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Albert Kong and Rosanne Di Stefano

Observational Evidence for Quasi-soft X-Ray Sources in Nearby Galaxies and the link to Intermediate-mass Black Holes. Harvard-Smithsonian Center for Astrophysics. Albert Kong and Rosanne Di Stefano. X-ray Point Sources in Nearby Galaxies.

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Albert Kong and Rosanne Di Stefano

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  1. Observational Evidence for Quasi-soft X-Ray Sources in NearbyGalaxiesand the link to Intermediate-mass Black Holes Harvard-Smithsonian Center for Astrophysics Albert Kong and Rosanne Di Stefano

  2. 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 > 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).

  3. 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)

  4. Very Soft X-ray SourcesSupersoft Sources and Quasi-soft Sources • QSSs typically have 100eV < kT < 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.

  5. 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.

  6. Recurrent SSS in NGC 300 kT=57 eV Lbol=1.5x1039 erg/s kTbb=91 eV kTRS=74 eV Lbol=7x1039 erg/s kT=71 eV Lbol=2.3x1038 erg/s kT=76 eV Lbol=4x1039 erg/s Supersoft Sources

  7. NGC1399-162 M83-35 QSS-σ QSS-MNOH kTMCD=0.35 keV L0.3-7=1.7x1039 erg/s kT=0.12 keV L0.3-7=1038 erg/s M101-114 M51-10 QSS-MNOH QSS-σ kT=0.13 keV L0.3-7=4.3x1037 erg/s kT=0.24 keV L0.3-7=3.6x1038 erg/s Quasi-soft Sources

  8. A Luminous SSSs in M31 kT=70eV, α=1.8 L0.3-7=4x1038 erg/s

  9. Red: SSSs Blue: QSSs M83 M101 NGC4697 M51

  10. 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 [(ξ/0.1) Lobs/3x1037 erg/s]1/2 where h is a factor of order unity. • They may represent a simple extension of observed properties of Galactic BHs.

  11. Cyg X-1 kT=0.3 keV, α=2.4 L0.5-200=7x1037 erg/s MBH=6.9-13.2 MSun XTE J1550-564 kT=0.8 keV, α=2.3 L0.5-10=1038 erg/s MBH=8.4-10.8MSun Chandra+RXTE (Miller et al. 2003)

  12. Kong 2003 kTMCD=0.2 keV α=1.8 L0.5-10=5x1039 erg/s IC 342 X-1 kTMCD=1.98 keV L0.5-10=1x1039 erg/s Miller et al. 2003

  13. Recurrent SSS in NGC 300 kT=57 eV Lbol=1.5x1039 erg/s A Recurrent SSS in NGC 300

  14. 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 Rosanne’s poster (#13) for details.

  15. 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).

  16. 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 Rosanne’s 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.

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