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?
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