59,387 research outputs found
A Model for Spectral States and Their Transition in Cyg X-1
A new accretion picture based on a small disk surrounding a black hole is
developed for the wind-fed source Cyg X-1. The hard and soft spectral states of
Cyg X-1 are interpreted in terms of co-spatial two component flows for the
innermost region of an accretion disk. The state transitions result from the
outward expansion and inward recession of this inner disk for the hard to soft
and soft to hard transition respectively. The theoretical framework for state
transitions in black hole X-ray binaries with high mass companions involving a
change in the inner disk size, thus, differs from systems with low mass
companions involving the change in the outer disk size. This fundamental
difference stems from the fact that matter captured and supplied to the black
hole in wind-fed systems has low specific angular momentum and is hot
essentially heated in the bow and spiral shocks, whereas it has high specific
angular momentum and is cool in Roche lobe overflow systems. The existence of a
weak cool disk around the ISCO region in the hard state allows for the presence
of a relativistically broadened Fe K line. The small disk fed by gas
condensation forms without an extensive outer disk, precluding thermal
instabilities and large outbursts, resulting in the lack of large amplitude
outbursts and hysteresis effects in the light curve of high mass black hole
X-ray binaries. Their relatively persistent X-ray emission is attributed to
their wind-fed nature.Comment: 13 pages, 2 figures. Accepted for publication in Ap
Crosstalk Correction in Atomic Force Microscopy
Commercial atomic force microscopes usually use a four-segmented photodiode
to detect the motion of the cantilever via laser beam deflection. This read-out
technique enables to measure bending and torsion of the cantilever separately.
A slight angle between the orientation of the photodiode and the plane of the
readout beam, however, causes false signals in both readout channels, so-called
crosstalk, that may lead to misinterpretation of the acquired data. We
demonstrate this fault with images recorded in contact mode on ferroelectric
crystals and present an electronic circuit to compensate for it, thereby
enabling crosstalk-free imaging
Two-temperature coronal flow above a thin disk
We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu,
& Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including
different temperatures in ions and electrons as well as Compton cooling. We
found that the mass evaporation rate and hence the fraction of accretion energy
released in the corona depend strongly on the rate of incoming mass flow from
outer edge of the disk, a larger rate leading to more Compton cooling, less
efficient evaporation and a weaker corona. We also found a strong dependence on
the viscosity, higher viscosity leading to an enhanced mass flow in the corona
and therefore more evaporation of gas from the disk below. If we take accretion
rates in units of the Eddington rate our results become independent on the mass
of the central black hole. The model predicts weaker contributions to the hard
X-rays for objects with higher accretion rate like narrow-line Seyfert 1
galaxies (NLS1s), in agreement with observations. For luminous active galactic
nuclei (AGN) strong Compton cooling in the innermost corona is so efficient
that a large amount of additional heating is required to maintain the corona
above the thin disk.Comment: 17 pages, 6 figures. ApJ accepte
A cool disk in the Galactic Center?
We study the possibility of a cool disk existing in the Galactic Center in
the framework of the disk-corona evaporation/condensation model. Assuming an
inactive disk, a hot corona should form above the disk since there is a
continuous supply of hot gas from stellar winds of the close-by massive stars.
Whether the cool disk can survive depends on the mass exchange between the disk
and corona. If the disk-corona interaction is dominated by evaporation and the
rate is larger than the Bondi accretion rate in the Galactic Center, the disk
will be depleted within a certain time period and no persistent disk will
exist. On the other hand, if the interaction results in hot gas steadily
condensing into the disk, an inactive cool disk might survive. For this case we
further investigate the Bremsstrahlung radiation from the hot corona and
compare it with the observed X-ray luminosity. Our model shows that, for
standard viscosity in the corona (alpha=0.3), the mass evaporation rate is much
higher than the Bondi accretion rate and the coronal density is much larger
than that inferred from Chandra observations. An inactive disk can not survive
such strong evaporation. For small viscosity (alpha<0.07) we find condensation
solutions. But detailed computations show that in this case there is too much
X-ray radiation from the corona to be in agreement with the observations.
Therefore, we conclude that there should be no thin/inactive disk presently in
the Galactic Center. However, we do not exclude that the alternative
non-radiative model of Nayakshin (2004) might instead be realized in nature.Comment: 8 pages, including 3 figures, accepted for publication in A&
Acoustical evaluation of the NASA Langley full-scale wind tunnel
Determining types of acoustical measurements suitable for test section of NASA Langley wind tunne
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