1,645 research outputs found
Operated by Universities Research Association Inc. under contract with the United States Department of Energy Low Tempera
Probing large-scale wind structures in Vela X-1 using off-states with INTEGRAL
Vela X-1 is the prototype of the class of wind-fed accreting pulsars in high
mass X-ray binaries hosting a supergiant donor. We have analyzed in a
systematic way ten years of INTEGRAL data of Vela X-1 (22-50 keV) and we found
that when outside the X-ray eclipse, the source undergoes several luminosity
drops where the hard X-rays luminosity goes below 3x10^35 erg/s, becoming
undetected by INTEGRAL. These drops in the X-ray flux are usually referred to
as "off-states" in the literature. We have investigated the distribution of
these off-states along the Vela X-1 ~8.9 d orbit, finding that their orbital
occurrence displays an asymmetric distribution, with a higher probability to
observe an off-state near the pre-eclipse than during the post-eclipse. This
asymmetry can be explained by scattering of hard X-rays in a region of ionized
wind, able to reduce the source hard X-ray brightness preferentially near
eclipse ingress. We associate this ionized large-scale wind structure with the
photoionization wake produced by the interaction of the supergiant wind with
the X-ray emission from the neutron star. We emphasize that this observational
result could be obtained thanks to the accumulation of a decade of INTEGRAL
data, with observations covering the whole orbit several times, allowing us to
detect an asymmetric pattern in the orbital distribution of off-states in Vela
X-1.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society (5 pages, 3 figures). A few typos fixed to match the published
versio
Information Technology and Sustained Competitive Advantage: A Resource-Based Analysis
This is the published version. Copyright 1995 MIS Quarterly.The concept of IT as a powerful competitive weapon has been strongly emphasized in the literature, yet the sustainability of the competitive advantage provided by IT applications is not well-explained. This work discusses the resource-based theory as a means of analyzing sustainability and develops a model founded on this resource-based view of the firm. This model is then applied to four attributes of IT -- capital requirements, proprietary technology, technical IT skills, and managerial IT skills -- which might be sources of sustained competitive advantage. From this resource-based analysis, we conclude that managerial IT skills is the only one of these attributes that can provide sustainability
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
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&
Variability in high-mass X-ray binaries
Strongly magnetized, accreting neutron stars show periodic and aperiodic
variability over a wide range of time scales. By obtaining spectral and timing
information on these different time scales, we can have a closer look into the
physics of accretion close to the neutron star and the properties of the
accreted material. One of the most prominent time scales is the strong
pulsation, i.e., the rotation period of the neutron star itself. Over one
rotation, our view of the accretion column and the X-ray producing region
changes significantly. This allows us to sample different physical conditions
within the column but at the same time requires that we have
viewing-angle-resolved models to properly describe them. In wind-fed high-mass
X-ray binaries, the main source of aperiodic variability is the clumpy stellar
wind, which leads to changes in the accretion rate (i.e., luminosity) as well
as absorption column. This variability allows us to study the behavior of the
accretion column as a function of luminosity, as well as to investigate the
structure and physical properties of the wind, which we can compare to winds in
isolated stars.Comment: 6 pages, 4 figures, accepted for publication in Astronomische
Nachrichten (proceedings of the XMM-Newton Workshop 2019
3-D GRMHD and GRPIC Simulations of Disk-Jet Coupling and Emission
We investigate jet formation in black-hole systems using 3-D General
Relativistic Particle-In-Cell (GRPIC) and 3-D GRMHD simulations. GRPIC
simulations, which allow charge separations in a collisionless plasma, do not
need to invoke the frozen condition as in GRMHD simulations. 3-D GRPIC
simulations show that jets are launched from Kerr black holes as in 3-D GRMHD
simulations, but jet formation in the two cases may not be identical.
Comparative study of black hole systems with GRPIC and GRMHD simulations with
the inclusion of radiate transfer will further clarify the mechanisms that
drive the evolution of disk-jet systems.Comment: 3 pages, 1 figure, Proceedings of the Eleventh Marcel Grossmann
Meeting on General Relativity, edited by H. Kleinert, R.T. Jantzen and R.
Ruffini, World Scientific, Singapore, 200
Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase
We present broad-band, multi-epoch X-ray spectroscopy of the pulsating
ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and
NuSTAR data from 2014 are best described by a multi-color black-body model with
a temperature gradient as a function of accretion disk radius significantly
flatter than expected for a standard thin accretion disk (T(r) ~ r^{-p}, with
p=0.608^{+0.014}_{-0.012}). Additionally, we detect a hard power-law tail at
energies above 10 keV, which we interpret as being due to Comptonization. We
compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from
2003, 2012, and 2013, and investigate possible spectral changes as a function
of phase over the 78d super-orbital period of this source. We find that
observations taken around phases 0.3-0.4 show very similar temperature
profiles, even though the observed flux varies significantly, while one
observation taken around phase 0 has a significantly steeper profile. We
discuss these findings in light of the recent discovery that the compact object
is a neutron star and show that precession of the accretion disk or the neutron
star can self-consistently explain most observed phenomena.Comment: 7 pages, 5 figures, submitted to ApJ; comments welcom
On the contribution of ULXs to stellar feedback: an intermediate mass black hole candidate and the population of ULXs in the low-metallicity starburst galaxy ESO 338-4
X-ray radiation from accreting compact objects is an important part of
stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous
starburst during the last 40 Myr and contains some of the most massive super
star clusters in the nearby Universe. Given its starburst age and its
star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures
of neutron stars and black holes, hence providing an excellent laboratory for
feedback studies. We compared X-ray images and spectra obtained by XMM-Newton
and Chandra telescopes with integral field spectroscopic VLT MUSE observations
in the optical to constrain the nature of strong X-ray emitters. X-ray
observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The
brightest among them, ESO 338~X-1, has X-ray luminosity in excess of 10^{40}
erg/s. We speculate that ESO 338-4 is powered by accretion on an
intermediate-mass (~300Msun) black hole. We show that X-ray radiation from ULXs
and hot superbubbles strongly contributes to HeII ionization and general
stellar feedback in this template starburst galaxy.Comment: A&A, in pres
- …