Inner Cool Disks in the Low/Hard State of Accreting Black Holes
In collaboration with R.E. Taam, F. Meyer, and E.
Meyer-Hofmeister
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X- - log f log h soft (high) state blackbody spec. with kT 1
keV hard (low) state power-law, f - with 0.7 cutoff at 100 keV
ADAF
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0.02 ADAF
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Interpreting the state transition m0.02, Disk-dominant
accretion Soft spectral srtate Transition rate: Transition is
thought to be the consequence of the disk corona interaction.
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New observation feature at low states Evidence pointing to
possible presence of a cool inner disk in low hard states of BH
transients GX339-4 ( Miller et al. 2006, ApJ 652, L113) X-ray
luminosity (3-100keV) of 0.05 Eddington luminosity Disk component
of 0.3keV from region around ISCO A broad iron line required to fit
the spectrum Swift J1753.5-0127 (Miller et al. 2006, ApJ 653,525)
X-ray luminosity (0.5-10keV) of 0.003 Eddington luminosity Disk
component of 0.2keV from region around ISCO
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New observation feature at low/states GX339-4 ( Tomsick et al.
2008 ApJ in press) X-ray luminosity (1-100keV) of 0.023 Eddington
luminosity Disk component of 0.201keV from region around ISCO A
broad iron line required to fit the spectrum and X-ray luminosity
(1-100keV) of 0.008 Eddington luminosity Disk component of 0.165keV
from region around ISCO A broad iron line required to fit the
spectrum New observations: (Ramadevi et al. 2007;Rykoff et
al.2007;Tomsick et al. 2008) Difficult to be explained by the
standard accretion picture in BH transient systems
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Previous disk-corona evaporation model At low states, disk
vanishes in the dominant evaporation region, instead an ADAF is the
dominant accretion process. An inner disk disappears within its
viscous time by accretion As a consequence, the accretion flow is
Inner ADAF +outer disk (Meyer, Liu, Meyer-Hofmeister, 2000,
A&A) BH ADAF DISK
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The new model To maintain the inner disk existing longer than
the viscous time, continuous mass feeding to the inner disk is
needed. One potential possibility for maintenance of the inner disk
is coronal mass continuously condensing into the inner disk. Can
mass condense from the corona to the disk at low hard states? Inner
disk The mass flow Outer disk corona The energy flow conduction BH
Comptonization Coronal gap
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Our model: assumptions and aims Consider an ADAF + cool disk at
low state in the inner region Study the interaction between the
disk and the overlying ADAF To check Could an inner disk exist at
low state? Under what condition? How strong is the inner disk?
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ADAF
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The ADAF above a cool disk ADAF structure is determined by
model parameters, i.e. viscosity, BH mass, accretion rate except
for the electron temperature. Electron temperature in the ADAF is
determined by the energy balance Vertical heat conduction + Compton
scattering (of the disk photons) = heating by coulomb collisions
with ions. Conductive flux to the transition layer is calculated
with the known ADAF structure.
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The radiating layer The coupling of ions and electrons The
energy balance in the radiating layer
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Condensation or evaporation? In the transition/radiation layer
the energy balance: Enthalpy flux+ conductive flux=radiation flux
Local evaporation/condensation rate With F c ADAF and the function
of viscosity, BH mass, accretion rate and radius. If evaporation;
If condensation C=1 determines the critical radius for condensation
and hence the inner disk size.
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Numerical results Condensation can occur in the innermost
region at low hard states in a certain parameter range The
condensation region is determined by The inner disk size depends ,
m, and the accretion rate The mass flow rate in the inner disk is
the integral of local condensation rate, determined also by , m,
and the accretion rate. The disk effective temperature depends on ,
m, and the accretion rate
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Model predictions Model predicts the inner disk size, the
maximal disk temperature, the disk luminosity, the coronal
luminosity, and the ratio of disk and corona luminosity as
functions of viscous, BH mass, accretion rate
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Comparison with Observations GX339-4 Input parameters From
Observation (Miller et al. 2006): m=10, L/L Edd =0.03, disk
temperature kT=0.3keV, Optional: =0.3 Model predictions Compton
dominant cooling Inner disk size: r d =66 L disk /L ADAF =12.7%
accretion rate=0.037, producing L/L Edd =0.03 kT eff =0.16keV
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Comparison with observations J1753.5-0127 Input parameters From
Observation (Miller et al. 2006): m=10, L/L Edd
=0.01(extrapolation), Soft component kT=0.2keV Optional:=0.3 Model
predictions Compton dominant cooling Inner disk size: r d =22 L
disk /L ADAF =10% accretion rate=0.034 kT eff =0.115keV
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The condensation rate as a function of accretion rate
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Fits to the observation of Tomsick et al. (2008)
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A mass-independent process The model does not depend on the
mass of black hole For the same accretion rate (in units of
Eddington rate), the predicted condensation rate (in units of
Eddington rate), inner disk size (in units of R S ), and luminosity
(in units of L Edd ) do not depend on the mass of black hole except
for the disk T eff. Applicable to AGNs Inner disk produces weak
UV-Optical radiation Inner disk provides a natural site of cool
material of Fe line emission
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Conclusion An inner cool disk could exist in the low hard state
of accreting black hole, fed by the coronal condensation The inner
disk is much weaker than the ADAF, but it provides a site for cool
materials close to BH, thus weak disk component and iron line
Roughly a very weak disk could exist at luminosity as low as
0.001.
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Future work Global solution The model in supermassive black
holes? Any relation between inner disk and jet? Compact jets
detected in GX339-4 (Tomsick et al. 2008)