wind from
the supergiant star due to tidal oscillations cannot be ruled out.Comment: Accepted for publication in the Astrophysical Journa
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