Black hole accretion flows

1. Black hole accretion flows Chris Done University of Durham

2. Modelling the behaviour of accretion flows in X-ray binariesorEverything you always wanted to know about accretion but were afraid to ask Chris Done, Marek Gierlinski, Aya Kubota Astronomy &Astrophysics Reviews 2007 (DGK07)

3. Stellar mass black hole binaries • Appearance of BH depends only on mass and spin (black holes have no hair!) • M~3-20 M (stellar evolution) - very homogeneous • Form observational template of variation of flow with L/LEdd

4. Spectral states • Dramatic changes in continuum – single object, different days • Underlying pattern in all systems • Low L/LEdd:hard spectrum, truncated disc, hot inner flow (Marek’s talk) • High L/LEdd:soft spectrum, peaks at kTmax often disc-like, plus tail (my talk) very high disk dominated high/soft

5. Spectra of accretion flow: disc • Differential Keplerian rotation • MRI: gravity  heat • Thermal emission: L = AsT4 • Temperature increases inwards until minimum radius Rlso(a*) For a*=0 and L~LEddRlso=6Rg Tmax~1keV (107 K) for 10 M • Extreme Kerr a*=0.998 Rlso=1.23Rg Tmax is 2.2x higher Log n f(n) Log n

6. Spectra of accretion flow: disc • Differential Keplerian rotation • MRI: gravity  heat • Thermal emission: L = AsT4 • Temperature increases inwards until minimum radius Rlso(a*) For a*=0 and L~LEddRlso=6Rg Tmax~1keV (107 K) for 10 M • Extreme Kerr a*=0.998 Rlso=1.23Rg Tmax is 2.2x higher Log n f(n) Log n

7. Observed disc spectra • Pick ONLY ones that look like a disc! • L/LEddT4max(Ebisawa et al 1993; Kubota et al 1999; 2001) • Constant size scale – last stable orbit!! • Proportionality constant gives a measure Rlso i.e. spin • Convert to real number: stress-free inner boundary condition, Teff=fcol T as little true opacity. GR corrections • Consistent with low to moderate spin not extreme/maximal Kerr (see also Shafee et al 2006) DGK07

8. Disc spectra: last stable orbit • Pick ONLY ones that look like a disc! • L/LEddT4max(Ebisawa et al 1993; Kubota et al 1999; 2001) • Constant size scale – last stable orbit!! • Proportionality constant gives a measure Rlso i.e. spin • stress-free inner boundary condition, Teff=fcol T as little true opacity. GR corrections • Consistent with low to moderate spin not extreme/maximal Kerr (see also Shafee et al 2006) Gierlinski & Done 2003

9. Observed disc spectra Done Gierlinski Kubota 2007

10. Theoretical disc spectra • Model disc including radiation transport through vertical structure of disc and SR/GR propagation to observer bhspec(Davis et al 2006) • Broader than diskbb due to GR • Atomic features not in kerrbb • NB: bandpass – especially in CCD data eg all ULX spectra Done & Davies 2008

11. Theoretical disc spectra a=0.01 • alpha discs (Q+ Ptot ) for a=0.1. disc goes effectively optically thin. Dissipate 10% energy in photosphere, see change in fcol • Observations close to LT4 so must have denser disc – • so a=0.01 ? • But all alpha discs unstable for L/LEdd > 0.05 yet observed discs are stable a=0.1 Done & Davies 2008 Davies, Blaes et al 2005

12. Theoretical disc spectra a=0.01 a=0.1 • Can’t yet do full MRI disc so try other stress prescriptions to see effect of vertical structure • beta (Q+ Pgas ) • Geometric mean(Q+ Ptot Pgas) • All dense discs give same small increase in fcol like most of data. • Disc is very optically thick. Photosphere is simply atmosphere – no significant energy dissipated here Done & Davies 2008 Davies, Blaes et al 2005

13. Observed disc spectra a*~0.6 a*~0.1 a*~0.3 • Not sensitive to stress scaling as long as not dissipating energy in photosphere. but does depend on system distance, mass, inc… • Nonetheless, very difficult to get maximal spin • Not much room for stress at inner boundary as in MRI Davies, Done & Blaes 2006

14. Boundary condition L(r) • Stress free inner boundary if material gets to ISCO and falls. But B field - MRI! • stress free and continuous • difference is factor 2 in temperature! • Measure low/moderate spins with stress-free so would have to be counter-rotating! • Or MRI not like this in thin discs – maybe only thick flows….. r

15. A physical model for transitions ? L(r) • Thick hot flow sees stress • Thin disc truncated but sees stress-free • Hot flow radiatively inefficient, with optical depth given by ADAF equations • Transition radius given by evaporation of disc (Rozanska & Czerny 2000) r

16. A physical model for transitions ?

17. Compared to Cyg X-1 • Range of low/hard state from Cyg X-1 • Hardest when disc first gets to size scale of hot flow • Softest when disc down to almost last stable orbit under flow – but still lots of energy as tapping stress

18. Compared to Cyg X-1 • Originally nonthermal injection thermalises • But need more thermalisation than simply coulomb collisions

19. Irradiation of truncated disc Done & Gierlinski & diaz Trigo 2008 • Irradiation dumps energy into photosphere!! Expect bigger fcol! Not all will thermalise – gives tail which may be ‘soft excess’ seen in LHS. • Makes determining extent of red wing very difficult! • Apparent extreme broad lines in LHS ‘truncated disc’ spectra eg GX339-4 (Miller et al 2006)

20. Spectral states • Dramatic changes in continuum – single object, different days • Underlying pattern in all systems • Low L/LEdd:hard spectrum, truncated disc, hot inner flow (Marek’s talk) • High L/LEdd:soft spectrum, peaks at kTmax often disc-like, plus tail (my talk) very high disk dominated high/soft

21. Very high state • Disk does not dominate! • Sometimes hard to see as separate component from continuum • Optically thick comptonisation with complex shape (thermal/non-thermal)

22. Very High State: Spectrum Kubota & Done 2004 • Disc AND tail have roughly equal power. BE CAREFUL!!! • Now depends on models - Comptonized spectrum is NOT a power law close to seed photons! Log n f(n) • Disc dominated(low L / high L) • Very high state (comp < disc) • Very high state (comp > disc) Log n

23. Very High State: photons Kubota & Done 2004 • But Comptonised photons come from the disc – optically thick so suppresses apparent disc emission • Correct for this Log n f(n) Log n

24. Very High State: energy Kubota & Done 2004 • But ENERGY of corona came from disc as well. Lower T under corona but more importantly lower L enhancing outer disc L(R) R-3 R

25. Very High State: energy Done & Kubota 2005 • But ENERGY of corona came from disc as well. Lower T under corona but more importantly lower L enhancing outer disc (Svensson & Zdziarski 1994) L(R) R-3 R

26. Intermediate mass BH? • Ultra - Luminous X-ray sources in spiral arms of nearby starforming galaxies – ULX • L~1039-40 ergs s-1so M~10-100 Mfor L <LEdd • Hard for stellar evolution to make BH > 50 M Gao et al 2003

27. Intermediate mass BH? • Fit spectra with disc models - Tmax  M -1/4 • low Tmax M ~1000M L/LEdd ~ 0.1 • BUT not pure disc spectrum. Lots of Compton tail. • Scale Comptonised GBH models up by factor 3-5 in mass of BH?(Kubota, Done & Makishima 2002; Kubota & Done 2005) Miller, Fabian & Miller 2005

28. BHB –NS spectra • RXTE archive of many GBH • Same spectral evolution 10-3 < L/LEdd < 1 • Truncated disc Rms qualitative and quantitative Done & Gierliński 2003 G (3-6.4) 4.5 3.0 1.5 1.5 1.5 3.0 3.0 4.5 4.5 G (6.4-16) G (6.4-16)

29. Lh/Ls LS Hard (low L/LEdd) Soft (high L/LEdd) 1 VHS HS US

30. Conclusions • LMXB very homogenous at ~10 Msun variable L/LEdd • Low/hard state: two separate structures. Truncated disc and hot inner flow – behaves like ADAF with stress on boundary like MRI • High-soft disc dominated state LT4max as disc models • Low to moderate spin + stress free in LMXB as expected • Corrections to GR from proper gravity must be smallish • Accretion flow NOT always simple disc – X-ray tail even at high – very high/steep power law state • Disc looks cooler than expected!! cf ULX’s… • Corona now same structure as disc?