681 research outputs found
Tracking the Orbital and Super-orbital Periods of SMC X-1
The High Mass X-ray Binary (HMXB) SMC X-1 demonstrates an orbital variation
of 3.89 days and a super-orbital variation with an average length of 55 days.
As we show here, however, the length of the super-orbital cycle varies by
almost a factor of two, even across adjacent cycles. To study both the orbital
and super-orbital variation we utilize lightcurves from the Rossi X-ray Timing
Explorer All Sky Monitor (RXTE-ASM). We employ the orbital ephemeris from
Wojdowski et al. (1998) to obtain the average orbital profile, and we show that
this profile exhibits complex modulation during non-eclipse phases.
Additionally, a very interesting ``bounceback'' in X-ray count rate is seen
during mid-orbital eclipse phases, with a softening of the emission during
these periods. This bounceback has not been previously identified in pointed
observations. We then define a super-orbital ephemeris (the phase of the
super-orbital cycle as a function of date) based on the ASM lightcurve and
analyze the trend and distribution of super-orbital cycle lengths. SMC X-1
exhibits a bimodal distribution of these lengths, similar to what has been
observed in other systems (e.g., Her X-1), but with more dramatic changes in
cycle length. There is some hint, but not conclusive evidence, for a dependence
of the super-orbital cycle length upon the underlying orbital period, as has
been observed previously for Her X-1 and Cyg X-2. Using our super-orbital
ephemeris we are also able to create an average super-orbital profile over the
71 observed cycles, for which we witness overall hardening of the spectrum
during low count rate times. We combine the orbital and super-orbital
ephemerides to study the correlation between the orbital and super-orbital
variations in the system.Comment: 10 pages, using emulateapj style. To be published in the
Astrophysical Journa
Photometric analysis of a space shuttle water venting
Presented here is a preliminary interpretation of a recent experiment conducted on Space Shuttle Discovery (Mission STS 29) in which a stream of liquid supply water was vented into space at twilight. The data consist of video images of the sunlight-scattering water/ice particle cloud that formed, taken by visible light-sensitive intensified cameras both onboard the spacecraft and at the AMOS ground station near the trajectory's nadir. This experiment was undertaken to study the phenomenology of water columns injected into the low-Earth orbital environment, and to provide information about the lifetime of ice particles that may recontact Space Shuttle orbits later. The findings about the composition of the cloud have relevance to ionospheric plasma depletion experiments and to the dynamics of the interaction of orbiting spacecraft with the environment
Sample Preparation Techniques for Grain Boundary Characterization of Annealed TRISO-Coated Particles
Crystallographic information about layers of silicon carbide (SiC) deposited by chemical vapor deposition is essential to understanding layer performance, especially when the the layers are in nonplanar geometries (e.g., spherical). Electron backscatter diffraction (EBSD) was used to analyze spherical SiC layers using a different sampling approach that applied focused ion beam (FIB) milling to avoid the negative impacts of traditional sample polishing and address the need for very small samples of irradiated materials for analysis. The mechanical and chemical grinding and polishing of sample surfaces can introduce lattice strain and result in the unequal removal of SiC and the surrounding layers of different materials due to the hardness differences among these materials. The nature of layer interfaces is thought to play a key role in the performance of SiC; therefore, the analysis of representative samples at these interfacial areas is crucial. In the work reported herein, a FIB was employed in a novel manner to prepare a more representative sample for EBSD analysis from tristructural-isotropic layers that are free of effects introduced by mechanical and chemical preparation methods. In addition, the difficulty of handling neutron-irradiated microscopic samples (such as those analyzed in this work) has been simplified using pretilted mounting stages. The results showed that while the average grain sizes of samples may be similar, the grain boundary characteristics can differ significantly. Furthermore, low-angle grain boundaries comprised 25% of all boundaries in the FIB-prepared sample compared to only 1% to 2% in the polished sample from the same particle. This study demonstrated that the characterization results from FIB-prepared samples provide more repeatable results due to the elimination of the effects of sample preparation
Three-dimensional Nonlinear Electromagnetic Field Computations, using Scalar Potentials”,
Abstract: A formulation based on scalar potentials for the numerical solution of three-dimensional nonlinear static electromagnetic field problems is presented. The resulting equations are solved using finite elements, based on a Galerkin procedure. A general-purpose package called TOSCA has been developed, implemented and tested. Results are presented for three cases and compared with measured values. List of principal symbols B = magnetic flux density H = magnetic field intensity J = current density H e = coercive field intensity µ = permeability ψ = magnetic scalar potential φ = reduced magnetic scalar potential n = unit outward normal vector t = unit surface tangent vector 1 Introductio
INTEGRAL deep observations of the Small Magellanic Cloud
Deep observations of the Small Magellanic Cloud (SMC) and region were carried
out in the hard X-ray band by the INTEGRAL observatory in 2008-2009. The field
of view of the instrument permitted simultaneous coverage of the entire SMC and
the eastern end of the Magellanic Bridge. In total, INTEGRAL detected seven
sources in the SMC and five in the Magellanic Bridge; the majority of the
sources were previously unknown systems. Several of the new sources were
detected undergoing bright X- ray outbursts and all the sources exhibited
transient behaviour except the supergiant system SMC X-1. They are all thought
to be High Mass X-ray Binary (HMXB) systems in which the compact object is a
neutron star.Comment: 7 pages, 10 figures Accepted for publication in MNRA
Apical and basolateral endosomes of MDCK cells are interconnected and contain a polarized sorting mechanism.
Sorting signals in the MHC class II invariant chain cytoplasmic tail and transmembrane region determine trafficking to an endocytic processing compartment.
Activation of Estrogen Receptor-α by E2 or EGF Induces Temporally Distinct Patterns of Large-Scale Chromatin Modification and mRNA Transcription
Estrogen receptor-α (ER) transcription function is regulated in a ligand-dependent (e.g., estradiol, E2) or ligand-independent (e.g., growth factors) manner. Our laboratory seeks to understand these two modes of action. Using a cell line that contains a visible prolactin enhancer/promoter array (PRL-HeLa) regulated by ER, we analyzed ER response to E2 and EGF by quantifying image-based results. Data show differential recruitment of GFP-ER to the array, with the AF1 domain playing a vital role in EGF-mediated responsiveness. Temporal analyses of large-scale chromatin dynamics, and accumulation of array-localized reporter mRNA over 24 hours showed that the EGF response consists of a single pulse of reporter mRNA accumulation concomitant with transient increase in array decondensation. Estradiol induced a novel cyclical pattern of mRNA accumulation with a sustained increase in array decondensation. Collectively, our work shows that there is a stimuli-specific pattern of large-scale chromatin modification and transcript levels by ER
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