Accretion to Neutron Stars and Black Holes in Binaries

1. Accretion to Neutron Stars and Black Holes in Binaries Li Xiang-Dong Department of Astronomy Nanjing University

2. 1. Accretion Torque of Magnetized Disks on Accreting Neutron Stars

3. Disk Accretion onto Magnetized Neutron Stars

4. Accretion Torque: The Standard Picture N = N0 + Nmag Spin-up Spin-up Spin-down

5. N = N0 f(w) w≡ Ws/W0 ≡(R0/Rc)3/2 N0 = 0 when w = wc Spin-up line P-dP/dt diagram for radio pulsars

6. Problems • Abrupt torque reversal • Spin-down rate increasing with accretion rate • Accretion in the propeller regime

7. Spin Reversal in Accreting X-ray Pulsars Chakrabarty et al. (1997) Bildsten et al. (1997)

8. Explanations • Change of the direction of disk rotation due to disk warping (van Kerkwijk et al. 1998). • Retrograde disk rotation (Makishima et al. 1988; Nelson et al. 1997) • Bimodal magnetic torque (Torkelsson 1998; Locsei & Melatos 2004). • Bimodal disk structure (Yi et al. 1997).

9. Spin-down Rate vs. X-ray Luminosity in GX 1+4 Chakrabarty et al. (1997)

10. Numerical Simulation of Disk Accretion to a Rapidly Rotating Magnetized Star The spin-down rate increases as the accretion rate increases. Accretion occurs in strong propeller regime. Romanova et al. (2004)

11. Dependence on Period and Mass Accretion Rate

12. 2. Ultraluminous X-ray Sources and Intermediate-Mass Black Holes

13. 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 1039 ergs-1 .

14. Distribution of ULXs in Galaxies • In spirals, ULXs are often near, but distinct from the dynamical centers of the galaxies. • In ellipticals, ULXs are almost exclusively in the halos of the galaxies. (Colbert & Ptak 2002)

15. ULXs as Accreting Black Holes • The X-ray spectra, spectral transitions, and variabilities strongly suggest that ULXs are black holes accreting from a surrounding disk.

16. Theoretical Models for ULXs • The majority of ULXs may be stellar-mass (~10 M⊙ ) black holes (SMBHs) , or intermediate-mass (~102-104 M⊙ ) black holes (IMBHs), accreting from their binary companion stars.

17. SMBHs as ULXs Rappaport et al. 2004

18. Evidence for the Beaming Model • A large fraction of ULXs are associated with star forming regions. • Similar spectral characteristics with the Galactic microquasars SS 433 and GRS 1915+105. • Possible massive optical counterparts. • Jet from a ULX(?)

19. ULXs and Star Formation The Antennae galaxies The Cartwheel galaxy

20. Difficulties for the Beaming Model • Detection of a strong narrow 54 mHz QPO in the starburst galaxy M82. • The broad Fe K line centered at 6.55 keV is also hard to understand in a beaming scenario (Strohmayer & Mushotzky 2003)

21. Difficulties for the Beaming Model • Periodic light change  eclipsing binary

22. Difficulties for the Beaming Model • Emission nebulae of a few hundred pc diameter are found to be present at or around several ULXs (isotropic emission?) (Pakull & Mirioni 2002)

23. IMBH Candidates • ULXs with very high X-ray luminosities (~1040 ergs-1), and relatively low color temperatures (0.05-0.3 keV) have been suggested to be IMBH candidates. (Miller, Fabian,, Miller 2004)

24. Formation of SMBHs (Heger et al. 2002)

25. Formation Scenarios for IMBHs • Merging of stars in a young stellar cluster followed by direct collapse into an IMBH (Portegies Zwart et al. 1999). • Merging of binaries that have a black hole with initial mass of ~50 M⊙in a globular cluster (Miller & Hamilton 2002). • Evolution of primordial population III stars (Madau & Rees 2001).

26. IMBHs in Compact Star Clusters (Portegies Zwart et al. 2004)

27. Formation and Evolution of IMBH Binaries • Black hole formation Runaway collision in dense young star clusters (Portegies Zwart et al. 2004) • X-ray binary formation Exchange encounters (Kalogera et al. 2004) Tidal capture (Hopman, Portegies Zwart, Alexander 2004)

28. X-ray Luminosities of IMBH Binaries stable transient

29. Implications • X-ray luminosities ~1040 ergs-1. • Stable X-ray lifetime ≤106 yr, much shorter than the main-sequence lifetime. • Hostless ULXs d ~ 100 (v/10 kms-1)(t0/107 yr)pc

30. The Effective Temperature Problem • For isotropic radiation, LX~ 1040 ergs-1 requires that the mass transfer rates are higher than ~10-6M⊙ yr-1. • The effective temperatures of supercritical accretion disks are ~ 1 keV, inconsistent with spectral analyzed results. (Ohsuga et al. 2002)

31. Conclusions • There may exist several types of ULXs with different nature. • The most luminous X-ray sources are likely to be IMBHs. • Transient behavior and beamed emission are not enough to distinguish between IMBHs and SMBHs.