Search Publications
Evolution of Nuclear Star Clusters
Merritt, David
Two-body relaxation times of nuclear star clusters are short enough that gravitational encounters should substantially affect their structure in 10 Gyr or less. In nuclear star clusters without massive black holes, dynamical evolution is a competition between core collapse, which causes densities to increase, and heat input from the surrounding ga…
Investigation of Thickness and Electrical Resistivity of the Current Sheets in Solar Eruptions
Raymond, J. C.; Ko, Y. -K.; Lin, J. +1 more
A discussion of the thickness of current sheets in solar eruptions, d, led Lin et al. in 2007 to estimate very large values for the effective resistivity, η e . Here, we address some questions raised by that paper. We apply the limb synoptic map technique and find d between 5.0 × 104 and 4.6 × 105 km, increasing wi…
The Velocity Distribution of Nearby Stars from Hipparcos Data. I. The Significance of the Moving Groups
Bovy, Jo; Hogg, David W.; Roweis, Sam T.
We present a three-dimensional reconstruction of the velocity distribution of nearby stars (lsim100 pc) using a maximum likelihood density estimation technique applied to the two-dimensional tangential velocities of stars. The underlying distribution is modeled as a mixture of Gaussian components. The algorithm reconstructs the error-deconvolved d…
Suzaku Observation of a Hard Excess in 1H 0419 - 577: Detection of a Compton-Thick Partial-Covering Absorber
Turner, T. J.; Kraemer, S. B.; Reeves, J. N. +2 more
We present results from a 200 ks Suzaku observation of 1H 0419 - 577 taken during 2007 July. The source shows a strong excess of counts above 10 keV compared to the extrapolation of models based on previous data in the 0.5-10 keV band. The "hard excess" in 1H 0419 - 577 can be explained by the presence of a Compton-thick partial-covering absorber …
Hinode/Extreme-Ultraviolet Imaging Spectrometer Observations of the Temperature Structure of the Quiet Corona
Warren, Harry P.; Brooks, David H.; Watanabe, Tetsuya +1 more
We present a differential emission measure (DEM) analysis of the quiet solar corona on disk using data obtained by the Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode. We show that the expected quiet-Sun DEM distribution can be recovered from judiciously selected lines, and that their average intensities can be reproduced to within 30%. W…
The Internal Structure of Coronal Mass Ejections: Are all Regular Magnetic Clouds Flux Ropes?
Poedts, S.; Lugaz, N.; Roussev, I. I. +1 more
In this Letter, we investigate the internal structure of a coronal mass ejection (CME) and its dynamics by invoking a realistic initiation mechanism in a quadrupolar magnetic setting. The study comprises a compressible three-dimensional magnetohydrodynamics simulation. We use an idealized model of the solar corona, into which we superimpose a quad…
Active Galactic Nuclei in Groups and Clusters of Galaxies: Detection and Host Morphology
Mulchaey, John S.; Martini, Paul; Jeltema, Tesla E. +2 more
The incidence and properties of active galactic nuclei (AGNs) in the field, groups, and clusters can provide new information about how these objects are triggered and fueled, similar to how these environments have been employed to study galaxy evolution. We have obtained new XMM-Newton observations of seven X-ray selected groups and poor clusters …
X-Ray Substructure Studies of Four Galaxy Clusters Using XMM-Newton Data
Sarazin, Craig L.; Reiprich, Thomas H.; Finoguenov, Alexis +2 more
Mahdavi et al. find that the degree of agreement between weak lensing and X-ray mass measurements is a function of cluster radius. Numerical simulations also point out that X-ray mass proxies do not work equally well at all radii. The origin of the effect is thought to be associated with cluster mergers. Recent work presenting the cluster maps sho…
High-Resolution Timing Observations of Spin-Powered Pulsars with the AGILE Gamma-Ray Telescope
Halpern, J.; Costa, E.; Mereghetti, S. +67 more
Astro-rivelatore Gamma ad Immagini LEggero (AGILE) is a small gamma-ray astronomy satellite mission of the Italian Space Agency dedicated to high-energy astrophysics launched in 2007 April. Its ~ 1 µs absolute time tagging capability coupled with a good sensitivity in the 30 MeV-30 GeV range, with simultaneous X-ray monitoring in the 18-60 k…
Relativistic Broadening of Iron Emission Lines in a Sample of Active Galactic Nuclei
Brenneman, Laura W.; Reynolds, Christopher S.
We present a uniform X-ray spectral analysis of eight type-1 active galactic nuclei that have been previously observed with relativistically broadened iron emission lines. Utilizing data from the XMM-Newton European Photon Imaging Camera (EPIC-pn) we carefully model the spectral continuum, taking complex intrinsic absorption and emission into acco…