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Galactic Black Hole Binaries

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					  Galactic Black Hole Binaries
              Miniature versions of AGNs




Emily Alicea-Muñoz
Astro 597A
18 October 2004
Outline
• Overview
   – SXTs, LMXBs, HMXBs
   – The case for BHBs and BHCs and not for NSBs
   – Accretion Mechanisms

• BHXRB states and their oh-so-confusing
  classification schemes (ugh, so many acronyms)
   – Luminosity classification scheme
   – McClintock-Remillard classification scheme

• Cyg X-1: NGC 4051’s Mini-me?
• Microquasars – AGN/QSO’s Mini-me’s; are they
  related to GRBs too?
Overview
Overview: SXTs
• Soft X-ray Transients (SXT)
   – short period binaries (5hrs – 6 days)
   – late-type (K-M) type secondaries
   – rare, dramatic x-ray outbursts (L > 1038 erg s-1)


• LMXB: low-mass x-ray binaries
   – most of them strong black hole candidates (BHC)
   – optical spectra are hot, blue continua (U-B ~ -1) with superposed
     broad H and He emission lines arising from the inner disk region


• HMXB: high-mass x-ray binaries
   – early-type (O-B) secondaries
   – most have NS instead of BH (with a few exceptions, e.g. Cyg X-1)
SXT Observations
• Outbursts
  –   once every ~10-20 yrs
  –   fast rise followed by exponential decay
  –   mini-outbursts sometimes in their way to quiescence
  –   ultra-soft x-ray spectra during outburst, with blackbody
      color temperature of kT ~ 0.5-1 keV superposed on
      hard power-law extending to higher energies

• Quiescence
  –   mass transfer continues at a very low rate
  –   inner accretion disk turns into hot, low density corona
  –   radiatively inefficient advective accretion
  –   SXTs with BH fainter than those with NS because of
      event horizon (energy is lost)
SXT Observations
• X-ray Spectroscopy and Variability
  – presence of type-I x-ray bursts indicates primary is NS
  – rapid quasi-periodic oscillations (QPOs) allow study of
    inner accretion disk; often indicates presence of BH
  – two different x-ray states
     • hard power-law spectrum extending to high energies
         – explained by Comptonization
         – predicts energy-dependent time delay
     • soft blackbody spectrum
         – arising from inner region of accretion disk
         – Multicolor Disk Model (MCD) – temperature varies with radius
  – situation not so simple as “two clearly defined” states;
    more details later...
SXT Observations
• Black Hole Spin
  – affects innermost stable circular orbit (ISCO)
  – maximal spin makes ISCO smaller


• Radial Velocity Curves
  – measured during quiescence
  – can also determine secondary’s spectral type and
    orbital period
  – mass function can be calculated from K-velocity
    amplitude          3        3   3
                     PK    M X sin i
            f (M )     
                     2G ( M X  M 2 )2
  – inclination is most uncertain parameter
BHB & BHC vs. NSB
Accretion Mechanisms
• Best Model: Thin Accretion Disk
  – secondary fills its Roche equipotential lobe
  – narrow stream of gas escapes through first Lagrangian
    point (L1)
  – gas with high angular momentum cannot directly
    accrete onto BH, thus forming an accretion disk
  – gas in disk moves in Keplerian orbits with angular
    velocity (GM/R3)1/2
  – viscosity transports angular momentum outward
  – gas gets hotter closer to the BH
  – disk terminates at ISCO:
     • RISCO = 6Rg = 6GM/c2, for a Schwarzschild BH
     • RISCO = RG = GM/c2, for a Kerr BH
Accretion Mechanisms
• Another Model: MCD
  – multi-temperature (multicolor) disk
  – used to describe thermal component in x-ray spectra
  – total disk luminosity in steady state
                             GMM   
                     Ldisk 
                              2 Rin
  – limitation: neglect of torque-free boundary condition at
    ISCO
     • MCD temperature profile T(R)  R-3/4 (maximum at ISCO)
     • proper boundary condition sets maximum at R > RISCO
  – relativistic MHD corrections – add a magnetic field and
    you extract energy from very near the horizon
X-Ray States: Luminosity
• Very High State (VHS)
   – strong ultra-soft (US) component and unbroken power-law (PL)
     component; strong QPOs at ~ 10Hz

• High/Soft State (HS)
   – US dominates; very weak PL component; high luminosity; MCD

• Intermediate State
   – US and steeper PL at high energies

• Low/Hard State (LH)
   – no US component; hard power-law PDS (power density
     spectrum); G ~ 1.7 (2-20keV); low luminosity; radio emission

• Quiescent State
   – truncated disk; ADAF down to the ISCO
X-Ray States: Luminosity
    “Unified MCD and ADAF model”
X-Ray States
• Limitations of luminosity classifications
   – unified MCD and ADAF model don’t account well for
     the VHS’s unbroken power-law
   – ordering states by accretion rate or luminosity is
     “naïve”
   – model does not account for dynamical behavior of
     corona (flares, QPOs, radio emission)
   – no quantitative model relating disk truncation to
     accretion rate

• McClintock & Remillard (2004) propose new
  scheme, based on a model consisting of a MCD
  and a power law component
X-Ray States: McC & R
• Quiescent State
  –   extraordinarily faint (Lx = 1030.5-1033.5 erg s-1)
  –   distinctly non-thermal, hard spectrum (G = 1.5 – 2.1)
  –   long period systems brighter than short period systems
  –   ADAF/MCD model accounts
      for observed properties
       • hard PL spectra
       • faintness of BHBs relative to
         NSBs
       • optical/UV time delay of X-ray
         novae
       • broadband spectrum
       • truncated accretion disk
X-Ray States: McC & R
• Thermal-Dominant (TD)
  – new name for the HS state
  – soft x-rays represent thermal
    emission from inner disk,
    dominant below 10keV
  – steep PL (G = 2.1 – 4.8)
  – PDS shows weak variability
    and power scaling roughly
    as n-1, indicative of
    turbulence
  – “The set of conditions for
    which the disk-flux fraction is
    above 75% (2-20keV), the
    PDS shows no QPOs, and
    weak power continuum”
X-Ray States: McC & R
• Hard X-Ray State
   – take out “low” from name
     (some sources show high
     luminosity)
   – PL with G ~ 1.7
   – broad enhancement at 20-
     100keV (reflection of PL
     from surface of inner disk)
   – steep cut-off near 100keV
   – compact quasi-steady radio
     jets present (disappear upon
     return to TD state)
   – physical conditions that give
     rise to this state are still
     debated
X-Ray States: McC & R
• Hard X-Ray State
   – blackbody radiation truncated at large radius ~100Rg
   – what’s going on inside this radius?
       • truncated disk, inner region filled by ADAF?
       • relativistic flow entrained in a jet?
       • disk intact but depleted of energy in some sort of Compton corona?
   – answer could be found by
       • optical/x-ray variability studies
       • spectral analysis focused on broad Fe emission features
   – origin of x-ray PL also debated – many possible mechanisms
   – “Association of hard state with radio jet is an important step
     forward. […] HS is well characterized by three conditions:
     spectrum dominated (>80% at 2-20keV) by power law, spectral
     index in the range 1.5 < G < 2.1, and a strong integrated power
     continuum”
X-Ray States: McC & R
• Steep Power-Law (SPL)
  – new name for VHS
  – often exceedingly bright
    (Lx>0.2LEdd), but not always
  – very steep unbroken (x-ray to
    gamma-ray) PL (G ≥ 2.4)
  – QPOs in 0.1–30 Hz range
  – no evidence for high-energy
    cutoff
  – transitions between TD and H
    states usually pass through
    SPL state
  – essentially radio-quiet; though
    sometimes shows impulsive
    jets
X-Ray States: McC & R
• Steep Power-Law (SPL)
   – physical origin still an outstanding problem
   – spectrum extends to ~1MeV, maybe higher
   – possible model:
       • inverse Compton scattering for a radiation mechanism
       • scattering occurs in a non-thermal corona
       • where do the Comptonizing electrons come from?
           – magnetic instabilities in accretion disk?
           – strongly magnetized disk, as in AGNs?
       • PL gets stronger and steeper as disk luminosity and radius decrease,
         while keeping high temperature

• Intermediate States
   – “State transitions and hybrid emission properties are to be
     expected; x-ray spectra and PDS should be interpreted as
     intermediate states when necessary, while specifying which states
     can be combined to yield he observed x-ray properties”
Cyg X-1
                                                    Cyg X-1
• Unusual spectrum
  – soft state dominated by PL
    instead of TD spectrum
  – transition from hard to soft x-ray
    spectrum considered as one
    from the hard state to the SPL
    state
  – weird SPL: no QPOs and low
    luminosity
  – reminiscent of NGC 4051
    (flashback to Week 5)
  – hints that the same physical
    mechanism generating variability
    regardless of BH size
                                         NGC 4051
Microquasars
• BHXRBs that eject
  plasma at relativistic
  speeds (jets)
• Fossil sources of
  GRBs?
• Analogy with
  AGN/QSO
   – length and time scales
     are proportional to BH
     mass
Microquasars
Microquasars
• A connection between x-ray flux and jets has
  been observed
  – jets appear when disk x-ray flux drops
  – jets are produced during replenishment of inner
    accretion disk
  – time delay between jet flares at different wavelengths
    consistent with adiabatically expanding cloud model
  – delay of few min between drop in x-ray flux and onset
    of jets could indicate absence of material border, thus
    making the case for a BH event horizon
     • however, absence of evidence is not evidence of absence, so
       this observation could have an alternative explanation
Microquasars
Microblazars
• Microquasars with jet axis with <10° angle with
  line of sight should be analogous to blazars
• Should appear as intense sources of high-energy
  photons with very fast variations in flux
• Very hard to observe
• It has been proposed that microblazars may be
  more frequently linked to HMXBs
  – gamma-rays produced by inverse Compton of the jet
    particles with the UV photons radiated by massive
    secondary
Microquasars
• X-ray/radio correlations
  – hard x-ray state w/ radio jets also proposed for AGNs
• Time variation correlations
  – duration of x-ray flares from stellar-mass BHs and
    AGNs (e.g. Sgr A*) seem proportional to BH mass
  – minimum frequencies of QPOs expected to be
    proportional to BH mass, for a given BH spin
• Iron Ka correlations
  – AGN: broad Fe Ka line, skewed to low energies
  – consistent with emission from surface of accretion disk
  – also observed for BHXRBs (post-Chandra era)
Microquasars and GRBs?
• Mirabel (2004) adopts the theory that long GRBs
  might be caused by formation of BH in collapsars
  or highly magnetized neutron stars

• The case for the microquasar connection:
  – spin-orbit interactions provide enough power for
    collapsar
  – GRBs seem associated with SN Ic, the ones that show
    no H or He lines
     • progenitor lost outer layers way before event
     • could be due to progenitor being in a binary system which
       underwent a common-envelope phase
Microquasars/AGNs/GRBs
                           Questions?
              (hopefully I’ll be able to answer them)




References:
P.A. Charles: astro-ph/9806217
J.E. McClintock & R.A. Remillard: astro-ph/0306213
I.F. Mirabel: astro-ph/0405433

				
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posted:1/29/2012
language:English
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