Irradiation models for ULXs and fits to HST data

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Irradiation models for ULXs and fits to HST data

• Chris Copperwheat (MSSL)
• Mark Cropper (MSSL), Roberto Soria (MSSL/CfA),
  Kinwah Wu (MSSL)

Contact cmc_at_mssl.ucl.ac.uk
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Aim

• Through examination of the optical counterparts
  of Ultraluminous X-ray Sources (ULXs) we can
  determine the masses of the black holes (BH) in
  these systems

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Ultraluminous X-Ray Sources

• A class of very luminous, non-nuclear, X-ray
  sources with apparent isotropic luminosities
  gt1039 erg s-1
• Much about their nature still unknown, although
  examination of X-ray spectra suggests they are
  black holes, accreting matter from a companion
  star.

From http//lheawww.gsfc.nasa.gov/users/white/xrb/
xrb.html
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Why are they interesting?

• Much more luminous than conventional X-ray
  binaries
• If emission is isotropic, they are defeating the
  Eddington limit for stellar-mass black holes (
  10 Msun )
• Possible explanations
• They are more massive black holes Intermediate
  Mass Black Holes ( gt100 Msun ) (eg. Colbert and
  Mushotzky, 1999)
• They are stellar-mass black holes with emission
  beamed towards the observer (eg. King et al.
  2001)
• They are stellar-mass black holes with
  super-Eddington accretion as a result of
  geometric considerations (inhomogeneous accretion
  disks) (Begelman 2002)

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What can we learn from optical observations?

• Studies of the X-ray data have not yet determined
  the nature of ULX
• We investigate instead the optical emission from
  these sources
• The optical emission is made up of components
  from the companion star and the accretion disk
• Both the star and the disk will have X-rays from
  the accreting black hole incident on their
  surface
• These X-rays will be attenuated by the star and
  disk, inducing a heating effect in their outer
  layers and driving changes in magnitude and colour

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Model

• Isotropic emission
• Accretion via Roche lobe overflow, with
  subsequent geometric constraints
• We consider the effects of radiative transport
  and radiative equilibrium in the irradiated
  surface of the star and disk
• Plane-parallel model with the radiative transport
  formulation of Milne (1926) and Wu et al. (2001)
• Total emergent radiation from a distorted, Roche
  lobe filling star determined numerically
• Gravity and limb darkening effects included
• Thin disk geometry used for accretion disk
• See Copperwheat et al. 2005 (astro-ph/050685) for
  details

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Intensity Maps
DISK STAR
Lx 1040 erg/s
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Effect of irradiation on an 05V star
Lx 1040 erg/s cos(i) 0.0
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Effect of irradiation on O5V star disk
Lx 1040 erg/s cos(i) 0.5
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Infrared wavelengths
Lx 1040 erg/s cos(i) 0.5 BH mass 100 Msun
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Counterpart of NGC 4559 X-7

• Soria et al. (2005)

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Counterpart candidates for NGC 4559 X-7
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Geneva Isocrones modified for irradiation model
Geneva isochrones from Lejeune Schaerer (2001)
1 Myr
High mass stars
20 Myr
Low mass stars
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cos(i) 0.0 0.5

• BH mass
• 10 Msun
• 100 Msun
• 1000 Msun

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NGC 4559 X-7 Counterpart stellar age vs. mass
of candidate 1
Other stars in error circle have ages of approx
20 Myr and masses of 10 - 15Msun
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NGC 4559 X-7 Counterpart stellar age vs. mass
Other stars in error circle have ages of approx
20 Myr and masses of 10 - 15Msun
When we include irradiation and take the age of
candidate 1 to also be 20 Myr, the fitted
stellar mass is reduced to 11-12 Msun
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NGC 4559 X-7 20Myr Isochrones
BH mass lt 400Msun
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NGC 3031 X-11
Liu et al. (2002)
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NGC 3031 X-11 Colour/magnitude diagrams
BH mass 10 Msun 100 Msun 1000 Msun
Isochrones range from 1 to 100 Myr
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NGC 3031 X-11 Colour/magnitude diagrams
BH mass 10 Msun 100 Msun 1000 Msun
Isochrones range from 1 to 100 Myr
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NGC 3031 X-11 Results
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M101 ULX-1
Kuntz et al. (2005)
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M101 ULX-1 Colour/magnitude diagrams
BH mass 10 Msun 100 Msun 1000 Msun
Isochrones range from 1 to 100 Myr
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M101 ULX-1 Colour/magnitude diagrams
BH mass 10 Msun 100 Msun 1000 Msun
Isochrones range from 1 to 100 Myr
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M101 ULX-1 Results
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Summary and Conclusions

• We have constructed a model to describe the
  irradiative effects of ULXs on their accretion
  disks and companion stars
• Our model suggests ULX counterparts are older and
  of later spectral type than currently thought
• In some cases, we can determine the mass range of
  the accreting black hole