14,531 research outputs found
On the area of the symmetry orbits in symmetric spacetimes with Vlasov matter
This paper treats the global existence question for a collection of general
relativistic collisionless particles, all having the same mass. The spacetimes
considered are globally hyperbolic, with Cauchy surface a 3-torus. Furthermore,
the spacetimes considered are isometrically invariant under a two-dimensional
group action, the orbits of which are spacelike 2-tori. It is known from
previous work that the area of the group orbits serves as a global time
coordinate. In the present work it is shown that the area takes on all positive
values in the maximal Cauchy development.Comment: 27 pages, version 2 minor changes and correction
Velocity-resolved observations of water in Comet Halley
High resolution (lambda/delta lambda approx. = 3 x 10 to the 5th power) near-infrared observations of H2O emission from Comet Halley were acquired at the time of maximum post-perihelion geocentric Doppler shift. The observed widths and absolute positions of the H2O line profiles reveal characteristics of the molecular velocity field in the coma. These results support H2O outflow from a Sun-lit hemisphere or the entire nucleus, but not from a single, narrow jet emanating from the nucleus. The measured pre- and post-perihelion outflow velocities were 0.9 + or - 0.2 and 1.4 + or - 0.2 km/s, respectively. Temporal variations in the kinematic properties of the outflow were inferred from changes in the spectral line shapes. These results are consistent with the release of H2O into the coma from multiple jets
Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN
Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we
study correlations between X-ray spectral features. The sample consists of 350
time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is
fitted to a model with an intrinsic powerlaw X-ray spectrum produced close to
the central black hole that is reprocessed and absorbed by material around the
black hole. To test the robustness of our results, we performed Monte Carlo
simulations of the spectral sample. We find a complex relationship between the
iron line equivalent width (EW) and the underlying power law index (Gamma). The
data reveal a correlation between Gamma and EW which turns over at Gamma <~ 2,
but finds a weak anti-correlation for steeper photon indices. We propose that
this relationship is driven by dilution of a disk spectrum (which includes the
narrow iron line) by a beamed jet component and, hence, could be used as a
diagnostic of jet-dominance. In addition, our sample shows a strong correlation
between the reflection fraction (R) and Gamma, but we find that it is likely
the result of modeling degeneracies. We also see the X-ray Baldwin effect (an
anti-correlation between the 2-10 keV X-ray luminosity and EW) for the sample
as a whole, but not for the individual galaxies and galaxy types.Comment: Accepted for publication in ApJ, 14 page
Observed transition from Richtmyer-Meshkov jet formation through feedout oscillations to Rayleigh-Taylor instability in a laser target
Experimental study of hydrodynamic perturbation evolution triggered by a
laser-driven shock wave breakout at the free rippled rear surface of a plastic
target is reported. At sub-megabar shock pressure, planar jets manifesting the
development of the Richtmyer-Meshkov-type instability in a non-accelerated
target are observed. As the shock pressure exceeds 1 Mbar, an oscillatory
rippled expansion wave is observed, followed by the "feedout" of the
rear-surface perturbations to the ablation front and the development of the
Rayleigh-Taylor instability, which breaks up the accelerated target.Comment: 12 pages, 4 figure
Detecting Compton Reflection and a Broad Iron Line in MCG-5-23-16 with the Rossi X-ray Timing Explorer
We report the detection with the Rossi X-ray Timing Explorer of a Compton
reflection signature in the Seyfert galaxy MCG-5-23-16. RXTE also resolves the
Fe K-alpha fluorescence line with FWHM ~48,000 km s^{-1}. This measurement
provides the first independent confirmation of ASCA detections in Seyfert
galaxies of broad Fe K-alpha lines that are thought to be the signature of
emission from the inner regions of an accretion disk orbiting a black hole.
Under the assumption that reflection arises from an isotropic source located
above a neutral accretion disk, and using a theoretical model that accounts for
the dependence of the reflected spectrum on inclination angle, we derive a 90%
confidence range for the disk inclination of i = 50 to 81 degrees. The large
inclination is consistent with that expected from the unified model for
MCG-5-23-16 based on its Seyfert 1.9 classification. If we assume that the
high-energy cutoff in the incident spectrum lies at energies larger than a few
hundred keV, then the equivalent width of the Fe K-alpha line is much larger
than predicted for the amount of reflection. This implies either an enhanced
iron abundance, a covering factor of reflecting material > 0.5, or a cutoff in
the incident spectrum at energies between ~60 and ~200 keV.Comment: Accepted for publication in ApJ, LaTeX. 14 pages including 3 figures,
with 1 table as a separate postscript file. Typo corrected in abstrac
A CSO Search for -CH: Detection in the Orion Bar PDR
The results of a Caltech Submillimeter Observatory (CSO) search for
-CH, first detected by Pety et al. (2012) in observations toward the
Horsehead photodissociation region (PDR), are presented. A total of 39 sources
were observed in the 1 mm window. Evidence of emission from -CH is
found in only a single source - the Orion Bar PDR region, which shows a
rotational temperature of 178(13) K and a column density of 7(2) x
cm. In the remaining sources, upper limits of ~10
cm are found. These results are discussed in the context of guiding
future observational searches for this species.Comment: 9 pages, 8 figures, 4 table
Fe XXV and Fe XXVI Diagnostics of the Black Hole and Accretion Disk in Active Galaxies: Chandra Time-Resolved Spectroscopy of NGC 7314
We report the detection of Fe XXV and Fe XXVI emission lines from a
Chandra HETGS observation of the Seyfert~1 galaxy NGC 7314, made simultaneously
with RXTE. The lines are redshifted (cz ~ 1500 km/s) relative to the systemic
velocity and unresolved. We argue that the lines originate in a near face-on
(<7 degrees) disk having a radial line emissivity flatter than r^-2. Line
emission from ionization states of Fe in the range ~Fe I up to Fe XXVI is
observed. The ionization balance of Fe responds to continuum variations on
timescales less than 12.5 ks, supporting an origin of the lines close to the
X-ray source. We present additional, detailed diagnostics from this rich data
set. These results identify NGC 7314 as a key source to study in the future if
we are to pursue reverberation mapping of space-time near black-hole event
horizons. This is because it is first necessary to understand the ionization
structure of accretion disks and the relation between the X-ray continuum and
Fe K line emission. However, we also describe how our results are suggestive of
a means of measuring black-hole spin without a knowledge of the relation
between the continuum and line emission. Finally, these data emphasize that one
{\it can} study strong gravity with narrow (as opposed to very broad) disk
lines. In fact narrow lines offer higher precision, given sufficient energy
resolution.Comment: Accepted for publication in the Astrophysical Journal. 30 pages, six
figures, five of them color. Abstract is abridge
Quasi Two-dimensional Transfer of Elastic Waves
A theory for multiple scattering of elastic waves is presented in a random
medium bounded by two ideal free surfaces, whose horizontal size is infinite
and whose transverse size is smaller than the mean free path of the waves. This
geometry is relevant for seismic wave propagation in the Earth crust. We derive
a time-dependent, quasi-2D radiative transfer equation, that describes the
coupling of the eigenmodes of the layer (surface Rayleigh waves, SH waves, and
Lamb waves). Expressions are found that relate the small-scale fluctuations to
the life time of the modes and to their coupling rates. We discuss a diffusion
approximation that simplifies the mathematics of this model significantly, and
which should apply at large lapse times. Finally, coherent backscattering is
studied within the quasi-2D radiative transfer equation for different source
and detection configurations.Comment: REVTeX, 36 pages with 10 figures. Submitted to Phys. Rev.
Variable Iron K-alpha Lines in Seyfert 1 Galaxies
We find that variability of the iron K-alpha line is common in Seyfert 1
galaxies. Using data from the ASCA archive for objects that have been observed
more than once during the mission, we study the time-averaged spectra from
individual observations, thereby probing variability on timescales that range
from days to years. Since the statistics of the data do not warrant searches
for line variability in terms of a complex physical model, we use a a simple
Gaussian to model the gross shape of the line, and then use the centroid
energy, intensity and equivalent width as robust indicators of changes in the
line profile. We find that ~70% of Seyfert 1s (ten out of fifteen) show
variability in at least one of these parameters: the centroid energy,
intensity, and equivalent width vary in six, four, and eight sources
respectively. Due to the low S/N, limited sampling and time averaging, we
consider these results to represent lower limits to the rate of incidence of
variability. In most cases changes in the line do not appear to track changes
in the continuum. In particular, we find no evidence for variability of the
line intensity in NGC 4151, suggesting an origin in a region larger than the
putative accretion disk, where most of the iron line has been thought to
originate. Mkn 279 is investigated on short timescales. The time-averaged
effective line energy is 6.5 keV in the galaxy rest frame. As the continuum
flux increases by 20% in a few hours, the Fe K line responds with the effective
line energy increasing by 0.22 keV (~10,500 km s^-1). Problems with the ASCA
and Rosat calibration that affect simultaneous spectral fits are discussed in
an appendix.Comment: 26 pages, 30 figures, accepted for publication in the Astrophysical
Journa
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