Black hole accretion flows

Chris DoneUniversity of Durham

Modelling the behaviour of accretion flows in X-ray binaries

orEverything you always wanted to know

about accretion but were afraid to ask

Chris Done, Marek Gierlinski, Aya KubotaAstronomy &Astrophysics Reviews 2007 (DGK07)

• 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

Stellar mass black hole binaries

• 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)

Spectral states

very high

disk dominated

high/soft

• Differential Keplerian rotation

• MRI: gravity heat

• Thermal emission: L = AT4

• Temperature increases inwards until minimum radius Rlso(a*) For a*=0 and L~LEdd Rlso=6Rg Tmax~1 keV (107 K) for 10 M

• Extreme Kerr a*=0.998 Rlso=1.23 Rg Tmax is 2.2x higher

Spectra of accretion flow: disc

Log

Log

f

(

• Differential Keplerian rotation

• MRI: gravity heat

• Thermal emission: L = AT4

• Temperature increases inwards until minimum radius Rlso(a*) For a*=0 and L~LEdd Rlso=6Rg Tmax~1 keV (107 K) for 10 M

• Extreme Kerr a*=0.998 Rlso=1.23 Rg Tmax is 2.2x higher

Spectra of accretion flow: disc

Log

Log

f

(

Observed disc spectra• Pick ONLY ones that look

like a disc! • L/LEdd T4

max (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

Gierlinski & Done 2003

Disc spectra: last stable orbit• Pick ONLY ones that look

like a disc! • L/LEdd T4

max (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)

Done Gierlinski Kubota 2007

Observed disc spectra

Done & Davies 2008

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

=0.1

=0.01

Theoretical disc spectra

• alpha discs (Q+ Ptot ) for =0.1. disc goes effectively optically thin. Dissipate 10% energy in photosphere, see change in fcol

• Observations close to LT4 so must have denser disc –

• so =0.01 ?

• But all alpha discs unstable for L/LEdd > 0.05 yet observed discs are stable Done & Davies 2008

Davies, Blaes et al 2005

=0.1

=0.01

Theoretical disc spectra

• 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

Dav

ies,

Don

e &

Bla

es 2

006

Observed disc spectra

• 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

a*~0.6 a*~0.1 a*~0.3

Boundary condition

• 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

L(r)

A physical model for transitions ?

• 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

L(r)

A physical model for transitions ?

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

Compared to Cyg X-1

• Originally nonthermal injection thermalises

• But need more thermalisation than simply coulomb collisions

Irradiation of truncated disc

• 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)

Done & Gierlinski & diaz Trigo 2008

• 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)

Spectral states

very high

disk dominated

high/soft

• Disk does not dominate!

• Sometimes hard to see as separate component from continuum

• Optically thick comptonisation with complex shape (thermal/non-thermal)

Very high state

• Disc AND tail have roughly equal power. BE CAREFUL!!!

• Now depends on models - Comptonized spectrum is NOT a power law close to seed photons!

Very High State: SpectrumKubota & Done 2004

• Disc dominated (low L / high L)• Very high state (comp < disc)• Very high state (comp > disc)

Log

Log

f

(

• But Comptonised photons come from the disc – optically thick so suppresses apparent disc emission

• Correct for this

Very High State: photonsKubota & Done 2004

Log

Log

f

(

• But ENERGY of corona came from disc as well. Lower T under corona but more importantly lower L enhancing outer disc

Very High State: energyKubota & Done 2004

L(R

)

R

R-3

• But ENERGY of corona came from disc as well. Lower T under corona but more importantly lower L enhancing outer disc (Svensson & Zdziarski 1994)

Very High State: energyDone & Kubota 2005

L(R

)

R

R-3

Intermediate mass BH?

• Ultra - Luminous X-ray sources in spiral arms of nearby starforming galaxies – ULX

• L~1039-40 ergs s-1 so M~10-100 M

for L <LEdd

• Hard for stellar evolution to make BH > 50 M

Gao

et a

l 200

3

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

1.53.04.5

BHB –NS spectra• Truncated disc Rms

qualitative and quantitative• RXTE archive of many GBH • Same spectral evolution

10-3 < L/LEdd < 1Done & Gierliński 2003

1.5

4.5

3.0

6.4-

16)

3-6.4)

1.5

4.5

3.0

6.4-

16)

LS

VHS

HS

US

Hard (low L/LEdd)

Soft (high L/LEdd)

Lh/Ls

1

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?