The Nature of the Supersoft Xray Sources

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The Nature of the Supersoft X-ray Sources

• Katherine McGowan (MSSL)

Phil Charles (SAAO), Alex Blustin (MSSL),
Mario Livio (STScI), Darragh
ODonoghue (SAAO), Bernard Heathcote (Barfold
Observatory)
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Supersoft Source Characteristics

• Luminous emission at very soft energies
• LX 1037 - 1038 erg s-1
• lt 0.5 keV

Greiner et al. 1991
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White Dwarf Model

• Binary system
• white dwarf compact object
• sub-giant companion
• High accretion rate
• 100 - 1000 higher than in CVs
• gt 10-7 Msun yr-1
• Near-Eddington luminosities achieved as white
  dwarf burns hydrogen steadily at its surface

van den Heuvel et al. 1992
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RX J0513.9-6951

• X-rays detected only during optical low states

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Model for X-ray Outbursts

• Regular thermonuclear flash model
• radius expansion
• increase in optical luminosity
• Contraction model
• accretion rate drops
• decrease in optical luminosity
• rise in effective temperature
• increase in X-ray luminosity

Southwell et al. 1996
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Optical Monitoring
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Multi-wavelength Data
Optical
UV
X-ray
(0.5-0.8 keV)
Hardness ratio
(0.3-0.5 keV)
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Low-resolution X-ray Spectra
Absorption
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Spectral Evolution
kT
LBB
RBB
LX
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High-resolution X-ray Spectra
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Summary 1

• Contraction model based on prediction that rise
  in X-rays occurs after drop in optical
• Detect X-rays in optical low state
• X-ray flux and temperature decrease as optical
  low state progresses
• As X-ray outburst evolves peak of emission moves
  into UV
• Radius decreases at start of observations
• As optical/UV intensity recover radius increases

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Summary 2

• Highly ionized gas outflowing at lt3000 km/s
• Optical depth of ionized absorption increases
• Viewing system close to pole-on through bipolar
  outflow, intrinsic covering factor of outflow
  changes?
• Seeing process of expansion, highly ionized nova
  by-products stream out from surface and build
  into optically-thick inflated surface?
• increase in optical/UV flux occurs as X-rays are
  down-scattered in inflated layer?