4,299 research outputs found
Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676
Gravitationally redshifted absorption lines of Fe XXVI, Fe XXV, and O VIII
were inferred recently in the X-ray spectrum of the bursting neutron star EXO
0748-676. We place an upper limit on the stellar magnetic field based on the
iron lines. The oxygen absorption feature shows a multiple component profile
that is consistent with Zeeman splitting in a magnetic field of ~(1-2)x10^9
gauss, and for which the corresponding Zeeman components of the iron lines are
expected to be blended together. In other systems, a field strength >5x10^{10}
gauss could induce a blueshift of the line centroids that would counteract
gravitational redshift and complicate the derivation of constraints on the
equation of state of the neutron star.Comment: 5 pages, submitted to Phys. Rev. Let
The clock paradox in a static homogeneous gravitational field
The gedanken experiment of the clock paradox is solved exactly using the
general relativistic equations for a static homogeneous gravitational field. We
demonstrate that the general and special relativistic clock paradox solutions
are identical and in particular that they are identical for finite
acceleration. Practical expressions are obtained for proper time and coordinate
time by using the destination distance as the key observable parameter. This
solution provides a formal demonstration of the identity between the special
and general relativistic clock paradox with finite acceleration and where
proper time is assumed to be the same in both formalisms. By solving the
equations of motion for a freely falling clock in a static homogeneous field
elapsed times are calculated for realistic journeys to the stars.Comment: Revision: Posted with the caption included with the figure
The equivalence principle, uniformly accelerated reference frames, and the uniform gravitational field
The relationship between uniformly accelerated reference frames in flat
spacetime and the uniform gravitational field is examined in a relativistic
context. It is shown that, contrary to previous statements in the pages of this
journal, equivalence does not break down in this context. No restrictions to
Newtonian approximations or small enclosures are necessary
Developing a Resilient, Robust and Efficient Supply Network in Africa
Purpose — Supply chains need to balance competing objectives; in addition to efficiency, supply chains need to be resilient to adversarial and environmental interference and robust to uncertainties in long-term demand. Significant research has been conducted designing efficient supply chains and recent research has focused on resilient supply chain design. However, the integration of resilient and robust supply chain design is less well studied. The purpose of the paper is to include resilience and robustness into supply chain design. Design/methodology/approach — The paper develops a method to include resilience and robustness into supply chain design. Using the region of West Africa, which is plagued with persisting logistical issues, the authors develop a regional risk assessment framework and then apply categorical risk to the countries of West Africa using publicly available data. A scenario reduction technique is used to focus on the highest risk scenarios for the model to be tractable. Next, the authors develop a mathematical model leveraging this framework to design a resilient supply network that minimizes cost while ensuring the network functions following a disruption. Finally, the authors examine the network\u27s robustness to demand uncertainty via several plausible emergency scenarios. Findings — The authors provide optimal sets of transshipment hubs with varying counts from 5 through 15 hubs. The authors determine there is no feasible solution that uses only five transshipment hubs. The authors\u27 findings reinforce those seven transshipment hubs – the solution currently employed in West Africa – is the cheapest architecture to achieve resilience and robustness. Additionally, for each set of feasibility transshipment hubs, the authors provide connections between hubs and demand spokes. Originality/value — While, at the time of this research, three other manuscripts incorporated both resilience and robustness of the authors\u27 research unique solved the problem as a network flow instead of as a set covering problem. Additionally, the authors establish a novel risk framework to guide the required amount of redundancy, and finally the out research proposes a scenario reduction heuristic to allow tractable exploration of 512 possible demand scenarios
Age and Prostate-Specific Antigen Level Prior to Diagnosis Predict Risk of Death from Prostate Cancer.
A single early prostate-specific antigen (PSA) level has been correlated with a higher likelihood of prostate cancer diagnosis and death in younger men. PSA testing in older men has been considered of limited utility. We evaluated prostate cancer death in relation to age and PSA level immediately prior to prostate cancer diagnosis. Using the Veterans Affairs database, we identified 230,081 men aged 50-89 years diagnosed with prostate cancer and at least one prior PSA test between 1999 and 2009. Prostate cancer-specific death over time was calculated for patients stratified by age group (e.g., 50-59 years, through 80-89 years) and PSA range at diagnosis (10 ranges) using Kaplan-Meier methods. Risk of 10-year prostate cancer mortality across age and PSA was compared using log-rank tests with a Bonferroni adjustment for multiple testing. 10.5% of men diagnosed with prostate cancer died of cancer during the 10-year study period (mean follow-up = 3.7 years). Higher PSA values prior to diagnosis predict a higher risk of death in all age groups (p < 0.0001). Within the same PSA range, older age groups are at increased risk for death from prostate cancer (p < 0.0001). For PSA of 7-10 ng/mL, cancer-specific death, 10 years after diagnosis, increased from 7% for age 50-59 years to 51% for age 80-89 years. Men older than 70 years are more likely to die of prostate cancer at any PSA level than younger men, suggesting prostate cancer remains a significant problem among older men (even those aged 80+) and deserves additional study
Analysis of Dislocation Mechanism for Melting of Elements: Pressure Dependence
In the framework of melting as a dislocation-mediated phase transition we
derive an equation for the pressure dependence of the melting temperatures of
the elements valid up to pressures of order their ambient bulk moduli. Melting
curves are calculated for Al, Mg, Ni, Pb, the iron group (Fe, Ru, Os), the
chromium group (Cr, Mo, W), the copper group (Cu, Ag, Au), noble gases (Ne, Ar,
Kr, Xe, Rn), and six actinides (Am, Cm, Np, Pa, Th, U). These calculated
melting curves are in good agreement with existing data. We also discuss the
apparent equivalence of our melting relation and the Lindemann criterion, and
the lack of the rigorous proof of their equivalence. We show that the would-be
mathematical equivalence of both formulas must manifest itself in a new
relation between the Gr\"{u}neisen constant, bulk and shear moduli, and the
pressure derivative of the shear modulus.Comment: 19 pages, LaTeX, 9 eps figure
Ferromagnetic redshift of the optical gap in GdN
We report measurements of the optical gap in a GdN film at temperatures from
300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is
1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split
bands below the Curie temperature. The paramagnetic gap is larger than was
suggested by very early experiments, and has permitted us to refine a
(LSDA+U)-computed band structure. The band structure was computed in the full
translation symmetry of the ferromagnetic ground state, assigning the
paramagnetic-state gap as the average of the majority- and minority-spin gaps
in the ferromagnetic state. That procedure has been further tested by a band
structure in a 32-atom supercell with randomly-oriented spins. After fitting
only the paramagnetic gap the refined band structure then reproduces our
measured gaps in both phases by direct transitions at the X point.Comment: 5 pages, 4 figure
School Counseling Interns’ Lived Experiences Addressing Social Determinants of Health
This phenomenological study explores school counselors in training (SCITs’) experiences addressing social determinants of health (SDOH), the leading causes of educational and wellness inequities. Interviews with eight SCITs revealed three core themes: (1) professional identity conflict between awareness, skills, and action; (2) social justice knowledge to practice gap; and (3) recommendations for knowledge to practice gap resolution. Themes explained participant preparedness for responding to disparities. Participants were aware of challenges related to SDOH, though struggled with addressing those challenges due to lack of preparation or perceived role constraints. Participants also experienced difficulty practically applying their understanding of social justice theory to SDOH-related challenges. The researchers noted various recommendations for how SDOH may be addressed in the counselor education curriculum and in practice. Keywords: school counselors in training (SCIT), social justice, social determinants of healt
Photoemission evidence for crossover from Peierls-like to Mott-like transition in highly strained VO
We present a spectroscopic study that reveals that the metal-insulator
transition of strained VO thin films may be driven towards a purely
electronic transition, which does not rely on the Peierls dimerization, by the
application of mechanical strain. Comparison with a moderately strained system,
which does involve the lattice, demonstrates the crossover from Peierls- to
Mott-like transitions
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