Observational Evidence for Quasisoft XRay Sources in Nearby Galaxies and the link to Intermediatemas

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Observational Evidence for Quasi-soft X-Ray
Sources in Nearby Galaxies and the link to
Intermediate-mass Black Holes

• Albert Kong and Rosanne Di Stefano

Harvard-Smithsonian Center for Astrophysics
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X-ray Point Sources in Nearby Galaxies

• Chandra and XMM-Newton found many point sources
  in nearby galaxies.
• They are mainly X-ray binaries, but some of them
  are SNRs or foreground/background objects.
• We found some extreme X-ray objects in nearby
  galaxies.
• Some off-nucleus point sources have luminosity gt
  1039 erg/s, often referred as ultra-luminous
  X-ray sources (ULXs).
• We also found some very soft X-ray sources
  typically with temperature of 100eV some of
  them are also ULXs (e.g., Mukai et al. 2003 Kong
  Di Stefano 2003 Fabbiano et al. 2003).

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Very Soft X-ray SourcesSupersoft Sources and
Quasi-soft Sources

• SSSs are sources with kT tens of eV, and with L
  typically between 1036-1038 erg/s.
• Some may be nuclear-burning WDs some of these
  may be progenitors of Type Ia supernovae.
• Because absorption is usually low comparing to
  our Galaxy, external galaxies are good to search
  for very soft sources (VSSs).
• We have developed a method to search for VSSs in
  external galaxies. (Di Stefano Kong 2003 a,b,c)

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Very Soft X-ray SourcesSupersoft Sources and
Quasi-soft Sources

• QSSs typically have 100eV lt kT lt 250 eV.
• Or else they have a softer dominant spectrum, but
  may include a small hard component.
• We have tested our algorithm on simulated data.

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• We have tested our algorithm on real data Chandra
  and some XMM-Newton from 20 galaxies.
• We find many SSSs and QSSs their luminosity is
  between 1036-1039 erg/s.
• QSSs and SSSs occupy between 11 and 45 of all
  X-ray sources.

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Supersoft Sources
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Quasi-soft Sources
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A Luminous SSSs in M31
kT70eV, a1.8 L0.3-74x1038 erg/s
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Red SSSs Blue QSSs
M83
M101
NGC4697
M51
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What are the Quasi-soft Sources?

• They are almost certainly not WDs.
• They may be SNRs, but variability can distinguish
  them from accreting objects.
• The most natural explanation may be that they are
  accreting IMBHs.
• If the accretion is mediated by a disk which is
  geometrically thin, but optically thick, the
  spectrum must be soft.
• (M/103MSun) h (42eV/kTlso)2 (?/0.1)
  Lobs/3x1037 erg/s1/2
• where h is a factor of order unity.
• They may represent a simple extension of observed
  properties of Galactic BHs.

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A Recurrent SSS in NGC 300
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• If the 5.4 hr variability is orbital origin, then
  the system is consistent with a 1000 MSun IMBH.
• If the donor fills its Roche lobe, then Porb
  (8.9 hr)(Md/M ), where Md is the mass of the
  donor star. If Porb 5.4 hr, then Md 0.61
  MSun.
• See Rosannes poster (13) for details.

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Alternative Models

• Neutron star the very soft X-ray emission would
  presumably emanate from a photosphere is much
  larger than the neutron star itself.
• Stellar-mass BH Compton thick outflow from a
  stellar-mass BH, accreting near the Eddington
  limit (King Pounds 2003).

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Summary

• Galaxies are rich in SSSs and in QSSs.
• We are faced with the challenge of understanding
  the nature(s) of the members of each of these
  classes of X-ray sources.
• Some SSSs and some QSSs may be IMBHs.
• Whatever the nature(s) of SSSs and QSSs
• (See Rosannes poster 13)
• IMBHs need not be ultraluminous.
• IMBHs can be found in anywhere in ellipticals and
  spirals.
• Binary evolution can provide important and
  testable predictions about the properties of
  accreting IMBHs.