537 research outputs found
Model Atmospheres for X-ray Bursting Neutron Stars
The hydrogen and helium accreted by X-ray bursting neutron stars is
periodically consumed in runaway thermonuclear reactions that cause the entire
surface to glow brightly in X-rays for a few seconds. With models of the
emission, the mass and radius of the neutron star can be inferred from the
observations. By simultaneously probing neutron star masses and radii, X-ray
bursts are one of the strongest diagnostics of the nature of matter at
extremely high densities. Accurate determinations of these parameters are
difficult, however, due to the highly non-ideal nature of the atmospheres where
X-ray bursts occur. Observations from X-ray telescopes such as RXTE and NuStar
can potentially place strong constraints on nuclear matter once uncertainties
in atmosphere models have been reduced. Here we discuss current progress on
modeling atmospheres of X-ray bursting neutron stars and some of the challenges
still to be overcome.Comment: 25 pages, 14 figure
The Epsilon Calculus and Herbrand Complexity
Hilbert's epsilon-calculus is based on an extension of the language of
predicate logic by a term-forming operator . Two fundamental
results about the epsilon-calculus, the first and second epsilon theorem, play
a role similar to that which the cut-elimination theorem plays in sequent
calculus. In particular, Herbrand's Theorem is a consequence of the epsilon
theorems. The paper investigates the epsilon theorems and the complexity of the
elimination procedure underlying their proof, as well as the length of Herbrand
disjunctions of existential theorems obtained by this elimination procedure.Comment: 23 p
Anatomy of quantum critical wave functions in dissipative impurity problems
Quantum phase transitions reflect singular changes taking place in a
many-body ground state, however, computing and analyzing large-scale critical
wave functions constitutes a formidable challenge. New physical insights into
the sub-Ohmic spin-boson model are provided by the coherent state expansion
(CSE), which represents the wave function by a linear combination of
classically displaced configurations. We find that the distribution of
low-energy displacements displays an emergent symmetry in the absence of
spontaneous symmetry breaking, while experiencing strong fluctuations of the
order parameter near the quantum critical point. Quantum criticality provides
two strong fingerprints in critical low-energy modes: an algebraic decay of the
average displacement and a constant universal average squeezing amplitude.
These observations, confirmed by extensive variational matrix product states
(VMPS) simulations and field theory arguments, offer precious clues into the
microscopics of critical many-body states in quantum impurity models.Comment: 11 pages, 8 figures. The paper was expanded in V
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Pair cascades in the magnetospheres of strongly-magnetized neutron stars
We present numerical simulations of electron-positron pair cascades in the
magnetospheres of magnetic neutron stars for a wide range of surface fields
(B_p = 10^{12}--10^{15} G), rotation periods (0.1--10 s), and field geometries.
This has been motivated by the discovery in recent years of a number of radio
pulsars with inferred magnetic fields comparable to those of magnetars.
Evolving the cascade generated by a primary electron or positron after it has
been accelerated in the inner gap of the magnetosphere, we follow the spatial
development of the cascade until the secondary photons and pairs leave the
magnetosphere, and we obtain the pair multiplicity and the energy spectra of
the cascade pairs and photons under various conditions. Going beyond previous
works, which were restricted to weaker fields (B < a few x 10^{12} G), we have
incorporated in our simulations detailed treatments of physical processes that
are potentially important (especially in the high field regime) but were either
neglected or crudely treated before, including photon splitting with the
correct selection rules for photon polarization modes, one-photon pair
production into low Landau levels for the e^+e^-, and resonant inverse Compton
scattering from polar cap hot spots. We discuss the implications of our results
for the radio pulsar death line and for the hard X-ray emission from magnetized
neutron stars.Comment: 28 pages, 18 figures, submitted to MNRA
An Integrated TCGA Pan-Cancer Clinical Data Resource to Drive High-Quality Survival Outcome Analytics
For a decade, The Cancer Genome Atlas (TCGA) program collected clinicopathologic annotation data along with multi-platform molecular profiles of more than 11,000 human tumors across 33 different cancer types. TCGA clinical data contain key features representing the democratized nature of the data collection process. To ensure proper use of this large clinical dataset associated with genomic features, we developed a standardized dataset named the TCGA Pan-Cancer Clinical Data Resource (TCGA-CDR), which includes four major clinical outcome endpoints. In addition to detailing major challenges and statistical limitations encountered during the effort of integrating the acquired clinical data, we present a summary that includes endpoint usage recommendations for each cancer type. These TCGA-CDR findings appear to be consistent with cancer genomics studies independent of the TCGA effort and provide opportunities for investigating cancer biology using clinical correlates at an unprecedented scale. Analysis of clinicopathologic annotations for over 11,000 cancer patients in the TCGA program leads to the generation of TCGA Clinical Data Resource, which provides recommendations of clinical outcome endpoint usage for 33 cancer types
Pan-Cancer Analysis of lncRNA Regulation Supports Their Targeting of Cancer Genes in Each Tumor Context
Long noncoding RNAs (lncRNAs) are commonly dys-regulated in tumors, but only a handful are known toplay pathophysiological roles in cancer. We inferredlncRNAs that dysregulate cancer pathways, onco-genes, and tumor suppressors (cancer genes) bymodeling their effects on the activity of transcriptionfactors, RNA-binding proteins, and microRNAs in5,185 TCGA tumors and 1,019 ENCODE assays.Our predictions included hundreds of candidateonco- and tumor-suppressor lncRNAs (cancerlncRNAs) whose somatic alterations account for thedysregulation of dozens of cancer genes and path-ways in each of 14 tumor contexts. To demonstrateproof of concept, we showed that perturbations tar-geting OIP5-AS1 (an inferred tumor suppressor) andTUG1 and WT1-AS (inferred onco-lncRNAs) dysre-gulated cancer genes and altered proliferation ofbreast and gynecologic cancer cells. Our analysis in-dicates that, although most lncRNAs are dysregu-lated in a tumor-specific manner, some, includingOIP5-AS1, TUG1, NEAT1, MEG3, and TSIX, synergis-tically dysregulate cancer pathways in multiple tumorcontexts
Pan-cancer Alterations of the MYC Oncogene and Its Proximal Network across the Cancer Genome Atlas
Although theMYConcogene has been implicated incancer, a systematic assessment of alterations ofMYC, related transcription factors, and co-regulatoryproteins, forming the proximal MYC network (PMN),across human cancers is lacking. Using computa-tional approaches, we define genomic and proteo-mic features associated with MYC and the PMNacross the 33 cancers of The Cancer Genome Atlas.Pan-cancer, 28% of all samples had at least one ofthe MYC paralogs amplified. In contrast, the MYCantagonists MGA and MNT were the most frequentlymutated or deleted members, proposing a roleas tumor suppressors.MYCalterations were mutu-ally exclusive withPIK3CA,PTEN,APC,orBRAFalterations, suggesting that MYC is a distinct onco-genic driver. Expression analysis revealed MYC-associated pathways in tumor subtypes, such asimmune response and growth factor signaling; chro-matin, translation, and DNA replication/repair wereconserved pan-cancer. This analysis reveals insightsinto MYC biology and is a reference for biomarkersand therapeutics for cancers with alterations ofMYC or the PMN
Genomic, Pathway Network, and Immunologic Features Distinguishing Squamous Carcinomas
This integrated, multiplatform PanCancer Atlas study co-mapped and identified distinguishing
molecular features of squamous cell carcinomas (SCCs) from five sites associated with smokin
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