X-ray Binaries in Nearby Galaxies

Vicky Kalogera Northwestern University

• Super Star Clusters

• Starburst galaxies

• Ultra-Luminous X-Ray Sources

• Elliptical galaxies

Chandra observations of XRBs - some of the puzzles -

How does XRB formation and evolution depend on star-formation history and metallicity ?

Do Super Star Clusters form High-Mass XRBs ?

What determines the shape of X-Ray Luminosity Functions (XLF) ? Is it due to a blend of different XRB populations ?

What is the nature of Ultra-Luminous X-ray Sources (ULX) ?

Super-Star Clusters (SSCs)

• Compact, young analog to globular clusters• Found frequently in starburst environments

• Masses range from ~104 to ~106 M

•Ages range from a few to tens of Myr

Kaaret et al. 2004

• Lx ≥ (0.5-3)x1036 erg/s

Distribution of X-Ray point sources

< 1 XRB per cluster!

Kaaret et al. 2004

• Lx ≥ 5x1035 erg/s

Distribution of X-Ray point sources

• XRBs closely associated with star clusters• Median distance ~30-100 pc

< 1 XRB per cluster!M82N5253

N1569

50%

Is this all due to Is this all due to Supernova Kicks ?Supernova Kicks ?

Theoretical XRB Distributions

• cluster mass: ~5x104 Mo

• LX > 5x1035 erg/s• average of 1,000 cluster simulations

• Significant age dependence

• < 1 XRB per cluster

Models: Population Syntheses of XRBs and Kinematic Orbit Evolution in Cluster Potential

Sepinsky et al. 2005, ApJL

Conclusions

XRB models without cluster dynamics appear in agreement with observations

Mean XRB number per SSC < 1 and spatial distribution: M < 105 Mo and 10-50Myr or more massive and ~50Myr

• Results do not appear sensitive to binary evolution assumptions, but extended parameter study is needed.• Explore role of dynamics for more massive and older clusters

Supernova kicks: eject XRBs @ D > 10pc especially for M < 105 Mo

NGC 1569NGC 1569(post-)starburst galaxy at 2.2Mpcwith well-constrained SF history: > ~100Myr-long episode, probably ended 5-10Myr ago, Z ~ 0.25 Zo

> older population with

continuous SF for ~ 1.5Gyr, Z ~ 0.004 or 0.0004, but weaker in SFR than

recent episode by factors of >10

Vallenari & Bomans 1996;Greggio et al. 1998;Aloisi et al. 2001; Martin et al. 2002

courtesySchirmer, HST

courtesyMartin, CXC,NOAO

Belczynski, VKet al. 2004, ApJL

NGC 1569 XLF modeling

Hybrid of 2 populations:

underlying old starburst young

Old: 1.5 GyrYoung: 110 MyrSFR Y/O: 20

Old: 1.5 GyrYoung: 70 MyrSFR Y/O: 20

Old: 1.3 GyrYoung: 70 MyrSFR Y/O: 40

Conclusions on Starbursts

Current understanding of XRB formation and evolution produces XLF properties consistent with observations Model XLFs can be used to constrain star-formation properties, e.g., age and metallicity

Shape of model XLFs appear robust against variations of most binary evolution parameters

Ultra-Luminous X-ray Sources

First discovered with Einstein X-ray telescope

Extragalactic sources with LX ≥ 1039 erg/s

Later observations determined many are off-nuclear & not associated with supernovae

What is the origin of these sources?

Intermediate-Mass Black Holes? (50 - 1000Mo)

Strongly Anisotropic XRB emission ?

IMBH Binaries ?

Cluster core simulations with binary evolution and multi-body dynamical interactions

Do IMBH acquire mass-transfering binary companions in cluster cores ?

Blecha, Ivanova, VK et al. 2005

IMBH Companions

Mass Mass distributiondistribution

Orbital separation Orbital separation distributiondistribution

Conclusions on IMBH Binaries

• Optimal IMBH mass range: 100-200 Mo

• MT is relatively rare; highest incidence at 100Mo

about 3-5% of cluster lifetime with MT IMBH binary

• MS mass-transferring companions are more common & spend more time in MT

BUT ARE THEY ULXs?Blecha, Ivanova, VK et al. 2005

Brightest XRBs in Elliptical Galaxies

Upper-end XLF slope (LX: from ~0.5 - 2 1039 erg/s) :

footprint of accreting BH mass spectrummodified by probability of XRB detection due to

transient mode of accretion

Ivanova & VK 2005, ApJ

What to Expect in the Future ?

For example:

* Long-term time monitoring will become possible; identification of X-ray transients and clues to ULX nature

* Bigger source samples will allow probing the rarebrightest sources and questions of BH formation

* Systematic modeling of galaxy samples will reveal sensitivity to SFR and Z …