68 research outputs found
The Dependence of Spectral State Transition and Disk Truncation on Viscosity Parameter $\alpha
A wealth of Galactic accreting X-ray binaries have been observed both in
low/hard state and high/soft state. The transition between these two states was
often detected. Observation shows that the transition luminosity between these
two states is different for different sources, ranging from 1% to 4% of the
Eddington luminosity. Even for the same source the transition luminosity at
different outbursts is also different. The transition can occur from 0.0069 to
0.15 Eddington luminosity. To investigate the underlying physics, we study the
influence of viscosity parameter on the transition luminosity on the
basis of the disk-corona model for black holes. We calculate the mass
evaporation rate for a wide range of viscosity parameter, . By fitting the numerical results, we obtain fitting formulae for both the
transition accretion rate and the corresponding radius as a function of
. We find that the transition luminosity is very sensitive to the value
of , . For ,
the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2
Eddington luminosity. Comparing with observations we find that the transition
luminosity can be fitted by adjusting the value of , and the model
determined values of are mostly in the range of observationally
inferred value. Meanwhile we investigate the truncation of the disk in the
low/hard state for some luminous sources. Our results are roughly in agreement
with the observations.Comment: 8 pages, 3 figures,accepted by PAS
The Spectral Features of Disk and Corona with Mass Evaporation in the Low/Hard State
We investigate the spectral features of accretion flows composed of an outer
cool, optically thick disk and inner hot, optically thin, advection dominated
accretion flows (ADAF) within the framework of disk and corona with mass
evaporation (Liu et al. 2002a). In this work, both the magnetic field and
Compton scattering of soft photons from the disk by electrons in the corona are
included to calculate the evaporation rates at different distances. The disk is
truncated at the distance where the evaporation rate equals to the accretion
rate (). For a series of accretion rates,
the corresponding truncation radii are calculated out, with which we are able
to calculate the emergent spectra from the inner ADAF + outer disk + corona. At
very low accretion rates, the spectra are similar to that of a pure ADAF
because the disk is truncated at large distances. The disk component becomes
important at high accretion rates since the truncation occurs at small
distances. An anti-correlation between the Eddington ratio and the hard X-ray photon index at low/hard states is predicted by the model. Comparing the theoretical
results with observations, we find that our model can reproduce the
anti-correlation between the Eddington ratio and the hard X-ray photon
index observed for the X-ray binary XTE J1118+480.Comment: 11 pages and 7 figures, published by PAS
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
- …