210 research outputs found
The 100-month Swift catalogue of supergiant fast X-ray transients I. BAT on-board and transient monitor flares
We investigate the characteristics of bright flares for a sample of
supergiant fast X-ray transients and their relation to the orbital phase. We
have retrieved all Swift/BAT Transient Monitor light curves, and collected all
detections in excess of from both daily- and orbital-averaged light
curves in the time range of 2005-Feb-12 to 2013-May-31. We also considered all
on-board detections as recorded in the same time span and selected those within
4 arcmin of each source in our sample and in excess of . We present a
catalogue of over a thousand BAT flares from 11 SFXTs, down to 15-150keV fluxes
of erg cm s (daily timescale) and
erg cm s (orbital timescale, averaging
s) and spanning 100 months. The great majority of these flares are
unpublished. This population is characterized by short (a few hundred seconds)
and relatively bright (in excess of 100mCrab, 15-50keV) events. In the hard
X-ray, these flares last in general much less than a day. Clustering of hard
X-ray flares can be used to indirectly measure the length of an outburst, even
when the low-level emission is not detected. We construct the distributions of
flares, of their significance (in terms of sigma) and their flux as a function
of orbital phase, to infer the properties of these binary systems. In
particular, we observe a trend of clustering of flares at some phases as
increases, as consistent with a progression from tight, circular
or mildly eccentric orbits at short periods, to wider and more eccentric orbits
at longer orbital periods. Finally, we estimate the expected number of flares
for a given source for our limiting flux and provide the recipe for calculating
them for the limiting flux of future hard X-ray observatories. (Abridged).Comment: Accepted for publication in Astronomy and Astrophysics. 23 pages, 8
figures. Full catalog files will be available at CDS and at
http://www.ifc.inaf.it/sfxt/ Fixed typos and updated reference
Searching for supergiant fast X-ray transients with Swift
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs)
hosting a neutron star and an OB supergiant companion. We examine the available
Swift data, as well as other new or archival/serendipitous data, on three
sources: IGR J17407-2808, 2XMM J185114.3-000004, and IGR J18175-2419, whose
X-ray characteristics qualify them as candidate SFXT, in order to explore their
properties and test whether they are consistent with an SFXT nature. As IGR
J17407-2808 and 2XMM J185114.3-000004 triggered the Burst Alert Telescope on
board Swift, the Swift data allow us to provide their first arcsecond
localisations, leading to an unequivocal identification of the source CXOU
J174042.0-280724 as the soft X-ray counterpart of IGR J17407-2808, as well as
their first broadband spectra, which can be fit with models generally
describing accreting neutron stars in HMXBs. While still lacking optical
spectroscopy to assess the spectral type of the companion, we propose 2XMM
J185114.3-000004 as a very strong SFXT candidate. The nature of IGR J17407-2808
remains, instead, more uncertain. Its broad band properties cannot exclude that
the emission originates from either a HMXB (and in that case, a SFXT) or, more
likely, a low mass X-ray binary. Finally, based on the deep non-detection in
our XRT monitoring campaign and a careful reanalysis of the original Integral
data in which the discovery of the source was first reported, we show that IGR
J18175-2419 is likely a spurious detection.Comment: Accepted for publication in Astronomy and Astrophysics. 12 pages, 11
figures, 6 table
Swift/XRT monitoring of the Supergiant Fast X-ray Transient IGR J18483-0311 for an entire orbital period
IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging
to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray
Transients. This system is one of the two members of this class, together with
IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are
known. We report on the first complete monitoring of the X-ray activity along
an entire orbital period of a Supergiant Fast X-ray Transient. These Swift
observations, lasting 28d, cover more than one entire orbital phase
consecutively. They are a unique data-set, which allows us to constrain the
different mechanisms proposed to explain the nature of this new class of X-ray
transients. We applied the new clumpy wind model for blue supergiants developed
by Ducci et al. (2009), to the observed X-ray light curve. Assuming an
eccentricity of e=0.4, the X-ray emission from this source can be explained in
terms of the accretion from a spherically symmetric clumpy wind, composed of
clumps with different masses, ranging from 10^{18}g to 5x 10^{21}g.Comment: Accepted for publication in MNRAS. 7 pages, 7 figures, 2 table
Monitoring Supergiant Fast X-ray Transients with Swift. Results from the first year
Swift has allowed the possibility to give Supergiant Fast X-ray Transients
(SFXTs), the new class of High Mass X-ray Binaries discovered by INTEGRAL, non
serendipitous attention throughout all phases of their life. We present our
results based on the first year of intense Swift monitoring of four SFXTs, IGR
J16479-4514, XTE J1739-302, IGR J17544-2619 and AX J1841.0-0536. We obtain the
first assessment of how long each source spends in each state using a
systematic monitoring with a sensitive instrument. The duty-cycle of inactivity
is 17, 28, 39, 55% (5% uncertainty), for IGR J16479-4514, AX J1841.0-0536, XTE
J1739-302, and IGR J17544-2619, respectively, so that true quiescence is a rare
state. This demonstrates that these transients accrete matter throughout their
life at different rates. AX J1841.0-0536 is the only source which has not
undergone a bright outburst during our campaign. Although individual sources
behave somewhat differently, common X-ray characteristics of this class are
emerging such as outburst lengths well in excess of hours, with a multiple
peaked structure. A high dynamic range (including bright outbursts) of 4 orders
of magnitude has been observed. We performed out-of-outburst intensity-based
spectroscopy. Spectral fits with an absorbed blackbody always result in
blackbody radii of a few hundred meters, consistent with being emitted from a
small portion of the neutron star surface, very likely the neutron star polar
caps. We also present the UVOT data of these sources. (Abridged)Comment: Accepted for publication in MNRAS. 20 pages, 9 figures, 8 table
The Swift Supergiant Fast X-Ray Transients Project:
We present a review of the Supergiant Fast X-ray Transients (SFXT) Project, a systematic investigation of the properties of SFXTs with a strategy that combines Swift monitoring programs with outburst follow-up observations. This strategy has quickly tripled the available sets of broad-band data of SFXT outbursts, and gathered a wealth of out-of-outburst data, which have led us to a broad-band spectral characterization, an assessment of the fraction of the time these sources spend in each phase, and their duty cycle of inactivity. We present some new observational results obtained through our outburst follow-ups, as fitting examples of the exceptional capabilities of Swift in catching bright flares and monitor them panchromatically
Variability of myocardial perfusion dark rim Gibbs artifacts due to sub-pixel shifts
<p>Abstract</p> <p>Background</p> <p>Gibbs ringing has been shown as a possible source of dark rim artifacts in myocardial perfusion studies. This type of artifact is usually described as transient, lasting a few heart beats, and localised in random segments of the myocardial wall. Dark rim artifacts are known to be unpredictably variable. This article aims to illustrate that a sub-pixel shift, i.e. a small displacement of the pixels with respect to the endocardial border, can result in different Gibbs ringing and hence different artifacts. Therefore a hypothesis for one cause of dark rim artifact variability is given based on the sub-pixel position of the endocardial border. This article also demonstrates the consequences for Gibbs artifacts when two different methods of image interpolation are applied (post-FFT interpolation, and pre-FFT zero-filling).</p> <p>Results</p> <p>Sub-pixel shifting of <it>in vivo </it>perfusion studies was shown to change the appearance of Gibbs artifacts. This effect was visible in the original uninterpolated images, and in the post-FFT interpolated images. The same shifted data interpolated by pre-FFT zero-filling exhibited much less variability in the Gibbs artifact. The <it>in vivo </it>findings were confirmed by phantom imaging and numerical simulations.</p> <p>Conclusion</p> <p>Unless pre-FFT zero-filling interpolation is performed, Gibbs artifacts are very dependent on the position of the subendocardial wall within the pixel. By introducing sub-pixel shifts relative to the endocardial border, some of the variability of the dark rim artifacts in different myocardial segments, in different patients and from frame to frame during first-pass perfusion due to cardiac and respiratory motion can be explained. Image interpolation by zero-filling can be used to minimize this dependency.</p
Polarisation Patterns and Vectorial Defects in Type II Optical Parametric Oscillators
Previous studies of lasers and nonlinear resonators have revealed that the
polarisation degree of freedom allows for the formation of polarisation
patterns and novel localized structures, such as vectorial defects. Type II
optical parametric oscillators are characterised by the fact that the
down-converted beams are emitted in orthogonal polarisations. In this paper we
show the results of the study of pattern and defect formation and dynamics in a
Type II degenerate optical parametric oscillator for which the pump field is
not resonated in the cavity. We find that traveling waves are the predominant
solutions and that the defects are vectorial dislocations which appear at the
boundaries of the regions where traveling waves of different phase or
wave-vector orientation are formed. A dislocation is defined by two topological
charges, one associated with the phase and another with the wave-vector
orientation. We also show how to stabilize a single defect in a realistic
experimental situation. The effects of phase mismatch of nonlinear interaction
are finally considered.Comment: 38 pages, including 15 figures, LATeX. Related material, including
movies, can be obtained from
http://www.imedea.uib.es/Nonlinear/research_topics/OPO
Women physicians in cardiovascular magnetic resonance: past, present, and future
Women's engagement in medicine, and more specifically cardiovascular imaging and cardiovascular MRI (CMR), has undergone a slow evolution over the past several decades. As a result, an increasing number of women have joined the cardiovascular imaging community to contribute their expertise. This collaborative work summarizes the barriers that women in cardiovascular imaging have overcome over the past several years, the positive interventions that have been implemented to better support women in the field of CMR, and the challenges that still remain, with a special emphasis on women physicians
Commoning mobility:Towards a new politics of mobility transitions
Scholars have argued that transitions to more sustainable and just mobilities require
moving beyond technocentrism to rethink the very meaning of mobility in cities,
communities, and societies. This paper demonstrates that such rethinking is inherently
political. In particular, we focus on recent theorisations of commoning practices
that have gained traction in geographic literatures. Drawing on our global
comparative research of low‐carbon mobility transitions, we argue that critical
mobilities scholars can rethink and expand the understanding of mobility through
engagement with commons–enclosure thinking. We present a new concept, “commoning
mobility,” a theorisation that both envisions and shapes practices that
develop fairer and greener mobilities and more inclusive, collaboratively governed
societies. Our analysis introduces three “logics” of mobility transition projects. First,
the paper discusses how a logic of scarcity has been a driver for mobility planning
as the scarcity of oil, finance, space, and time are invoked across the world as stimuli
for aspiring to greener, “smarter,” and cheaper mobilities. The paper then identifies
two responses to the logic of scarcity: the logics of austerity and the logics of commoning.
Austere mobilities are examined to problematise the distribution of responsibility
for emissions and ensuing injustices and exclusion in low‐carbon transitions.
The logics of commoning shows a potential to reassess mobility not only as an individual
freedom but also as a collective good, paving the way for fairer mobility transitions
and a collaborative tackling of sustainable mobility challenges.info:eu-repo/semantics/publishedVersio
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