100 research outputs found
Calibration of the Gamma-RAy Polarimeter Experiment (GRAPE) at a Polarized Hard X-Ray Beam
The Gamma-RAy Polarimeter Experiment (GRAPE) is a concept for an astronomical
hard X-ray Compton polarimeter operating in the 50 - 500 keV energy band. The
instrument has been optimized for wide-field polarization measurements of
transient outbursts from energetic astrophysical objects such as gamma-ray
bursts and solar flares. The GRAPE instrument is composed of identical modules,
each of which consists of an array of scintillator elements read out by a
multi-anode photomultiplier tube (MAPMT). Incident photons Compton scatter in
plastic scintillator elements and are subsequently absorbed in inorganic
scintillator elements; a net polarization signal is revealed by a
characteristic asymmetry in the azimuthal scattering angles. We have
constructed a prototype GRAPE module containing a single CsI(Na) calorimeter
element, at the center of the MAPMT, surrounded by 60 plastic elements. The
prototype has been combined with custom readout electronics and software to
create a complete "engineering model" of the GRAPE instrument. This engineering
model has been calibrated using a nearly 100% polarized hard X-ray beam at the
Advanced Photon Source at Argonne National Laboratory. We find modulation
factors of 0.46 +/- 0.06 and 0.48 +/- 0.03 at 69.5 keV and 129.5 keV,
respectively, in good agreement with Monte Carlo simulations. In this paper we
present details of the beam test, data analysis, and simulations, and discuss
the implications of our results for the further development of the GRAPE
concept.Comment: 35 pages, 14 figures, accepted for publication in NIM-
Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths
We review observations of several classes of neutron-star-powered outflows:
pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe
interacting directly with interstellar medium (ISM), and magnetar-powered
outflows. We describe radio, X-ray, and gamma-ray observations of PWNe,
focusing first on integrated spectral-energy distributions (SEDs) and global
spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering
array of morphologies, with jets, trails, and other structures. Several of the
23 so far identified magnetars show evidence for continuous or sporadic
emission of material, sometimes associated with giant flares, and a few
possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published
in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray
Bursts and Blazars: Physics of Extreme Energy Release
Catching Element Formation In The Act ; The Case for a New MeV Gamma-Ray Mission: Radionuclide Astronomy in the 2020s
High Energy Astrophysic
Interaction of membrane-bound MBP with Fyn-SH3 domain: role of membrane-surface charge and post-translational modifications of MBP
Synthesis and secondary structure of loop 4 of myelin proteolipid protein: effect of a point mutation found in Pelizaeus-Merzbacher disease
Critical and Off-Critical Miscibility Transitions in Model Extracellular and Cytoplasmic Myelin Lipid Monolayers
Monolayers based on the composition of the cytoplasmic (CYT) or extracellular (EXT) sides of the myelin bilayer form coexisting immiscible liquid phases similar to the liquid-ordered/liquid-disordered phases in phospholipid/cholesterol monolayers. Increasing the temperature or surface pressure causes the two liquid phases to mix, although in significantly different fashion for the CYT and EXT monolayers. The cerebroside-rich EXT monolayer is near a critical composition and the domains undergo coalescence and a circle-to-stripe transition along with significant roughening of the domain boundaries before mixing. The phase transition in the cerebroside-free cytoplasmic side occurs abruptly without domain coalescence; hence, the cytoplasmic monolayer is not near a critical composition, although the domains exhibit shape instabilities within 1–2 mN/m of the transition. The change in mixing pressure decreases significantly with temperature for the EXT monolayer, with dΠcrit/dT ∼ 1.5 mN/m/°C, but the mixing pressure of the CYT monolayer varies little with temperature. This is due to the differences in the nonideality of cholesterol interactions with cerebrosides (EXT) relative to phospholipids (CYT). EXT monolayers based on the composition of white matter from marmosets with experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis, remain phase-separated at higher surface pressures than control, while EAE CYT monolayers are similar to control. Myelin basic protein, when added to the CYT monolayer, increases lipid miscibility in CYT monolayers; likely done by altering the dipole density difference between the two phases
Analysis of Solute Transport in Flow Fields Influenced by Preferential Flowpaths at the Decimeter Scale
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