126 research outputs found
The transition from a cool disk to an ion supported flow
We show that the inner regions of a cool accretion disk in an X-ray binary
can transform into an advective, ion supported accretion flow (an optically
thin ADAF, here called ISAF), through events involving only the known
properties of the Coulomb interaction in a two-temperature plasma, standard
radiation processes, and viscous heating. The optically thin inner edge of the
disk is heated to a few 100 keV by the strong flux of hot ions from the
surrounding hot ISAF. We show that he resident ions in this `warm' disk are
thermally unstable due to internal viscous heating, and heat up to their virial
temperature. The innermost disk regions thus evaporate and feed the ISAF. These
processes are demonstrated with time dependent calculations of a
two-temperature plasma in vertical hydrostatic equilibrium, including heating
by external ions, internal proton--electron energy exchange, and viscous
heating. The process complements the `coronal' evaporation mechanism which
operates at larger distances from the central object.Comment: Final version as accepted A&A. Includes new subzection on
applicability of Spitzer's energy loss formula for the incident proton
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
X-Ray spectra from accretion disks illuminated by protons
The X-ray spectrum from a cool accretion disk heated by virialized protons is
computed. The cool disk is either embedded in a magnetically heated accretion
disk corona or partly extends into an ion supported torus (or ADAF). We
calculate the stationary equilibrium between proton heating, electron thermal
conduction and the radiative losses by bremsstrahlung and Compton scattering. A
heated surface layer on top of the accretion disk is produced with temperatures
between 60--90 keV above a cool layer with temperatures of 0.01 keV (AGN) and
1keV (galactic black hole candidates). The spectra produced by the surface
layer are reminiscent of hard state spectra, but a bit too steep, especially
for AGN's. Near the inner edge of the disk, where the optical depth of the disk
, we find that the cool component of the disk disappears.
Instead, the hot protons from the corona/ADAF heat the disk, on a dynamical
time-scale, to temperatures of several 100 keV, limited by pair production.
This region, here called a `warm disk', could contribute significantly to the
hard X-ray spectra and could be important for feeding material into an ADAF.Comment: Accepted by A&A. Includes new subsection on the applicability of
Spitzer's energy loss formula for the incident proton
X-Ray spectra from protons illuminating a neutron star
We consider the interaction of a slowly rotating unmagnetized neutron star
with a hot (ion supported, ADAF) accretion flow. The virialized protons of the
ADAF penetrate into the neutron star atmosphere, heating a surface layer.
Detailed calculations are presented of the equilibrium between heating by the
protons, electron thermal conduction, bremsstrahlung and multiple Compton
scattering in this layer. Its temperature is of the order 40-70 keV. Its
optical depth increases with the incident proton energy flux, and is of the
order unity for accretion at -- of the Eddington rate. At
these rates, the X-ray spectrum produced by the layer has a hard tail extending
to 100 keV, and is similar to the observed spectra of accreting neutron stars
in their hard states. The steep gradient at the base of the heated layer gives
rise to an excess of photons at the soft end of the spectrum (compared to a
blackbody) through an `inverse photosphere effect'. The differences with
respect to previous studies of similar problems are discussed, they are due
mostly to a more accurate treatment of the proton penetration process and the
vertical structure of the heated layer.Comment: Accepted for publication in A&
Ein neuer Genort für eine autosomal-dominante, nichtsyndromale Schwerhörigkeit (DFNA33) liegt auf Chromosom 13q34-qte
Bei der Untersuchung einer deutschen Familie mit nichtsyndromalem Hörverlust mit frühem Beginn und autosomal-dominantem Erbgang konnten wir eine Kopplung zu bekannten DFNA-Loci ausschließen und die Existenz eines neuen Locus (DFNA33) bestätigen. Mit einem nachfolgenden Genom-Scan wurde der Phänotyp auf einem 6-cM-Intervall auf Chromosom 13q34-qter kartiert. Für den Marker D13S285 wurde ein maximaler 2-Punkt-Lodscore von 2,96 erreicht, der maximale Lodscore in der Multipoint-Analyse betrug 3,28 bei 124,56 cM. = By investigation of a German family pedigree with non-syndromic hearing impairment of early onset and autosomal-dominant mode of inheritance, linkage to known DFNA loci was excluded, and the existence of a new locus (DFNA33) was revealed. In a subsequent genomic scan the phenotype was mapped to a 6 cM interval on chromosome 13q34-qter. A maximum two-point lod score of 2.96 was obtained for the marker D13S285 with a maximum lod score in the multipoint analysis of 3.28 at 124.56 cM
Excitation of low-frequency QPOs in black-hole accretion
We study possible mechanisms of excitation of quasiperiodic oscillations in
the accretion flow of black hole accreters in their hard spectral states, in
the context of the `truncated disk' model. Quasi-spherical oscillations of the
inner ion-supported accretion flow (ISAF) can be excited by the interaction of
this hot flow with the cool disk extending outward from it. The fundamental
mode of (p-mode) oscillation is most easily excited, and has a frequency near
the Kepler frequency at the inner edge of the cool disk. The strongest
excitation mechanism is a feedback loop involving cooling of the ISAF by soft
photons from the cool disk and heating of the cool disk by the ISAF, while
synchrotron emission can be a relatively strong damping effect. Growth times
are computed by detailed Comptonization calculations of the interaction of an
idealized ISAF with a cool disk. Typical growth times as short as a few
dynamical times are found, while amplitudes can be as large as 10%.Comment: 11 pages, 5 figures, accepted for publication in A&
Transient jets in V617 Sagittarii
Some of the luminous Compact Binary Supersoft X-Ray sources (CBSS) have shown
indications of jets, also called satellites due to their appearance in the
spectra. In V Sagittae (V Sge) stars, the galactic counterparts of the CBSS,
such features have been reported only for WX Cen. If V Sge stars are indeed the
analogs of CBSS, one may expect transient jet emission in other objects of this
class. Spectroscopic observations of the V Sge star V617 Sgr have been made,
both at high photometric state and at decline. We show that V617 Sgr presents
Halpha satellites at high photometric state with velocities of +/-780 km/s.
This feature confirms, once more, the CBSS nature of the V Sge stars, however
the details of the spectral characteristics also suggest that the two groups of
stars display some intrinsic spectroscopic differences, which are likely to be
due to a selection effect related to chemical abundance.Comment: Four pages, accepted to be published as a Letter in A&
The Structure and Evolution of Circumbinary Disks in Cataclysmic Variable Systems
We investigate the structure and evolution of a geometrically thin viscous
Keplerian circumbinary (CB) disk, using detailed models of their
radiative/convective vertical structure. We use a simplified description for
the evolution of the cataclysmic binary and focus on cases where the
circumbinary disk causes accelerated mass transfer (> 1e-8 Msun/yr). The inner
edge of the disk is assumed to be determined by the tidal truncation radius and
the mass input rate into the disk is assumed to be a small fraction (1e-5-0.01)
of the mass transfer rate. Under the action of the viscous stresses in the disk
the matter drifts outward with the optically thick region extending to several
AU. The inner part of the disk is cool with maximum effective temperatures <
3,000 K while the outermost parts of the disk are < 30 K and optically thin. We
calculate the effects of thermal instability on a sufficiently massive CB disk.
It leads to outbursts reminiscent of those in thermally unstable accretion
disks, with the instability remaining confined to the inner regions of the CB
disk. However, for most of the evolutionary sequences the surface densities
required to trigger instability are not reached. The spectral energy
distributions from circumbinary disks are calculated, and the prospects for the
detection of such disks in the infrared and submm wavelength regions are
discussed.Comment: 16 pages, 12 figures, accepted for publication by Ap
The Contribution of Particle Impact to the Production of Fe K Emission from Accreting Black Holes
The iron K line is perhaps the most important spectral diagnostic available
in the study of accreting black holes. The line is thought to result from the
reprocessing of external X-rays by the surface of the accretion disk. However,
as is observed in the solar corona, illumination by energetic particles may
also produce line emission. In principle, such a process may be uncorrelated
with the observed X-rays and could explain some of the unexpected variability
behavior of the Fe line. This paper compares predictions of iron K flux
generated by impacting electrons and protons to that from photoionization.
Non-thermal power-laws of electrons are considered as well as thermal
distributions of electrons and virialized protons. The electrons are thought to
originate in a magnetically dominated accretion disk corona, while the protons
are considered in the context of a two phase (hot/cold) accretion scenario. In
each case, the Fe K flux from particle impact is found to be < 1% of that
produced by photoionization by a hard X-ray power-law (normalized to the same
energy flux as the particles). Thus, the electrons or protons must strike the
disk with 100--10,000 times more energy flux than radiation for particle impact
to be a significant producer of Fe K flux. This situation is difficult to
reconcile with the observations of hard X-ray spectra, or the proposed particle
acceleration mechanisms in the accretion disk corona. Truncated accretion flows
must be externally illuminated by hard X-rays in order to produce the Fe line,
as proton impact is very inefficient in generating line emission. In contrast
to the Sun, our conclusion is that, with the possible exception for localized
regions around magnetic footpoints, particle impact will not be an important
contributor to the X-ray emission in accreting black holes.Comment: 27 pages, 6 figures, ApJ accepte
Circumbinary disks and cataclysmic variable evolution
The influence of a circumbinary (CB) disk on the evolution of cataclysmic
variable (CV) binary systems is investigated. We show that CB mass surface
densities sufficient to influence the evolution rate are plausibly provided by
the outflows observed in CVs, if the net effect of these winds is to deliver
-- of the mass transfer rate to the CB disk. The torque
exerted by the CB disk provides a positive feedback between mass transfer rate
and CB disk mass which can lead to mass transfer rates of \sim 10^{-8}
-10^{-7} \mpy. This mechanism may be responsible for causing the range of
variation of mass transfer rates in CV's. In particular, it may explain rates
inferred for the novalike variables and the supersoft X-ray binary systems
observed near the upper edge of the period gap ( hr), as well as
the spread in mass transfer rates above and below the period gap. Consquences
and the possible observability of such disks are discussed.Comment: submitted to Ap
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