235 research outputs found
The aperiodic X-ray variability of the accreting millisecond pulsar SAX J1808.4-3658
We have studied the aperiodic variability of the 401 Hz accreting millisecond
X-ray pulsar SAX J1808.4-3658 using the complete data set collected with the
Rossi X-ray Timing Explorer over 14 years of observation. The source shows a
number of exceptional aperiodic timing phenomena that are observed against a
backdrop of timing properties that show consistent trends in all five observed
outbursts and closely resemble those of other atoll sources. We performed a
detailed study of the enigmatic ~410 Hz QPO, which has only been observed in
SAX J1808.4-3658. We find that it appears only when the upper kHz QPO frequency
is less than the 401 Hz spin frequency. The difference between the ~410 Hz QPO
frequency and the spin frequency follows a similar frequency correlation as the
low frequency power spectral components, suggesting that the ~410 Hz QPO is a
retrograde beat against the spin frequency of a rotational phenomenon in the 9
Hz range. Comparing this 9 Hz beat feature with the Low-Frequency QPO in SAX
J1808.4-3658 and other neutron star sources, we conclude that these two might
be part of the same basic phenomenon. We suggest that they might be caused by
retrograde precession due to a combination of relativistic, classical and
magnetic torques. Additionally we present two new measurements of the lower kHz
QPO, allowing us, for the first time, to measure the frequency evolution of the
twin kHz QPOs in this source. The twin kHz QPOs are seen to move together over
150 Hz, maintaining a centroid frequency separation of .Comment: 18 pages, 9 figures, 7 tables. Accepted for publication in Ap
Pulse amplitude depends on kHz QPO frequency in the accreting millisecond pulsar SAX J1808.4-3658
We study the relation between the 300-700 Hz upper kHz quasi-periodic
oscillation (QPO) and the 401 Hz coherent pulsations across all outbursts of
the accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the Rossi
X-ray Timing Explorer. We find that the pulse amplitude systematically changes
by a factor of ~2 when the upper kHz QPO frequency passes through 401 Hz: it
halves when the QPO moves to above the spin frequency and doubles again on the
way back. This establishes for the first time the existence of a direct effect
of kHz QPOs on the millisecond pulsations and provides a new clue to the origin
of the upper kHz QPO. We discuss several scenarios and conclude that while more
complex explanations can not formally be excluded, our result strongly suggests
that the QPO is produced by azimuthal motion at the inner edge of the accretion
disk, most likely orbital motion. Depending on whether this azimuthal motion is
faster or slower than the spin, the plasma then interacts differently with the
neutron-star magnetic field. The most straightforward interpretation involves
magnetospheric centrifugal inhibition of the accretion flow that sets in when
the upper kHz QPO becomes slower than the spin.Comment: 5 pages, 4 figures, Accepted for publication in ApJ
The stochastic X-ray variability of the accreting millisecond pulsar MAXI J0911-655
In this work, I report on the stochastic X-ray variability of the 340 Hz accreting millisecond pulsar MAXI J0911–655. Analyzing pointed observations of the XMM-Newton and NuSTAR observatories, I find that the source shows broad band-limited stochastic variability in the 0.01-10 Hz range with a total fractional variability of ~24% rms in the 0.4-3 keV energy band that increases to ~40% rms in the 3–10 keV band. Additionally, a pair of harmonically related quasi-periodic oscillations (QPOs) are discovered. The fundamental frequency of this harmonic pair is observed between frequencies of 62 and 146 mHz. Like the band-limited noise, the amplitudes of the QPOs show a steep increase as a function of energy; this suggests that they share a similar origin, likely the inner accretion flow. Based on their energy dependence and frequency relation with respect to the noise terms, the QPOs are identified as low-frequency oscillations and discussed in terms of the Lense–Thirring precession model
Quasi-periodic pulse amplitude modulation in the accreting millisecond pulsar IGR J00291+5934
We introduce a new method for analysing the aperiodic variability of coherent
pulsations in accreting millisecond X-ray pulsars. Our method involves applying
a complex frequency correction to the time-domain light curve, allowing for the
aperiodic modulation of the pulse amplitude to be robustly extracted in the
frequency domain. We discuss the statistical properties of the resulting
modulation spectrum and show how it can be correlated with the non-pulsed
emission to determine if the periodic and aperiodic variability are coupled
processes. Using this method, we study the 598.88 Hz coherent pulsations of the
accreting millisecond X-ray pulsar IGR J00291+5934 as observed with the Rossi
X-ray Timing Explorer and XMM-Newton. We demonstrate that our method easily
confirms the known coupling between the pulsations and a strong 8 mHz QPO in
XMM-Newton observations. Applying our method to the RXTE observations, we
further show, for the first time, that the much weaker 20 mHz QPO and its
harmonic are also coupled the pulsations. We discuss the implications of this
coupling and indicate how it may be used to extract new information on the
underlying accretion process.Comment: 12 pages, 7 figures, 1 table. Accepted for publication in Ap
The magnetic-field strengths of accreting millisecond pulsars
In this work we have estimated upper and lower limits to the strength of the magnetic dipole
moment of all 14 accreting millisecond X-ray pulsars observed with the Rossi X-ray Timing
Explorer (RXTE). For each source we searched the archival RXTE data for the highest and
lowest flux levels with a significant detection of pulsations. We assume these flux levels to
correspond to the closest and farthest location of the inner edge of the accretion disc at which
channelled accretion takes place. By estimating the accretion rate from the observed luminosity
at these two flux levels, we place upper and lower limits on the magnetic dipole moment of
the neutron star, using assumptions from standard magnetospheric accretion theory. Finally,
we discuss how our field strength estimates can be further improved as more information on
these pulsars is obtained.
Key words: stars: magnetic field – stars: neutron – pulsars: general – X-rays: binar
Context-aware stacked convolutional neural networks for classification of breast carcinomas in whole-slide histopathology images
Automated classification of histopathological whole-slide images (WSI) of
breast tissue requires analysis at very high resolutions with a large
contextual area. In this paper, we present context-aware stacked convolutional
neural networks (CNN) for classification of breast WSIs into normal/benign,
ductal carcinoma in situ (DCIS), and invasive ductal carcinoma (IDC). We first
train a CNN using high pixel resolution patches to capture cellular level
information. The feature responses generated by this model are then fed as
input to a second CNN, stacked on top of the first. Training of this stacked
architecture with large input patches enables learning of fine-grained
(cellular) details and global interdependence of tissue structures. Our system
is trained and evaluated on a dataset containing 221 WSIs of H&E stained breast
tissue specimens. The system achieves an AUC of 0.962 for the binary
classification of non-malignant and malignant slides and obtains a three class
accuracy of 81.3% for classification of WSIs into normal/benign, DCIS, and IDC,
demonstrating its potentials for routine diagnostics
The representation of protein complexes in the Protein Ontology
Representing species-specific proteins and protein complexes in ontologies that are both human and machine-readable facilitates the retrieval, analysis, and interpretation of genome-scale data sets. Although existing protin-centric informatics resources provide the biomedical research community with well-curated compendia of protein sequence and structure, these resources lack formal ontological representations of the relationships among the proteins themselves. The Protein Ontology (PRO) Consortium is filling this informatics resource gap by developing ontological representations and relationships among proteins and their variants and
modified forms. Because proteins are often functional only as members of stable protein complexes, the PRO Consortium, in collaboration with existing protein and pathway databases, has launched a new initiative to implement logical and consistent representation of protein complexes. We describe here how the PRO Consortium is meeting the challenge of representing species-specific protein complexes, how protein complex representation in PRO supports annotation of protein complexes and comparative biology, and how PRO is being integrated into existing community bioinformatics resources. The PRO resource is accessible at http://pir.georgetown.edu/pro/
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