53 research outputs found
The Physical Interpretation of X-ray Phase Lags and Coherence: RXTE Observations of Cygnus X--1 as a Case Study
There have been a number of recent spectral models that have been successful
in reproducing the observed X-ray spectra of galactic black hole candidates
(GBHC). However, there still exists controversy over such issues as: what are
the sources of hard radiation, what is the system's geometry, is the accretion
efficient or inefficient, etc. A potentially powerful tool for distinguishing
among these possibilities, made possible by the Rossi X-ray Timing Explorer
(RXTE), is the variability data, especially the observed phase lags and
variability coherence. These data, in conjunction with spectral modeling, have
the potential of determining physical sizes of the system, as well as placing
strong constraints on both Compton corona and advection models. As an example,
we present RXTE variability data of Cygnus X-1Comment: To Appear in the Proceedings of the Symposium "The Active X-ray Sky",
held October 21-24, 1997, Rom
Polarimetric Properties of Flux-Ropes and Sheared Arcades in Coronal Prominence Cavities
The coronal magnetic field is the primary driver of solar dynamic events.
Linear and circular polarization signals of certain infrared coronal emission
lines contain information about the magnetic field, and to access this
information, either a forward or an inversion method must be used. We study
three coronal magnetic configurations that are applicable to polar-crown
filament cavities by doing forward calculations to produce synthetic
polarization data. We analyze these forward data to determine the
distinguishing characteristics of each model. We conclude that it is possible
to distinguish between cylindrical flux ropes, spheromak flux ropes, and
sheared arcades using coronal polarization measurements. If one of these models
is found to be consistent with observational measurements, it will mean
positive identification of the magnetic morphology that surrounds certain
quiescent filaments, which will lead to a greater understanding of how they
form and why they erupt.Comment: 22 pages, 8 figures, Solar Physics topical issue: Coronal Magnetis
Observations of Coronal Mass Ejections with the Coronal Multichannel Polarimeter
The Coronal Multichannel Polarimeter (CoMP) measures not only the
polarization of coronal emission, but also the full radiance profiles of
coronal emission lines. For the first time, CoMP observations provide
high-cadence image sequences of the coronal line intensity, Doppler shift and
line width simultaneously in a large field of view. By studying the Doppler
shift and line width we may explore more of the physical processes of CME
initiation and propagation. Here we identify a list of CMEs observed by CoMP
and present the first results of these observations. Our preliminary analysis
shows that CMEs are usually associated with greatly increased Doppler shift and
enhanced line width. These new observations provide not only valuable
information to constrain CME models and probe various processes during the
initial propagation of CMEs in the low corona, but also offer a possible
cost-effective and low-risk means of space weather monitoring.Comment: 6 figures. Will appear in the special issue of Coronal Magnetism,
Sol. Phy
Observing the First Stars and Black Holes
The high sensitivity of JWST will open a new window on the end of the
cosmological dark ages. Small stellar clusters, with a stellar mass of several
10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun
should be directly detectable out to redshift z=10, and individual supernovae
(SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible
beyond this redshift. Dense primordial gas, in the process of collapsing from
large scales to form protogalaxies, may also be possible to image through
diffuse recombination line emission, possibly even before stars or BHs are
formed. In this article, I discuss the key physical processes that are expected
to have determined the sizes of the first star-clusters and black holes, and
the prospect of studying these objects by direct detections with JWST and with
other instruments. The direct light emitted by the very first stellar clusters
and intermediate-mass black holes at z>10 will likely fall below JWST's
detection threshold. However, JWST could reveal a decline at the faint-end of
the high-redshift luminosity function, and thereby shed light on radiative and
other feedback effects that operate at these early epochs. JWST will also have
the sensitivity to detect individual SNe from beyond z=10. In a dedicated
survey lasting for several weeks, thousands of SNe could be detected at z>6,
with a redshift distribution extending to the formation of the very first stars
at z>15. Using these SNe as tracers may be the only method to map out the
earliest stages of the cosmic star-formation history. Finally, we point out
that studying the earliest objects at high redshift will also offer a new
window on the primordial power spectrum, on 100 times smaller scales than
probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", Astrophysics & Space Science Library, Eds. H.
Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Diving into the vertical dimension of elasmobranch movement ecology
Knowledge of the three-dimensional movement patterns of elasmobranchs is vital to understand their ecological roles and exposure to anthropogenic pressures. To date, comparative studies among species at global scales have mostly focused on horizontal movements. Our study addresses the knowledge gap of vertical movements by compiling the first global synthesis of vertical habitat use by elasmobranchs from data obtained by deployment of 989 biotelemetry tags on 38 elasmobranch species. Elasmobranchs displayed high intra- and interspecific variability in vertical movement patterns. Substantial vertical overlap was observed for many epipelagic elasmobranchs, indicating an increased likelihood to display spatial overlap, biologically interact, and share similar risk to anthropogenic threats that vary on a vertical gradient. We highlight the critical next steps toward incorporating vertical movement into global management and monitoring strategies for elasmobranchs, emphasizing the need to address geographic and taxonomic biases in deployments and to concurrently consider both horizontal and vertical movements
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