Presentation on theme: "Disk corona in AGN: what do we expect? Bifang Liu Yunnan Observatory, CAS The disk corona evaporation model The model for X-ray binaries Similarities between."— Presentation transcript:
Disk corona in AGN: what do we expect? Bifang Liu Yunnan Observatory, CAS The disk corona evaporation model The model for X-ray binaries Similarities between AGNs and BHXBs Disk corona: what do we expect in AGNs? Unified accretion scheme of BHXBs and AGNs?
The disk corona evaporation model Interaction of the disk and the corona: Mass and energy exchange Steady accreting corona: Gas accretes to the BH, re- supplied by evaporating gas from the disk corona BH/NS/WD X-rays evaporation disk Conductive heat Meyer & Meyer-Hofmeister 1994
The radius-dependent evaporation rate The maximal evaporation rate 0.02Eddington rate The corresponding radius 300RS, depending on magnetic field and heat conduction (see Qian, L., P-ID110) The disk is depleted if the feeding accretion rate to the disk is less than evaporation rate log m log r Meyer, Liu, Meyer-Hofmeister,1998, 2000
Consequences of disk-corona interaction: two basic accretion modes
Hysteresis in X-ray binaries: Additional support to the disk evaporation model Schematic light curve of an X-ray nova outburst CGX339-4
Explanation by disk corona model Different irradiation in hard and soft state leads to different evaporation rate and transition rate Meyer-Hofmeister, Liu, Meyer 2005 Liu, Meyer, Meyer-Hofmeister 2005
The hysteresis in truncation radius The values of hysteresis and disk inner radius can be changed by Hard or soft irradiation efficiency of energy conversion Magnetic field
Disk corona model in X-ray binaries Provides a physical mechanism for BHXBs Truncation of the disk at low accretion rate (Liu et al 1999; Liu, Meyer-Hofmeister 2001) Spectral transition between hard and soft states (Meyer, Liu, Meyer-Hofmeister 2000) The hysteresis between the transitions of hard-to-soft and soft-to-hard states (Meyer-Hofmeister, Liu, Meyer 2005; Liu, Meyer, Meyer-Hofmeister 2005) Occurrence of intermediate state (Liu, Meyer, Meyer-Hofmeister 2006)
Observational similarities between AGNs and BHXBs The fundamental plane for AGNs and BHXBs (Merloni et al. 2003) log L R =0.6log L X +0.78log M+7.33 Falcke et al. 2004: Proposed unification scheme
Observational similarities between AGNs and BHXBs Similarities in spectral states Similarity between high-luminosity AGNs and soft-state BHXBs (Maccarone et al. 2003) Similarity between low-luminosity AGNs and hard-state BHXBs (Narayan 2004; Falcke et al. 2004) Similarity between NLS1s and very high state (steep power law) BHXBs Timescale in AGN much longer than in BHXB Galactic sources: a laboratory for AGN?
Disk corona model in AGNs The accretion mode does not depend on M Critical accretion rate (in units of Eddington) for soft and hard spectral states is 0.02 Hard state: m<0.02 the Galactic Center, LLAGN, LINER, etc Soft state: m>0.02 e.g. quasars, some of the Seyferts An example of tentative spectral state transition Seyfert-LINER transition galaxy NGC7589 (Yuan et al. 2005) shows a possible low-to-high state transition in X-rays Observations in AGN show the transition rate m 0.02 (Maccarone et al 2003) · · ·
Importance of corona in AGN Luminous AGN and quasars (at soft state) SED (optical-UV, soft and hard X-ray, fluorescent iron lines) indicates coexistence of hot corona and cold thin disk. The high X-ray luminosity means a large fraction of accretion energy released in the hot corona
The disk corona model in luminous AGN The corona at high accretion rate is either over-cooled by inverse Compton effect or blown away by radiation-driven winds. Additional heating is required to keep the corona The most promising mechanism: Magnetic field transports accretion energy to the corona which is released by reconnection Liu, Mineshige, Ohsuga 2003
Unifying BHXBs and AGNs on the basis of disk corona model? Difficulty: the difference between BHXBs and AGNs For all BHXBs, jets are quenched once the spectrum changes from hard to soft For AGNs, there exist ~10% radio-loud quasars at high state
Questions for both BHXB and AGNs In the soft state, the disk is radiation pressure dominant and unstable, but observations show no corresponding variation in light curves The very high state: What causes the transition from high state to very high state? Activity of magnetic fields?
Magnetic activity Magnetic field in the disk Parker instability (Magnetic buoyant instability) Magnetorotational instability Bring accretion energy upwards and outwards which dissipated in the outer disk surface layers or in the corona Critical accretion rate for P r dominant m=0.04(αm) -1/8 (1-f) -9/8
Effect of magnetic activity The critical accretion rate is raised by (1-f) -9/8 m cr =0.04(αm) -1/8 (1-f) -9/8 Disk in BHXBs at soft state is stable Consistent with stable disk radiations in BHXBs at accretion rate up to 0.5 Disk in a part of AGNs at soft state is unstable Inner RIAF+SSD Formation of Jets RLQs ·
Unified accretion scheme 10 9 M/M P=Pr m 10 RIAF/hard state SSD/soft state SSD+corona/very high state RIAF+SSD AGNs and BHXBs are under the same accretion scheme The hard-soft transition caused by disk-corona interaction The high-very high state transition caused by strong magnetic activity The RLQs caused by radiation pressure instability Magnetic field plays important roles
Summary: the disk corona in AGNs Answers why the accretion mode and spectral state change with the accretion rate Magnetic fields seem essential in the disk and corona Source of viscosity Cause of flares and variability Jet & outflow formation Heat the corona Stabilize the disk Trigger transition from high state to very high state? Interpret why jets occur in quasar but not in soft-state BHXBs Unification of accretion in BHXBs and AGNs