13,196 research outputs found
X-Ray Fluctuations from Locally Unstable Advection-Dominated Disks
The response of advection-dominated accretion disks to local disturbances is
examined by one-dimensional numerical simulations. It is generally believed
that advection-dominated disks are thermally stable. We, however, find that any
disurbance added onto accretion flow at large radii does not decay so rapidly
that it can move inward with roughly the free-fall velocity. Although
disturbances continue to be present, the global disk structure will not be
modified largely. This can account for persistent hard X-ray emission with
substantial variations observed in active galactic nuclei and stellar black
hole candidates during the hard state. Moreover, when the disturbance reaches
the innermost parts, an acoustic wave emerges, propagating outward as a shock
wave. The resultant light variation is roughly (time) symmetric and is quite
reminiscent of the observed X-ray shots of Cygnus X-1.Comment: plain TeX, 11 pages, without figures; to be published in ApJ Lette
Modeling The Time Variability of Accreting Compact Sources
We present model light curves for accreting Black Hole Candidates (BHC) based
on a recently proposed model for their spectro-temporal properties. According
to this model, the observed light curves and aperiodic variability of BHC are
due to a series of soft photon injections at random (Poisson) intervals near
the compact object and their reprocessing into hard radiation in an extended
but non-uniform hot plasma corona surrounding the compact object. We argue that
the majority of the timing characteristics of these light curves are due to the
stochastic nature of the Comptonization process in the extended corona, whose
properties, most notably its radial density dependence, are imprinted in them.
We compute the corresponding Power Spectral Densities (PSD), autocorrelation
functions, time skewness of the light curves and time lags between the light
curves of the sources at different photon energies and compare our results to
observation. Our model light curves compare well with observations, providing
good fits to their overall morphology, as manifest by the autocorrelation and
skewness functions. The lags and PSDs of the model light curves are also in
good agreement with those observed (the model can even accommodate the presence
of QPOs). Finally, while most of the variability power resides at time scales
\gsim a few seconds, at the same time, the model allows also for shots of a
few msec in duration, in accordance with observation. We suggest that
refinements of this type of model along with spectral and phase lag information
can be used to probe the structure of this class of high energy sources.Comment: 23 pages Latex, 15 encapsulated postscript figures, to appear in the
Astrophysical Journa
Analysis of model Titan atmospheric components using ion mobility spectrometry
The Gas Chromatograph-Ion Mobility Spectrometer (GC-IMS) was proposed as an analytical technique for the analysis of Titan's atmosphere during the Cassini Mission. The IMS is an atmospheric pressure, chemical detector that produces an identifying spectrum of each chemical species measured. When the IMS is combined with a GC as a GC-IMS, the GC is used to separate the sample into its individual components, or perhaps small groups of components. The IMS is then used to detect, quantify, and identify each sample component. Conventional IMS detection and identification of sample components depends upon a source of energetic radiation, such as beta radiation, which ionizes the atmospheric pressure host gas. This primary ionization initiates a sequence of ion-molecule reactions leading to the formation of sufficiently energetic positive or negative ions, which in turn ionize most constituents in the sample. In conventional IMS, this reaction sequence is dominated by the water cluster ion. However, many of the light hydrocarbons expected in Titan's atmosphere cannot be analyzed by IMS using this mechanism at the concentrations expected. Research at NASA Ames and PCP Inc., has demonstrated IMS analysis of expected Titan atmospheric components, including saturated aliphatic hydrocarbons, using two alternate sample ionizations mechanisms. The sensitivity of the IMS to hydrocarbons such as propane and butane was increased by several orders of magnitude. Both ultra dry (waterless) IMS sample ionization and metastable ionization were successfully used to analyze a model Titan atmospheric gas mixture
Probing the Structure of Accreting Compact Sources Through X-Ray Time Lags and Spectra
We exhibit, by compiling all data sets we can acquire, that the Fourier
frequency dependent hard X-ray lags, first observed in the analysis of
aperiodic variability of the light curves of the black hole candidate Cygnus
X-1, appear to be a property shared by several other accreting black hole
candidate sources and also by the different spectral states of this source. We
then present both analytic and numerical models of these time lags resulting by
the process of Comptonization in a variety of hot electron configurations. We
argue that under the assumption that the observed spectra are due to
Comptonization, the dependence of the lags on the Fourier period provides a
means for mapping the spatial density profile of the hot electron plasma, while
the period at which the lags eventually level--off provides an estimate of the
size of the scattering cloud. We further examine the influence of the location
and spatial extent of the soft photon source on the form of the resulting lags
for a variety of configurations; we conclude that the study of the X-ray hard
lags can provide clues about these parameters of the Comptonization process
too. Fits of the existing data with our models indicate that the size of the
Comptonizing clouds are quite large in extent ( 1 light second) with
inferred radial density profiles which are in many instances inconsistent with
those of the standard dynamical models, while the extent of the source of soft
photons appears to be much smaller than those of the hot electrons by roughly
two orders of magnitude and its location consistent with the center of the hot
electron corona.Comment: 20 pages Latex, 11 postscript figures, to appear in the Astrophysical
Journal, Vol 512, Feb 20 issu
Far-Ultraviolet and Far-Infrared Bivariate Luminosity Function of Galaxies: Complex Relation between Stellar and Dust Emission
Far-ultraviolet (FUV) and far-infrared (FIR) luminosity functions (LFs) of
galaxies show a strong evolution from to , but the FIR LF
evolves much stronger than the FUV one. The FUV is dominantly radiated from
newly formed short-lived OB stars, while the FIR is emitted by dust grains
heated by the FUV radiation field. It is known that dust is always associated
with star formation activity. Thus, both FUV and FIR are tightly related to the
star formation in galaxies, but in a very complicated manner. In order to
disentangle the relation between FUV and FIR emissions, we estimate the UV-IR
bivariate LF (BLF) of galaxies with {\sl GALEX} and {\sl AKARI} All-Sky Survey
datasets. Recently we invented a new mathematical method to construct the BLF
with given marginals and prescribed correlation coefficient. This method makes
use of a tool from mathematical statistics, so called "copula". The copula
enables us to construct a bivariate distribution function from given marginal
distributions with prescribed correlation and/or dependence structure. With
this new formulation and FUV and FIR univariate LFs, we analyze various FUV and
FIR data with {\sl GALEX}, {\sl Spitzer}, and {\sl AKARI} to estimate the UV-IR
BLF. The obtained BLFs naturally explain the nonlinear complicated relation
between FUV and FIR emission from star-forming galaxies. Though the faint-end
of the BLF was not well constrained for high- samples, the estimated linear
correlation coefficient was found to be very high, and is remarkably
stable with redshifts (from 0.95 at to 0.85 at ). This implies
the evolution of the UV-IR BLF is mainly due to the different evolution of the
univariate LFs, and may not be controlled by the dependence structure.Comment: 10 pages, 7 figures, Earth, Planets and Space, in pres
Hall Effect in Nested Antiferromagnets Near the Quantum Critical Point
We investigate the behavior of the Hall coefficient in the case of
antiferromagnetism driven by Fermi surface nesting, and find that the Hall
coefficient should abruptly increase with the onset of magnetism, as recently
observed in vanadium doped chromium. This effect is due to the sudden removal
of flat portions of the Fermi surface upon magnetic ordering. Within this
picture, the Hall coefficient should scale as the square of the residual
resistivity divided by the impurity concentration, which is consistent with
available data.Comment: published version; an accidental interchange in the quoting of
analytic dependencies was correcte
Geometrical classification of Killing tensors on bidimensional flat manifolds
Valence two Killing tensors in the Euclidean and Minkowski planes are
classified under the action of the group which preserves the type of the
corresponding Killing web. The classification is based on an analysis of the
system of determining partial differential equations for the group invariants
and is entirely algebraic. The approach allows to classify both characteristic
and non characteristic Killing tensors.Comment: 27 pages, 20 figures, pictures format changed to .eps, typos
correcte
Monte Carlo studies of supersymmetric matrix quantum mechanics with sixteen supercharges at finite temperature
We present the first Monte Carlo results for supersymmetric matrix quantum
mechanics with sixteen supercharges at finite temperature. The recently
proposed non-lattice simulation enables us to include the effects of fermionic
matrices in a transparent and reliable manner. The internal energy nicely
interpolates the weak coupling behavior obtained by the high temperature
expansion, and the strong coupling behavior predicted from the dual black hole
geometry. The Polyakov line takes large values even at low temperature
suggesting the absence of a phase transition in sharp contrast to the bosonic
case. These results provide highly non-trivial evidences for the gauge/gravity
duality.Comment: REVTeX4, 4 pages, 3 figure
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