8,873 research outputs found
Multi-Messenger Astronomy: Cosmic Rays, Gamma-Rays, and Neutrinos
Although cosmic rays were discovered a century ago, we do not know where or
how they are accelerated. There is a realistic hope that the oldest problem in
astronomy will be solved soon by ambitious experimentation: air shower arrays
of 10,000 kilometer-square area, arrays of air Cerenkov telescopes and
kilometer- scale neutrino observatories. Their predecessors are producing
science. We will review the highlights:
- Cosmic rays: the highest energy particles and the GZK cutoff, the search
for cosmic accelerators and the the Cygnus region, top-down mechanisms: photons
versus protons?
- TeV-energy gamma rays: blazars, how molecular clouds may have revealed
proton beams, first hints of the diffuse infrared background?
- Neutrinos: first results and proof of concept for technologies to construct
kilometer-scale observatories.Comment: 26 pages, Latex2e with ws-procs9x6.cls (included), 13 postscript
illustrations (placed using graphicx.sty). Talk presented at "Texas in
Tuscany", 21st Symposium oon Relavitistic Astrophysics, Florence, Italy, Dec.
200
Edge states on graphene ribbon in magnetic field: interplay between Dirac and ferromagnetic-like gaps
By combining analytic and numerical methods, edge states on a finite width
graphene ribbon in a magnetic field are studied in the framework of low-energy
effective theory that takes into account the possibility of quantum Hall
ferromagnetism (QHF) gaps and dynamically generated Dirac-like masses. The
analysis is done for graphene ribbons with both zigzag and armchair edges. The
characteristic features of the spectrum of the edge states in both these cases
are described. In particular, the conditions for the existence of the gapless
edge states are established. Implications of these results for the
interpretation of recent experiments are discussed.Comment: 13 pages, 7 figures. v2: analysis for ribbons with armchair edges
added, to appear in Phys. Rev.
Lysophosphatidic acid mediates myeloid differentiation within the human bone marrow microenvironment.
Lysophosphatidic acid (LPA) is a pleiotropic phospholipid present in the blood and certain tissues at high concentrations; its diverse effects are mediated through differential, tissue specific expression of LPA receptors. Our goal was to determine if LPA exerts lineage-specific effects during normal human hematopoiesis. In vitro stimulation of CD34+ human hematopoietic progenitors by LPA induced myeloid differentiation but had no effect on lymphoid differentiation. LPA receptors were expressed at significantly higher levels on Common Myeloid Progenitors (CMP) than either multipotent Hematopoietic Stem/Progenitor Cells (HSPC) or Common Lymphoid Progenitors (CLP) suggesting that LPA acts on committed myeloid progenitors. Functional studies demonstrated that LPA enhanced migration, induced cell proliferation and reduced apoptosis of isolated CMP, but had no effect on either HSPC or CLP. Analysis of adult and fetal human bone marrow sections showed that PPAP2A, (the enzyme which degrades LPA) was highly expressed in the osteoblastic niche but not in the perivascular regions, whereas Autotaxin (the enzyme that synthesizes LPA) was expressed in perivascular regions of the marrow. We propose that a gradient of LPA with the highest levels in peri-sinusoidal regions and lowest near the endosteal zone, regulates the localization, proliferation and differentiation of myeloid progenitors within the bone marrow marrow
Tunable nonlinearity in atomic response to a bichromatic field
Atomic response to a probe beam can be tailored, by creating coherences
between atomic levels with help of another beam. Changing parameters of the
control beam will change the nature of coherences and hence the nature of
atomic response as well. Such change can depend upon intensity of both probe
and control beams, in a nonlinear fashion. We present a situation where this
nonlinearity in dependence can be precisely controlled, as to obtain different
variations as desired. We also present a detailed analysis of how this
nonlinear dependency arises and show that this is an interesting effect of
several Coherent Population Trap(CPT) states that exist and a competition among
them to trap atomic population in them.Comment: 16 pages and 6 figure
Sinking of a magnetically confined mountain on an accreting neutron star
We perform ideal-magnetohydrodynamic axisymmetric simulations of magnetically
confined mountains on an accreting neutron star, with masses less than ~0.12
solar masses. We consider two scenarios, in which the mountain sits atop a hard
surface or sinks into a soft, fluid base. We find that the ellipticity of the
star, due to a mountain grown on a hard surface, approaches ~2e-4 for accreted
masses greater than ~1.2e-3 solar masses, and that sinking reduces the
ellipticity by between 25% and 60%. The consequences for gravitational
radiation from low-mass x-ray binaries are discussed.Comment: 13 pages, 12 figures, and 3 tables; accepted for publication in MNRA
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