3,871 research outputs found
Learning Transferable Architectures for Scalable Image Recognition
Developing neural network image classification models often requires
significant architecture engineering. In this paper, we study a method to learn
the model architectures directly on the dataset of interest. As this approach
is expensive when the dataset is large, we propose to search for an
architectural building block on a small dataset and then transfer the block to
a larger dataset. The key contribution of this work is the design of a new
search space (the "NASNet search space") which enables transferability. In our
experiments, we search for the best convolutional layer (or "cell") on the
CIFAR-10 dataset and then apply this cell to the ImageNet dataset by stacking
together more copies of this cell, each with their own parameters to design a
convolutional architecture, named "NASNet architecture". We also introduce a
new regularization technique called ScheduledDropPath that significantly
improves generalization in the NASNet models. On CIFAR-10 itself, NASNet
achieves 2.4% error rate, which is state-of-the-art. On ImageNet, NASNet
achieves, among the published works, state-of-the-art accuracy of 82.7% top-1
and 96.2% top-5 on ImageNet. Our model is 1.2% better in top-1 accuracy than
the best human-invented architectures while having 9 billion fewer FLOPS - a
reduction of 28% in computational demand from the previous state-of-the-art
model. When evaluated at different levels of computational cost, accuracies of
NASNets exceed those of the state-of-the-art human-designed models. For
instance, a small version of NASNet also achieves 74% top-1 accuracy, which is
3.1% better than equivalently-sized, state-of-the-art models for mobile
platforms. Finally, the learned features by NASNet used with the Faster-RCNN
framework surpass state-of-the-art by 4.0% achieving 43.1% mAP on the COCO
dataset
Constraining the equation of state of supra-nuclear dense matter from XMM-Newton observations of neutron stars in globular clusters
We report on the detailed modelling of the X-ray spectra of three likely
neutron stars. The neutron stars, observed with XMM-Newton are found in three
quiescent X-ray binaries in the globular clusters: omega Cen, M 13 and NGC
2808. Whether they are accreting at very low rates or radiating energy from an
accretion heated core, their X-ray spectra are expected to be those of a
hydrogen atmosphere. We use and compare publicly available hydrogen atmosphere
models, with constant and varying surface gravities to constrain the masses and
radii of the neutron stars. Thanks to the high XMM-Newton throughput, and the
accurate distances available for these clusters, using the latest science
analysis software release and calibration of the XMM-Newton EPIC cameras, we
derive the most stringent constraints on the masses and radii of the neutron
stars obtained to date from these systems. A comparison of the models indicate
that previously used hydrogen atmosphere models (assuming constant surface
gravity) tend to underestimate the mass and overestimate the radius of neutron
stars. Our data constrain the allowed equations of state to those which concern
normal nucleonic matter and one possible strange quark matter model, thus
constraining radii to be from 8 km and masses up to 2.4 M.Comment: 10 pages, 8 figures, accepted to be published in The Astrophysical
Journa
Hard X-ray Bursts Recorded by the IBIS Telescope of the INTEGRAL Observatory in 2003-2009
To find X-ray bursts from sources within the field of view of the
IBIS/INTEGRAL telescope, we have analysed all the archival data of the
telescope available at the time of writing the paper (the observations from
January 2003 to April 2009). We have detected 834 hard (15-25 keV) X-ray
bursts, 239 of which were simultaneously recorded by the JEM-X/INTEGRAL
telescope in the standard X-ray energy range. More than 70% of all bursts (587
events) have been recorded from the well-known X-ray burster GX 354-0. We have
found upper limits on the distances to their sources by assuming that the
Eddington luminosity limit was reached at the brightness maximum of the
brightest bursts.Comment: 18 pages, 2 figures, 2 table
INTEGRAL observations of the Be/X-ray binary EXO 2030+375 during outburst
We present a type-I outburst of the high-mass X-ray binary EXO 2030+375,
detected during INTEGRAL's Performance and Verification Phase in December 2002
(on-source time about 10e+06 seconds). In addition, six more outbursts have
been observed during INTEGRAL's Galactic Plane Scans. X-ray pulsations have
been detected with a pulse period of 41.691798+-0.000016 s. The X-ray
luminosity in the 5-300 keV energy range was 9.7*10e+36 erg/s, for a distance
of 7.1 kpc. Two unusual features were found in the light curve, with an initial
peak before the main outburst and another possible spike after the maximum.
RXTE observations confirm only the existence of the initial spike. Although the
initial peak appears to be a recurrent feature, the physical mechanisms
producing it and the possible second spike are unknown. Moreover, a four-day
delay between periastron passage and the peak of the outburst is observed. We
present for the first time a 5-300 keV broad-band spectrum of this source. It
can be modelled by the sum of a disk black body (kT_bb~8 keV) with either a
power law model with Gamma=2.04+-0.11 keV or a Comptonized component (spherical
geometry, kT_e=30 keV, tau=2.64, kT_W=1.5 keV).Comment: 10 pages, 11 figures, to be published in A&
Update on the ICUD-SIU consultation on multi-parametric magnetic resonance imaging in localised prostate cancer
Introduction: Prostate cancer (PCa) imaging is a rapidly evolving field. Dramatic improvements in prostate MRI during the last decade will probably change the accuracy of diagnosis. This chapter reviews recent current evidence about MRI diagnostic performance and impact on PCa management. Materials and methods: The International Consultation on Urological Diseases nominated a committee to review the literature on prostate MRI. A search of the PubMed database was conducted to identify articles focussed on MP-MRI detection and staging protocols, reporting and scoring systems, the role of MP-MRI in diagnosing PCa prior to biopsy, in active surveillance, in focal therapy and in detecting local recurrence after treatment. Results: Differences in opinion were reported in the use of the strength of magnets [1.5 Tesla (T) vs. 3T] and coils. More agreement was found regarding the choice of pulse sequences; diffusion-weighted MRI (DW-MRI), dynamic contrast-enhanced MRI (DCE MRI), and/or MR spectroscopy imaging (MRSI) are recommended in addition to conventional T2-weighted anatomical sequences. In 2015, the Prostate Imaging Reporting and Data System (PI-RADS version 2) was described to standardize image acquisition and interpretation. MP-MRI improves detection of clinically significant PCa (csPCa) in the repeat biopsy setting or before the confirmatory biopsy in patients considering active surveillance. It is useful to guide focal treatment and to detect local recurrences after treatment. Its role in biopsy-naive patients or during the course of active surveillance remains debated. Conclusion: MP-MRI is increasingly used to improve detection of csPCa and for the selection of a suitable therapeutic approach
Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst
We present results from 16 snapshots of Aql X-1 with RXTE during the rising
phase of its recent outburst. The observations were carried out at a typical
rate of once or twice per day. The source shows interesting spectral evolution
during this period. Phenomenologically, it bears remarkable similarities to
``atoll'' sources. Shortly after the onset of the outburst, the source is seen
to be in an ``island'' state, but with little X-ray variability. It then
appears to have made a rapid spectral transition (on a time scale less than
half a day) to another ``island'' state, where it evolves slightly and stays
for 4 days. In this state, the observed X-ray flux becomes increasingly
variable as the source brightens. Quasi-period oscillation (QPO) in the X-ray
intensity is detected in the frequency range 670--870 Hz. The QPO frequency
increases with the X-ray flux while its fractional rms decreases. The QPO
becomes undetectable following a transition to a ``banana'' state, where the
source continues its evolution by moving up and down the ``banana'' branch in
the color-color diagram as the flux (presumably, the mass accretion rate)
fluctuates around the peak of the outburst. Throughout the entire period, the
power density spectrum is dominated by very-low frequency noises. Little power
can be seen above ~1 Hz, which is different from typical ``atoll'' sources. In
the ``banana'' state, the overall X-ray variability remains low (with
fractional rms ~3--4%) but roughly constant. The observed X-ray spectrum is
soft with few photons from above 25 keV, implying the thermal origin of
the emission. The evolution of both spectral and temporal X-ray properties is
discussed in the context of disk-instability models.Comment: 13 pages, including one table and five figures. To appear in ApJ
Letters (July 20
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
RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05
We present the results of detailed spectral studies of the ultra-compact low
mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing
Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to
exhibit a ~199-day modulation in its 2--12 keV flux. Observations were
performed with the PCA and HEXTE instruments on RXTE at roughly one-month
intervals to sample this long-term period and study accretion rate-related
spectral changes. We obtain good fits with a model consisting of a blackbody
and an exponentially cut-off power law. The spectral parameters are strongly
correlated with both the broad-band (2--50 keV) luminosity and the position in
the color-color diagram, with the source moving from a low hard state to a high
soft state as the accretion rate increases. The blackbody component appears to
drive the spectral evolution. Our results are consistent with a geometry in
which the soft component arises from an optically thick boundary layer and the
hard component from an extended Comptonizing corona. Comparing our results with
those of a similar study of the brighter source 4U 1820-30 (Bloser et al.
2000), we find that the two ultra-compact LMXBs occupy similar spectral states
even though the transitions occur at very different total luminosities.Comment: 27 pages LaTeX, 8 figures, accepted to the Astrophysical Journa
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