Accretion to Neutron Stars Accretion to Neutron Stars and Black Holes in Binariesand Black Holes in Binaries

Li Xiang-DongLi Xiang-Dong

Department of AstronomyDepartment of Astronomy

Nanjing UniversityNanjing University

1. Accretion Torque of 1. Accretion Torque of Magnetized Disks on Magnetized Disks on

Accreting Neutron StarsAccreting Neutron Stars

Disk Accretion onto Magnetized Disk Accretion onto Magnetized Neutron StarsNeutron Stars

Accretion Torque: The Standard Accretion Torque: The Standard PicturePicture

N N0 Nmag

0

0

2mag

2 2

d

d dc

c

zR

R

z zR R

N R B B R

R B B R R B B R

Spin-up Spin-down

Spin-up

N = N00 f()

≡ss/0 0 ≡ (R00/Rcc)3/2

N0 0 = 0 when = cc

Spin-up line

P-dP/dt diagram for radio pulsars

ProblemsProblems

• Abrupt torque reversalAbrupt torque reversal

• Spin-down rate increasing with accretion Spin-down rate increasing with accretion raterate

• Accretion in the propeller regimeAccretion in the propeller regime

Spin Reversal in Accreting Spin Reversal in Accreting X-ray PulsarsX-ray Pulsars

Bildsten et al. (1997)

Chakrabarty et al. (1997)

ExplanationsExplanations

• Change of the direction of disk rotation due Change of the direction of disk rotation due to disk warping (van Kerkwijk et al. 1998).to disk warping (van Kerkwijk et al. 1998).

• Retrograde disk rotation (Makishima et al. Retrograde disk rotation (Makishima et al. 1988; Nelson et al. 1997)1988; Nelson et al. 1997)

• Bimodal magnetic torque (Torkelsson 1998; Bimodal magnetic torque (Torkelsson 1998; Locsei & Melatos 2004).Locsei & Melatos 2004).

• Bimodal disk structure (Yi et al. 1997).Bimodal disk structure (Yi et al. 1997).

Spin-down Rate vs. X-ray Spin-down Rate vs. X-ray Luminosity in GX 1+4Luminosity in GX 1+4

Chakrabarty et al. (1997)

Numerical Simulation of Disk Accretion to Numerical Simulation of Disk Accretion to a Rapidly Rotating Magnetized Stara Rapidly Rotating Magnetized Star

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

Romanova et al. (2004)Romanova et al. (2004)

Accretion occurs Accretion occurs in strong propeller in strong propeller regime.regime.

Dependence on Period and Mass Dependence on Period and Mass Accretion RateAccretion Rate

2. Ultraluminous X-ray Sources and Intermediate-

Mass Black Holes

IntroductionIntroduction

Ultraluminous X-ray sources (ULXs) are point-Ultraluminous X-ray sources (ULXs) are point-like, extra-nuclear X-ray sources found in nearby like, extra-nuclear X-ray sources found in nearby galaxies, with galaxies, with isotropicisotropic X-ray luminosities in X-ray luminosities in excess of 10excess of 103939 ergs ergs-1 -1 ..

Distribution of ULXs in GalaxiesDistribution of ULXs in Galaxies

• In spirals, ULXs are In spirals, ULXs are often near, but distinct often near, but distinct from the dynamical from the dynamical centers of the galaxies.centers of the galaxies.

• In ellipticals, ULXs are In ellipticals, ULXs are almost exclusively in the almost exclusively in the halos of the galaxies.halos of the galaxies.

(Colbert & Ptak 2002)(Colbert & Ptak 2002)

ULXs as Accreting Black HolesULXs as Accreting Black Holes

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

Theoretical ModelsTheoretical Models for ULXs for ULXs

• The majority of ULXs may be stellar-mass (~10 The majority of ULXs may be stellar-mass (~10 MM⊙⊙ ) black holes (SMBHs) , or intermediate- ) black holes (SMBHs) , or intermediate-

mass (~10mass (~1022-10-1044 M M⊙⊙ ) black holes (IMBHs), ) black holes (IMBHs),

accreting from their binary companion stars.accreting from their binary companion stars.

SMBHs as ULXsSMBHs as ULXs

Rappaport et al. 2004

Evidence for the Beaming ModelEvidence for the Beaming Model

• A large fraction of ULXs are associated A large fraction of ULXs are associated with star forming regions.with star forming regions.

• Similar spectral characteristics with the Similar spectral characteristics with the Galactic microquasars SS 433 and GRS Galactic microquasars SS 433 and GRS 1915+105.1915+105.

• Possible massive optical counterparts.Possible massive optical counterparts.

• Jet from a ULX(?)Jet from a ULX(?)

ULXs and Star FormationULXs and Star Formation

The Antennae galaxies The Cartwheel galaxy

Difficulties for the Beaming ModelDifficulties for the Beaming Model

• Detection of a strong Detection of a strong narrow 54 mHz QPO in narrow 54 mHz QPO in the starburst galaxy the starburst galaxy M82.M82.

• The broad Fe K line The broad Fe K line centered at 6.55 keV is centered at 6.55 keV is also hard to understand also hard to understand in a beaming scenarioin a beaming scenario

(Strohmayer & Mushotzky 2003)

Difficulties for the Beaming ModelDifficulties for the Beaming Model

• Periodic light change Periodic light change eclipsing binary eclipsing binary

Difficulties for the Beaming ModelDifficulties for the Beaming Model

• Emission nebulae of a few hundred pc diameter Emission nebulae of a few hundred pc diameter are found to be present at or around several ULXs are found to be present at or around several ULXs ((isotropic emission?)isotropic emission?)

(Pakull & Mirioni 2002)

IMBH CandidatesIMBH Candidates

• ULXs with very high X-ULXs with very high X-ray luminosities (~10ray luminosities (~104040 ergsergs-1-1), and relatively low ), and relatively low color temperatures (0.05-color temperatures (0.05-0.3 keV) have been 0.3 keV) have been suggested to be IMBH suggested to be IMBH candidates.candidates.

(Miller, Fabian,, Miller (Miller, Fabian,, Miller 2004)2004)

Formation of SMBHs

(Heger et al. 2002)

Formation Scenarios for IMBHsFormation Scenarios for IMBHs

• Merging of stars in a young stellar cluster Merging of stars in a young stellar cluster followed by direct collapse into an IMBH followed by direct collapse into an IMBH (Portegies Zwart et al. 1999).(Portegies Zwart et al. 1999).

• Merging of binaries that have a black hole Merging of binaries that have a black hole with initial mass of ~50 with initial mass of ~50 MM⊙⊙ in a globular in a globular cluster (Miller & Hamilton 2002).cluster (Miller & Hamilton 2002).

• Evolution of primordial population III Evolution of primordial population III stars (Madau & Rees 2001).stars (Madau & Rees 2001).

IMBHs in Compact Star ClustersIMBHs in Compact Star Clusters

(Portegies Zwart et al. 2004)

Formation and Evolution of IMBH Formation and Evolution of IMBH BinariesBinaries

• Black hole formationBlack hole formation

Runaway collision in dense young star Runaway collision in dense young star clusters (Portegies Zwart et al. 2004)clusters (Portegies Zwart et al. 2004)

• X-ray binary formationX-ray binary formation

Exchange encounters (Kalogera et al. 2004)Exchange encounters (Kalogera et al. 2004)

Tidal capture (Hopman, Portegies Zwart, Tidal capture (Hopman, Portegies Zwart, Alexander 2004) Alexander 2004)

X-ray Luminosities of IMBH BinariesX-ray Luminosities of IMBH Binaries

stable transient

ImplicationsImplications

• X-ray luminosities ~10X-ray luminosities ~104040 ergs ergs-1-1..

• Stable X-ray lifetime ≤10Stable X-ray lifetime ≤1066 yr, much shorter yr, much shorter than the main-sequence lifetime.than the main-sequence lifetime.

• Hostless ULXsHostless ULXs

dd ~ 100 ( ~ 100 (vv/10 kms/10 kms-1-1)) ((tt00/10/1077 yr) yr) pcpc

The Effective Temperature The Effective Temperature ProblemProblem

• For isotropic radiation, For isotropic radiation, LLXX

10104040 ergs ergs-1-1 requires that the requires that the mass transfer rates are higher mass transfer rates are higher than than MM⊙⊙ yr yr..

• The effective temperatures of The effective temperatures of supercritical accretion disks are supercritical accretion disks are ~ 1 keV, inconsistent with ~ 1 keV, inconsistent with spectral analyzed results.spectral analyzed results. (Ohsuga et al. 2002)

ConclusionsConclusions

• There may exist several types of ULXs with There may exist several types of ULXs with different nature.different nature.

• The most luminous X-ray sources are likely The most luminous X-ray sources are likely to be IMBHs.to be IMBHs.

• Transient behavior and beamed emission Transient behavior and beamed emission are not enough to distinguish between are not enough to distinguish between IMBHs and SMBHs.IMBHs and SMBHs.