369 research outputs found
Galaxy Formation in Triaxial Halos: Black Hole-Bulge-Dark Halo Correlation
The masses of supermassive black holes (SBHs) show correlations with bulge
properties in disk and elliptical galaxies. We study the formation of galactic
structure within flat-core triaxial haloes and show that these correlations can
be understood within the framework of a baryonic component modifying the
orbital structure in the underlying potential. In particular, we find that
terminal properties of bulges and their central SBHs are constrained by the
destruction of box orbits in the harmonic cores of dark haloes and the
emergence of progressively less eccentric loop orbits there. SBH masses, M_SBH,
should exhibit a tighter correlation with bulge velocity dispersions, sigma_B,
than with bulge masses, M_B, in accord with observations, if there is a
significant scatter in the M_H-sigma_H relation for the halo. In the context of
this model the observed M_SBH-sigma_B relation implies that haloes should
exhibit a Faber-Jackson type relationship between their masses and velocity
dispersions. The most important prediction of our model is that halo properties
determine the bulge and SBH parameters. The model also has important
implications for galactic morphology and the process of disk formation.Comment: 20 pp, 10 postscript figures, submitted to the Astrophysical Journa
Rms-flux relation of Cyg X-1 with RXTE: dipping and nondipping cases
The rms (root mean square) variability is the parameter for understanding the
emission temporal properties of X-ray binaries (XRBs) and active galactic
nuclei (AGN).
The rms-flux relation with Rossi X-ray Timing Explorer (RXTE) data for the
dips and nondip of black hole Cyg X-1 has been investigated in this paper. Our
results show that there exist the linear rms-flux relations in the frequency
range 0.1-10 Hz for the dipping light curve. Moreover, this linear relation
still remains during the nondip regime, but with the steeper slope than that of
the dipping case in the low energy band. For the high energy band, the slopes
of the dipping and nondipping cases are hardly constant within errors. The
explanations of the results have been made by means of the ``Propagating
Perturbation'' model of Lyubarskii (1997).Comment: 15 pages, 12 figures, Accepted for publication in Astrophysics &
Space Scienc
X-ray continuum variability of MCG-6-30-15
This paper presents a comprehensive examination of the X-ray continuum
variability of the bright Seyfert 1 galaxy MCG-6-30-15. The source clearly
shows the strong, linear correlation between rms variability amplitude and flux
first seen in Galactic X-ray binaries. The high frequency power spectral
density (PSD) of MCG-6-30-15 is examined in detail using a Monte Carlo fitting
procedure and is found to be well represented by a steep power-law at high
frequencies (with a power-law index alpha ~ 2.5), breaking to a flatter slope
(alpha ~ 1) below f_br ~ 0.6 - 2.0 x 10^-4 Hz, consistent with the previous
results of Uttley, McHardy & Papadakis. The slope of the power spectrum above
the break is energy dependent, with the higher energies showing a flatter PSD.
At low frequencies the variations between different energy bands are highly
coherent while at high frequencies the coherence is significantly reduced. Time
lags are detected between energy bands, with the soft variations leading the
hard. The magnitude of the lag is small (<200 s for the frequencies observed)
and is most likely frequency dependent. These properties are remarkably similar
to the temporal properties of the Galactic black hole candidate Cygnus X-1. The
characteristic timescales in these two types of source differ by ~10^5;
assuming that these timescales scale linearly with black hole mass then
suggests a black hole mass ~10^6 M_sun for MCG-6-30-15. We speculate that the
timing properties of MCG-6-30-15 may be analogous to those of Cyg X-1 in its
high/soft state and discuss a simple phenomenological model, originally
developed to explain the timing properties of Cyg X-1, that can explain many of
the observed properties of MCG-6-30-15.Comment: 19 pages, 24 figures, accepted for publication in MNRA
The correlation timescale of the X-ray flux during the outbursts of soft X-ray transients
Recent studies of black hole and neutron star low mass X-ray binaries (LMXBs)
show a positive correlation between the X-ray flux at which the
low/hard(LH)-to-high/soft(HS) state transition occurs and the peak flux of the
following HS state. By analyzing the data from the All Sky Monitor (ASM)
onboard the Rossi X-ray Timing Explorer (RXTE), we show that the HS state flux
after the source reaches its HS flux peak still correlates with the transition
flux during soft X-ray transient (SXT) outbursts. By studying large outbursts
or flares of GX 339-4, Aql X-1 and 4U 1705-44, we have found that the
correlation holds up to 250, 40, and 50 d after the LH-to-HS state transition,
respectively. These time scales correspond to the viscous time scale in a
standard accretion disk around a stellar mass black hole or a neutron star at a
radius of ~104-5 Rg, indicating that the mass accretion rates in the accretion
flow either correlate over a large range of radii at a given time or correlate
over a long period of time at a given radius. If the accretion geometry is a
two-flow geometry composed of a sub-Keplerian inflow or outflow and a disk flow
in the LH state, the disk flow with a radius up to ~105 Rg would have
contributed to the nearly instantaneous non-thermal radiation directly or
indirectly, and therefore affects the time when the state transition occurs.Comment: 7 pages, 3 figure
Predictive significance of the six-minute walk distance for long-term survival in chronic hypercapnic respiratory failure
Background: The 6-min walk distance ( 6-MWD) is a global marker of functional capacity and prognosis in chronic obstructive pulmonary disease ( COPD), but less explored in other chronic respiratory diseases. Objective: To study the role of 6-MWD in chronic hypercapnic respiratory failure ( CHRF). Methods: In 424 stable patients with CHRF and non-invasive ventilation ( NIV) comprising COPD ( n = 197), restrictive diseases ( RD; n = 112) and obesity-hypoventilation- syndrome ( OHS; n = 115), the prognostic value of 6-MWD for long- term survival was assessed in relation to that of body mass index (BMI), lung function, respiratory muscle function and laboratory parameters. Results: 6-MWD was reduced in patients with COPD ( median 280 m; quartiles 204/350 m) and RD ( 290 m; 204/362 m) compared to OHS ( 360 m; 275/440 m; p <0.001 each). Overall mortality during 24.9 (13.1/40.5) months was 22.9%. In the 424 patients with CHRF, 6-MWD independently predicted mortality in addition to BMI, leukocytes and forced expiratory volume in 1 s ( p <0.05 each). In COPD, 6-MWD was strongly associated with mortality using the median {[} p <0.001, hazard ratio ( HR) = 3.75, 95% confidence interval (CI): 2.24-6.38] or quartiles as cutoff levels. In contrast, 6-MWD was only significantly associated with impaired survival in RD patients when it was reduced to 204 m or less (1st quartile; p = 0.003, HR = 3.31, 95% CI: 1.73-14.10), while in OHS 6-MWD had not any prognostic value. Conclusions: In patients with CHRF and NIV, 6-MWD was predictive for long- term survival particularly in COPD. In RD only severely reduced 6-MWD predicted mortality, while in OHS 6-MWD was relatively high and had no prognostic value. These results support a disease-specific use of 6-MWD in the routine assessment of patients with CHRF. Copyright (C) 2007 S. Karger AG, Basel
Electronic states and quantum transport in double-wall carbon nanotubes
Electronic states and transport properties of double-wall carbon nanotubes
without impurities are studied in a systematic manner. It is revealed that
scattering in the bulk is negligible and the number of channels determines the
average conductance. In the case of general incommensurate tubes, separation of
degenerated energy levels due to intertube transfer is suppressed in the energy
region higher than the Fermi energy but not in the energy region lower than
that. Accordingly, in the former case, there are few effects of intertube
transfer on the conductance, while in the latter case, separation of
degenerated energy levels leads to large reduction of the conductance. It is
also found that in some cases antiresonance with edge states in inner tubes
causes an anomalous conductance quantization, , near the Fermi
energy.Comment: 24 pages, 13 figures, to be published in Physical Review
Slow dynamics near glass transitions in thin polymer films
The -process (segmental motion) of thin polystyrene films supported
on glass substrate has been investigated in a wider frequency range from
10 Hz to 10 Hz using dielectric relaxation spectroscopy and thermal
expansion spectroscopy. The relaxation rate of the -process increases
with decreasing film thickness at a given temperature above the glass
transition. This increase in the relaxation rate with decreasing film thickness
is much more enhanced near the glass transition temperature. The glass
transition temperature determined as the temperature at which the relaxation
time of the -process becomes a macroscopic time scale shows a distinct
molecular weight dependence. It is also found that the Vogel temperature has
the thickness dependence, i.e., the Vogel temperature decreases with decreasing
film thickness. The expansion coefficient of the free volume is
extracted from the temperature dependence of the relaxation time within the
free volume theory. The fragility index is also evaluated as a function of
thickness. Both and are found to decrease with decreasing film
thickness.Comment: 9 pages, 7 figures, and 2 table
Equilibrium Initialization and Stability of Three-Dimensional Gas Disks
We present a new systematic way of setting up galactic gas disks based on the
assumption of detailed hydrodynamic equilibrium. To do this, we need to specify
the density distribution and the velocity field which supports the disk. We
first show that the required circular velocity has no dependence on the height
above or below the midplane so long as the gas pressure is a function of
density only. The assumption of disks being very thin enables us to decouple
the vertical structure from the radial direction. Based on that, the equation
of hydrostatic equilibrium together with the reduced Poisson equation leads to
two sets of second-order non-linear differential equation, which are easily
integrated to set-up a stable disk. We call one approach `density method' and
the other one `potential method'. Gas disks in detailed balance are especially
suitable for investigating the onset of the gravitational instability. We
revisit the question of global, axisymmetric instability using fully
three-dimensional disk simulations. The impact of disk thickness on the disk
instability and the formation of spontaneously induced spirals is studied
systematically with or without the presence of the stellar potential. In our
models, the numerical results show that the threshold value for disk
instability is shifted from unity to 0.69 for self-gravitating thick disks and
to 0.75 for combined stellar and gas thick disks. The simulations also show
that self-induced spirals occur in the correct regions and with the right
numbers as predicted by the analytic theory.Comment: 17 pages, 10 figures, accepted by MNRA
Gravitating discs around black holes
Fluid discs and tori around black holes are discussed within different
approaches and with the emphasis on the role of disc gravity. First reviewed
are the prospects of investigating the gravitational field of a black
hole--disc system by analytical solutions of stationary, axially symmetric
Einstein's equations. Then, more detailed considerations are focused to middle
and outer parts of extended disc-like configurations where relativistic effects
are small and the Newtonian description is adequate.
Within general relativity, only a static case has been analysed in detail.
Results are often very inspiring, however, simplifying assumptions must be
imposed: ad hoc profiles of the disc density are commonly assumed and the
effects of frame-dragging and completely lacking. Astrophysical discs (e.g.
accretion discs in active galactic nuclei) typically extend far beyond the
relativistic domain and are fairly diluted. However, self-gravity is still
essential for their structure and evolution, as well as for their radiation
emission and the impact on the environment around. For example, a nuclear star
cluster in a galactic centre may bear various imprints of mutual star--disc
interactions, which can be recognised in observational properties, such as the
relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be
available from http://www.iop.org/EJ/journal/CQ
Ab-initio structural, elastic, and vibrational properties of carbon nanotubes
A study based on ab initio calculations is presented on the estructural,
elastic, and vibrational properties of single-wall carbon nanotubes with
different radii and chiralities. We use SIESTA, an implementation of
pseudopotential-density-functional theory which allows calculations on systems
with a large number of atoms per cell. Different quantities like bond
distances, Young moduli, Poisson ratio and the frequencies of different phonon
branches are monitored versus tube radius. The validity of expectations based
on graphite is explored down to small radii, where some deviations appear
related to the curvature effects. For the phonon spectra, the results are
compared with the predictions of the simple zone-folding approximation. Except
for the known defficiencies of this approximation in the low-frequency
vibrational regions, it offers quite accurate results, even for relatively
small radii.Comment: 13 pages, 7 figures, submitted to Phys. Rev. B (11 Nov. 98
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