260,039 research outputs found
Research Questions and Answers
RQ1: What is the impact of SSD on buffer management? RQ2: How could we deal with that
The hard to soft spectral transition in LMXBs - affected by recondensation of gas into an inner disk
Soft and hard spectral states of X-ray transient sources reflect two modes of
accretion, accretion via a geometrically thin, optically thick disk or an
advection-dominated accretion flow (ADAF). The luminosity at transition between
these two states seems to vary from source to source, or even for the same
source during different outbursts, as observed for GX 339-4. We investigate how
the existence of an inner weak disk in the hard state affects the transition
luminosity. We evaluate the structure of the corona above an outer truncated
disk and the resulting disk evaporation rate for different irradiation. In some
cases, recent observations of X-ray transients indicate the presence of an
inner cool disk during the hard state. Such a disk can remain during quiescence
after the last outburst as long as the luminosity does not drop to very low
values (10^-4 to 10^-3 of the Eddington luminosity). Consequently, as part of
the matter accretes via the inner disk, the hard irradiation is reduced. The
hard irradiation is further reduced, occulted and partly reflected by the inner
disk. This leads to a hard-soft transition at a lower luminosity if an inner
disk exists below the ADAF. This seems to be supported by observations for GX
339-4.Comment: 9 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Frustration of freezing in a two dimensional hard-core fluid due to particle shape anisotropy
The freezing mechanism suggested for a fluid composed of hard disks [Huerta
et al., Phys. Rev. E, 2006, 74, 061106] is used here to probe the
fluid-to-solid transition in a hard-dumbbell fluid composed of overlapping hard
disks with a variable length between disk centers. Analyzing the trends in the
shape of second maximum of the radial distribution function of the planar
hard-dumbbell fluid it has been found that the type of transition could be
sensitive to the length of hard-dumbbell molecules. From the Monte
Carlo simulations data we show that if a hard-dumbbell length does not exceed
15% of the disk diameter, the fluid-to-solid transition scenario follows the
case of a hard-disk fluid, i.e., the isotropic hard-dumbbell fluid experiences
freezing. However, for a hard-dumbbell length larger than 15% of disk diameter,
there is evidence that fluid-to-solid transition may change to continuous
transition, i.e., such an isotropic hard-dumbbell fluid will avoid freezing.Comment: 9 pages, 7 figure
Soft X-ray components in the hard state of accreting black holes
Recent observations of two black hole candidates (GX 339-4 and J1753.5-0127)
in the low-hard state (L_X/L_Edd ~ 0.003-0.05) suggest the presence of a cool
accretion disk very close to the innermost stable orbit of the black hole. This
runs counter to models of the low-hard state in which the cool disk is
truncated at a much larger radius. We study the interaction between a
moderately truncated disk and a hot inner flow. Ion-bombardment heats the
surface of the disk in the overlap region between a two-temperature
advection-dominated accretion flow and standard accretion disk, producing a hot
(kT_e ~70 keV) layer on the surface of the cool disk. The hard X-ray flux from
this layer heats the inner parts of the underlying cool disk, producing a soft
X-ray excess. Together with interstellar absorption these effects mimic the
thermal spectrum from a disk extending to the last stable orbit. The results
show that soft excesses in the low-hard state are a natural feature of
truncated disk models.Comment: 12 pages, 8 figures, accepted by Astronomy & Astrophysics, reference
added, minor typos correcte
Magnetic flares in accretion disc coronae and the Spectral States of black hole candidates: the case of GX 339-4
We present a model for the different X-ray spectral states displayed by
Galactic Black Hole Candidates (GBHC). We discuss the physical and spectral
implications for a magnetically structured corona in which magnetic flares
result from reconnection of flux tubes rising from the accretion disk by the
magnetic buoyancy instability. Using observations of one of the best studied
examples, GX339-4, we identify the geometry and the physical conditions
characterizing each of these states. We find that, in the Soft state, flaring
occurs at small scale heights above the accretion disk. The soft thermal-like
spectrum is the result of heating and consequent re-radiation of the hard
X-rays produced by such flares. The hard tail is produced by Comptonization of
the soft field radiation. Conversely, the hard state is the result of flares
triggered high above the underlying accretion disk which produce X-rays via
Comptonization of either internal synchrotron radiation or soft disk photons.
The spectral characteristics of the different states are naturally accounted
for by the choice of geometry: when flares are triggered high above the disk
the system is photon-starved, hence the hard Comptonized spectrum of the hard
state. Intense flaring close to the disk greatly enhances the soft-photon field
with the result that the spectrum softens. We interpret the two states as being
related to two different phases of magnetic energy dissipation. In the Soft
state, Parker instability in the disk favours the emergence of large numbers of
relatively low magnetic field flux tubes. In the hard state, only intense
magnetic fields become buoyant. The model can also qualitatively account for
the observed short timescale variability and the characteristics of the X-ray
reflected component of the hard state.Comment: submitted to MNRAS, Feb. 1998, 10 pages, 3 figures in MNRAS LaTex
styl
The Disk-Wind-Jet Connection in the Black Hole H 1743-322
X-ray disk winds are detected in spectrally soft, disk-dominated phases of
stellar-mass black hole outbursts. In contrast, compact, steady, relativistic
jets are detected in spectrally hard states that are dominated by non-thermal
X-ray emission. Although these distinctive outflows appear to be almost
mutually exclusive, it is possible that a disk wind persists in hard states but
cannot be detected via X-ray absorption lines owing to very high ionization.
Here, we present an analysis of a deep, 60 ksec Chandra/HETGS observation of
the black hole candidate H 1743-322 in the low/hard state. The spectrum shows
no evidence of a disk wind, with tight limits, and within the range of ionizing
flux levels that were measured in prior Chandra observations wherein a wind was
clearly detected. In H 1743-322, at least, disk winds are actually diminished
in the low/hard state, and disk winds and jets are likely state-dependent and
anti-correlated. These results suggest that although the launching radii of
winds and jets may differ by orders of magnitude, they may both be tied to a
fundamental property of the inner accretion flow, such as the mass accretion
rate and/or the magnetic field topology of the disk. We discuss these results
in the context of disk winds and jets in other stellar-mass black holes, and
possible launching mechanisms for black hole outflows.Comment: Accepted for publication in ApJ Letter
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
The Emission from Inner Disk and Corona in the Low and Intermediate Spectral States of Black Hole X-ray Binaries
Recent observations reveal that a cool disk may survive in the innermost
stable circular orbit (ISCO) for some black hole X-ray binaries in the
canonical low/hard state. The spectrum is characterized by a power law with a
photon index in the range of 2-10 keV and a weak disk
component with temperature of keV. In this work, We revisit the
formation of such a cool, optically thick, geometrically thin disk in the most
inner region of black hole X-ray binaries at the low/hard state within the
context of disk accretion fed by condensation of hot corona. By taking into
account the cooling process associated with both Compton and conductive
processes in a corona, and the irradiation of the hot corona to the disk, we
calculate the structure of the corona. For viscosity parameter ,
it's found that the inner disk can exist for accretion rate ranging from , over which the electron temperatures of
the corona are in the range of producing the hard X-ray
emission. We calculate the emergent spectra of the inner disk and corona for
different mass accretion rates. The effect of viscosity parameter and
albedo ( is defined as the energy ratio of reflected radiation from the
surface of the thin disk to incident radiation upon it from the corona) to the
emergent spectra are also presented. Our model is used to explain the recent
observations of GX 339-4 and Cyg X-1, in which the thin disk may exist at ISCO
region in the low/hard state at luminosity around a few percent of . It's found that the observed maximal effective temperature of the
thermal component and the hard X-ray photon index can be matched well
by our model.Comment: Accepted for publication by Ap
- …