Interpreting the state transition m<0.02, RIAF-dominant accretion Hard spectral state m>0.02, Disk-dominant accretion Soft spectral srtate Transition rate: Transition is thought to be the consequence of the disk corona interaction.
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
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
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
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
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?
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.
The radiating layer The coupling of ions and electrons The energy balance in the radiating layer
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.
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 α, m, and the accretion rate The mass flow rate in the inner disk is the integral of local condensation rate, determined also by α, m, and the accretion rate. The disk effective temperature depends on α, m, and the accretion rate
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
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:α =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
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:α=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
The condensation rate as a function of accretion rate
Fits to the observation of Tomsick et al. (2008)
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
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.
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)