Accretion to Neutron Stars and Black Holes in Binaries

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Accretion to Neutron Stars and Black Holes in
Binaries

• Li Xiang-Dong
• Department of Astronomy
• Nanjing University

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1. Accretion Torque of Magnetized Disks on
Accreting Neutron Stars
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Disk Accretion onto Magnetized Neutron Stars
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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
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Problems

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

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

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Spin-down Rate vs. X-ray Luminosity in GX 14
Chakrabarty et al. (1997)
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Numerical Simulation of Disk Accretion to a
Rapidly Rotating Magnetized Star
Accretion occurs in strong propeller regime.

• The spin-down rate increases as the accretion
  rate increases.

Romanova et al. (2004)
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Dependence on Period and Mass Accretion Rate
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2. Ultraluminous X-ray Sources and
Intermediate-Mass Black Holes
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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 .

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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)
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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.

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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.

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SMBHs as ULXs
Rappaport et al. 2004
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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 1915105.
• Possible massive optical counterparts.
• Jet from a ULX(?)

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ULXs and Star Formation
The Antennae galaxies
The Cartwheel galaxy
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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)
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Difficulties for the Beaming Model

• Periodic light change ? eclipsing binary

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

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

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X-ray Luminosities of IMBH Binaries
stable
transient
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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

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The Effective Temperature Problem

• For isotropic radiation, LX 1040 ergs-1
  requires that the mass transfer rates are higher
  than 10-6 M? yr-1.
• The effective temperatures of supercritical
  accretion disks are 1 keV, inconsistent with
  spectral analyzed results.

(Ohsuga et al. 2002)
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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.