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Ultraluminous X-ray Sources

Ultraluminous X-ray Sources. Zhaoyu Zuo 2001,Apj, 552,L109 astro-ph:0402677. Introduction. Ultraluminous X-ray sources (ULXs) are point-like, extra-nuclear X-ray sources found in nearby galaxies, with isotropic X-ray luminosities in excess of 10^39 ergs -1 .

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Ultraluminous X-ray Sources

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  1. Ultraluminous X-ray Sources Zhaoyu Zuo 2001,Apj, 552,L109astro-ph:0402677

  2. Introduction • Ultraluminous X-ray sources (ULXs) are point-like, extra-nuclear X-ray sources found in nearby galaxies, with isotropic X-ray luminosities in excess of 10^39 ergs-1 . • The majority of ULXs may be stellar-mass (~10 M⊙ ) black holes (SMBHs) , or intermediate-mass (~10^2-10^4 M⊙ ) black holes (IMBHs), accreting from their binary companion stars, or in few cases, perhaps from the fallback material originating from supernovae that have produced those sources.

  3. Eddington Limit: Evasions of the limit are possible but rare! 1)A0538-66(transient): magnetic NS+Be star companion 10^11G magnetic field of the NS decreasedLx>10^39erg/s 2) Accreting black holes of masses 10^2 – 10^4 M⊙ if they emit at 0.1–0.01 of the Eddington luminosity, typical of Galactic X-ray binaries (Colbert & Mushotzky 1999).

  4. Model • M1:accretor • M2:donor • n: mean observed number of ULXs per galaxy • b: beaming factor • d: duty cycle • a: acceptance rate • Lsph: apparent X-ray (assumed bolometric) luminosity of a source, given by the assumption of isotropic(spherical) emission

  5. Unbeamed Model(b=1)   Unbeamed models for the ULXs class thus have to invoke a class of extremely massive X-ray binaries.

  6. Unbeamed Model(b=1) Constrains: (1)The binary must be wide enough so that the progenitor of the compact star(here an ~100Msun BH) is able to fit inside its Roche lobe (otherwise it will provoke common envelope) (2)The binary must be able to provide the inferred minimum accretion rate: MS of masses >100Msun have radii >1000Rsun

  7. EXAMPLE

  8. Implications 1)stellar evolution: a high mass  winds and pulsations,  black hole of mass M < 20M⊙ (e.g., Fryer & Kalogera 2001) 2)According to Salpeter IMF The local stellar density XRBs(NS and LMBHs) >>XRBs(massive BHs) The X-ray luminosities of the systems observed in the Antennae (Fabbiano et al. 2001) contradict this.

  9. Compared with Antennae • Merging of stars in a young dense stellar cluster followed by direct collapse into an IMBH (Portegies Zwart et al. 1999) or gaining a stellar companion by some dynamical process (for example tidal capture). • successive merger of lower mass objects deep potential well of the clusterabsence of black hole binary systems in globular clusters (Sigurdsson & ernquist 1993; Kulkarni, Hut, & McMillan 1993). • Evolution of effectively zero-metallicity stars seems unlikely to explain the ULXs observed in the Antennae (Fabbiano et al. 2001) since these black holes would be distributed throughout the galactic halo

  10. Beamed Model mechanism : accretion disk around an accreting black hole has a much lower scattering optical depth over a restricted range of solid angles (e.g., the rotational poles) than in other directions.   NS value  thermaltimescale mass transfer. when the donor has a radiative envelope (a) more massive than the accretor or (b) first fills its Roche lobe as it expands across the Hertzsprung gap. both cases give rise to highly super-Eddington masstransfer rates.

  11.  ULXs descending from high-mass X-ray binaries would naturally be associated with a young stellar population, as required by observation.

  12. Difficulties for the Beaming Model • Detection of a strong narrow 54 mHz QPO in the starburst galaxy M82. • The broad Fe K line (in M82)centered at 6.55 keV is also hard to understand in a beaming scenario • Periodic light change  eclipsing binary • Emission nebulae of a few hundred pc diameter are found to be present at or around several ULXs .

  13. Conclusions 1)Unbeamed models probably require a black hole of M>100Msun,in an ∼1 yr binary orbit with an evolved donor star. 2) the likely transient behavior of the accretion disk in such a wide system is hard to reconcile with observation. 3)an individual ULX may contain a very massive black hole , perhaps accreting from the ISM.

  14. Conclusions 4) mild beaming (b ∼ 0.1–0.01) reduces M to values already observed for Galactic X-ray binaries  ULXs represent a short-lived phase of their evolution. 5) candidate  thermal-timescale mass transfer  intermediate- and high-mass X-ray binaries. 6)short donor lifetime in HMXBs ULXs are associated with young stellar populations.

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