Black hole accretion flows

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Black hole accretion flows
Chris Done University of Durham
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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!)
• M3-20 M? (stellar evolution) - very homogeneous
• Form observational template of variation of flow
  with L/LEdd

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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
• (Mareks talk)
• High L/LEdd soft spectrum, peaks at kTmax often
  disc-like, plus tail (my talk)

very high
disk dominated
high/soft
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Spectra of accretion flow disc

• Differential Keplerian rotation
• MRI gravity ? heat
• Thermal emission L AsT4
• Temperature increases inwards until minimum
  radius Rlso(a) For a0 and LLEdd Rlso6Rg
  Tmax1 keV (107 K) for 10 M?
• Extreme Kerr a0.998 Rlso1.23 Rg Tmax is 2.2x
  higher

Log n f(n)
Log n
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Spectra of accretion flow disc

• Differential Keplerian rotation
• MRI gravity ? heat
• Thermal emission L AsT4
• Temperature increases inwards until minimum
  radius Rlso(a) For a0 and LLEdd Rlso6Rg
  Tmax1 keV (107 K) for 10 M?
• Extreme Kerr a0.998 Rlso1.23 Rg 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, Tefffcol T as little true
  opacity. GR corrections
• Consistent with low to moderate spin not
  extreme/maximal Kerr (see also Shafee et al 2006)

DGK07
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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, Tefffcol 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
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Observed disc spectra
Done Gierlinski Kubota 2007
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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
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Theoretical disc spectra

• alpha discs (Q? Ptot ) for a0.1. 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 a0.01 ?
• But all alpha discs unstable for L/LEdd gt 0.05
  yet observed discs are stable

a0.01
a0.1
Done Davies 2008
Davies, Blaes et al 2005
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Theoretical disc spectra

• Cant 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
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Observed disc spectra
a0.6
a0.1
a0.3
Davies, Done Blaes 2006

• 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

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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
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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
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A physical model for transitions ?
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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

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Compared to Cyg X-1

• Originally nonthermal injection thermalises
• But need more thermalisation than simply coulomb
  collisions

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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
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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
• (Mareks talk)
• High L/LEdd soft spectrum, peaks at kTmax often
  disc-like, plus tail (my talk)

very high
disk dominated
high/soft
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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)

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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 lt disc)
• Very high state (comp gt disc)

Log n
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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
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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
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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
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Intermediate mass BH?

• Ultra - Luminous X-ray sources in spiral arms of
  nearby starforming galaxies ULX
• L1039-40 ergs s-1 so M10-100 M? for L ltLEdd
• Hard for stellar evolution to make BH gt 50 M?

Gao et al 2003
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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
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BHB NS spectra

• RXTE archive of many GBH
• Same spectral evolution 10-3 lt
  L/LEdd lt 1

• Truncated disc?? Rms qualitative and quantitative

Done Gierlinski 2003
1.5
3.0
4.5
G (3-6.4)
1.5
1.5
3.0
3.0
4.5
4.5
G (6.4-16)
G (6.4-16)
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Lh/Ls
LS
Hard (low L/LEdd) Soft (high L/LEdd)
1
VHS
HS
US
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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 ULXs
• Corona now same structure as disc?