6,594 research outputs found
A Chandra Proper Motion for PSR J1809-2332
We report on a new Chandra exposure of PSR J1809-2332, the recently
discovered pulsar powering the bright EGRET source 3EG J1809-2328. By
registration of field X-ray sources in an archival exposure, we measure a
significant proper motion for the pulsar point source over an ~11 year
baseline. The shift of 0.30+/-0.06" (at PA= 153.3+/-18.4) supports an
association with proposed SNR parent G7.5-1.7. Spectral analysis of diffuse
emission in the region also supports the interpretation as a hard wind nebula
trail pointing back toward the SNR.Comment: To Appear in the Astrophysical Journal, Sept 1 (v. 756
Simulation of MeV/atom cluster correlations in matter
We present an efficient algorithm able to predict the trajectories of individual cluster constituents as they penetrate relatively thick amorphous targets. Our algorithm properly treats both the intracluster Coulomb repulsion and the collisions between cluster constituents and target atoms. We have compared our simulation predictions to experimental measurements of the distribution of lateral exit velocities, and demonstrated that the in-target Coulomb explosion of 2MeV/atom carbon clusters in carbon foils must be shielded with a screening length of less than 2.5 Å. We also present a simple phenomenological model for the suppression of the exit-side charge of ions in clusters which depends on the enhanced ionization potential that an electron near an ion feels due to the ion’s charged comoving neighbors. By using our simulation algorithm we have predicted the exit correlations of the cluster constituents and verified that the charge suppression model fits the observed charge suppression of ions in clusters to within the experimental uncertainties
Regional vitiligo induced by imiquimod treatment for in-transit melanoma metastases
Imiquimod is a topical immunomodulator used for the treatment of viral warts and superficial basal cell carcinoma and as an emerging therapy for lentigo maligna and cutaneous melanoma metastases. Vitiligo-like depigmentation has been described as a local adverse effect of topical imiquimod therapy for melanoma1 and at distant sites in patients treated with combination topical monobenzone-imiquimod for metastatic melanoma.2 We present a case in which topical imiquimod resulted in both the resolution of in-transit metastatic melanoma and the depigmentation of local and regional skin, suggesting a regional lymphatic effect of this localized topical treatment
Computation of a contraction metric for a periodic orbit using meshfree collocation
Contraction analysis uses a local criterion to prove the long-term behaviour of a dynamical system. We consider a contraction metric, i.e. a Riemannian metric with respect to which the distance between adjacent solutions contracts. If adjacent solutions in all directions perpendicular to the flow are contracted, then there exists a unique periodic orbit, which is exponentially stable. In this paper we propose a construction method using meshfree collocation to approximately solve a matrix-valued PDE problem. We derive error estimates and show that the approximation is itself a matrix-valued PDE problem. We derive error estimates and show that the approximation is itself a contraction metric if the collocation points are sufficiently dense. We apply the method to several examples
Enceladus: Cassini observations and implications for the search for life
Aims. The recent Cassini discovery of water vapor plumes ejected from the south pole of the Saturnian satellite, Enceladus, presents a unique window of opportunity for the detection of extant life in our solar system.
Methods. With its significant geothermal energy source propelling these plumes >80 km from the surface of the moon and the ensuing large temperature gradient with the surrounding environment, it is possible to have the weathering of rocks by liquid water at the rock/liquid interface. For the cases of the putatively detected salt-water oceans beneath the ice crusts of Europa and Callisto, an isolated subsurface ocean without photosynthesis or contact with an oxidizing atmosphere will approach chemical equilibrium and annihilate any ecosystems dependent on redox gradients unless there is a substantial alternative energy source. This thermodynamic tendency imposes severe constraints on any biota that is based on chemical energy. On Enceladus, the weathering of rocks by liquid water and any concomitant radioactive emissions are possible incipient conditions for life. If there is CO, CO2 and NH3 present in the spectra obtained from the plume, then this is possible evidence that amino acids could be formed at the rock/liquid interface of Enceladus. The combination of a hydrological cycle, chemical redox gradient and geochemical cycle give favorable conditions for life.
Results. We discuss the search for signatures of these species and organics in the Cassini UVIS spectra of the plume and implications for the possible detection of life
Jet opening angles and gamma-ray brightness of AGN
We have investigated the differences in apparent opening angles between the
parsec-scale jets of the active galactic nuclei (AGN) detected by the Fermi
Large Area Telescope (LAT) during its first three months of operations and
those of non-LAT-detected AGN. We used 15.4 GHz VLBA observations of sources
from the 2 cm VLBA MOJAVE program, a subset of which comprise the statistically
complete flux density limited MOJAVE sample. We determined the apparent opening
angles by analyzing transverse jet profiles from the data in the image plane
and by applying a model fitting technique to the data in the (u,v) plane. Both
methods provided comparable opening angle estimates. The apparent opening
angles of gamma-ray bright blazars are preferentially larger than those of
gamma-ray weak sources. At the same time, we have found the two groups to have
similar intrinsic opening angle distributions, based on a smaller subset of
sources. This suggests that the jets in gamma-ray bright AGN are oriented at
preferentially smaller angles to the line of sight resulting in a stronger
relativistic beaming. The intrinsic jet opening angle and bulk flow Lorentz
factor are found to be inversely proportional, as predicted by standard models
of compact relativistic jets. If a gas dynamical jet acceleration model is
assumed, the ratio of the initial pressure of the plasma in the core region P_0
to the external pressure P_ext lies within the range 1.1 to 34.6, with a best
fit estimate of P_0/P_ext=2.Comment: 4 pages, 3 figures; accepted for publication in the A&A Letters;
table in electronic form can be extracted from the preprint sourc
An Exploration of Student Perceptions of the Financial Aid Appeal Process from the Perspectives of Quality Management and Financial Aid Issues
In this study, we report findings from a sample of students at a traditionally African American public institution in the South on a key retention issue – appealing denial of financial aid. We present background which suggests that the financial aid area is in a period of uncertainty where good information may be difficult for students to find. Our findings suggest that the students in our sample were relatively successful in their appeals but that they did not appear to fully realize the influence of college services in helping them to attain the results
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