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Luminosity function of accreting neutron stars

Luminosity function of accreting neutron stars. Konstantin Postnov Sternberg Astronomical Institute Zeldovich-90, IKI, 23.12.2004. Outlook. Introduction Power-law luminosity functions from population of sources LMXB and HMXB Evolutionary explanation of LMXB luminosity function.

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Luminosity function of accreting neutron stars

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  1. Luminosity function of accreting neutron stars Konstantin Postnov Sternberg Astronomical Institute Zeldovich-90, IKI, 23.12.2004

  2. Outlook • Introduction • Power-law luminosity functions from population of sources • LMXB and HMXB • Evolutionary explanation of LMXB luminosity function

  3. 4 UHURU X-ray map

  4. Chandra view of X-ray sources in galaxies M83, Chandra+VLT

  5. Chandra view of X-ray sources in galaxies NGC 4261

  6. Chandra view of X-ray sources in galaxies NGC 4697

  7. X-ray luminosity function Grimm et al 2002, Gilfanov 2004, Kim and Fabbiano 2004

  8. “Universal” X-ray LF (Gilfanov 2004)

  9. XLF is naturally explained by accretion in binaries:HMXB: stellar wind accretion from early-typeoptical companion (Postnov 2003)LMXB: accretion from Roche-lobe filling low-masslate-type companion (Postnov & Kuranov 2004)Main assumptions: if then For example, Salpeter IMF β=2.35, Miller-Scalo β=2.5

  10. Stationary mass distribution of optical components • If dM/dt ~ Mα then stationary mass distribution for a population of sources with initial mass distribution dN/dM ~ M-β reads • dN/dMst ~ M-(β-1+α), βst = β-1+α

  11. HMXB: stellar wind accretion + proportionality of the wind rate to the optical luminosity and mass of early-type companion dM/dt ~ Lopt ~ M03 yieldsLx~ M02.5, α≈2.5, for β~2.35…2.5, βst=β-1+α ~ 4ΓHMXB = (βst -1)/α ~ 0.54…0.6(cf observed value Γ~0.6!)

  12. BUT: LMXBs are highly variable. Can the observed XLF be simply shaped by individual XLF of sources?Let L0 be mean luminosity corresponding to accretion rate supplied by the optical star, F(Lx-Lo) be theindividual luminosity function of sources. Then for a population of sources It is easy to prove that if then power-law shape of dN/dL0 is preserved

  13. Variability of individual LMXBs (ASM RXTE)

  14. Short-period LMXBs

  15. Bright LMXBs:

  16. Mean LF of LMXBs (RXTE ASM) Lorentzian A/((Lx-L0)2+BL20), or Gaussian,both satisfy

  17. LMXB evolution: Magnetic stellar wind and gravitational radiation a) Magnetic braking (0.4<M<1.5 M)

  18. LMXB evolution: Magnetic stellar wind and gravitational radiation b) Gravitational radiation (M<0.4 M) dN/dlnL0 ~L0-0.16…0.3ΓGR~0.16…0.3

  19. TransitionMSW-GR: τMSW = τGR

  20. Numerical results: population synthesis Pfahl et al. 2003 PK & Kuranov 2005

  21. Conclusions • Observed XLF of HMXB in galaxies are explained by wind accretion in binaries from early-type companions • XLF of LMXB can be explained by evolution of mass accretion rate (Roche lobe overflow of late-type companions) due to magnetic braking and gravitational radiation giving different slopes of the observed XLF below and above ~1037erg/s • PL shape of XLF of LMXB is not affected by individual LF of sources for the observed distribution of actual luminosity distribution wrt to the mean value

  22. Thank you very much and Merry Christmas!

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