X-ray Binaries in Nearby Galaxies

1
X-ray Binaries in Nearby Galaxies

• Vicky Kalogera
• Northwestern University
• with
• Chris Belczynski (NU)
• Andreas Zezas and Pepi Fabbiano (CfA)

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X-Ray Binaries in Nearby Galaxies
Outline

• Observations Past and Present

• Questions and Puzzles

• Theoretical models of X-ray Binaries

• What can they tell us ?
• How can we use them ?

• Population Synthesis Tutorial

• Results and
• Comparisons with Observations

• What's next...

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X-Ray Binary Populations
the Milky Way

• first discovered in our Galaxy
• 100 known 'low-mass' XRBs (Roche-lobe
  overflow)
• 30 known 'high-mass' XRBs (wind
  accretion)
• long-standing problem with distance estimates
• very hard to study the X-ray luminosity
  function
• and spatial distribution
• other properties, e.g., orbital period, donor
  masses
• known only for a few systems

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X-Ray Binary Populations
other galaxies pre-Chandra ...

• discovered in the LMC/SMC, M31,
• and another 15 galaxies (all spirals), most
  of them
• with only a handful of point X-ray sources (lt
  10)
• gt very limited spectral information
• due to low X-ray counts
• long-standing problems with
• low angular resolution and source confusion
• gt XLF reliably constructed only for M31 and
  M101
• gt 'super-Eddington' sources were tentatively
• identified

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X-Ray Binary Populations
other galaxies post-Chandra ...

• more than 100 galaxies observed
• they cover a wide range of galaxy types
• and star-formation histories
• 10-100 point sources in each
• population studies become feasible
• known sample distance great advantage for
• studies of X-ray luminosity functions
• and spatial distributions

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The Antennae 80 point sources!
Chandra
ROSAT
courtesy Fabbiano,Zezas et al.
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X-Ray Binary Populations
other galaxies post-Chandra ...
cont

• typical sensitivity limits down to 1036-1037
  erg/s
• spectral information useful for identification
  of
• point-source types LMXBs, HMXBs, SNRs
• X-ray luminosity functions (XLF)
• power-laws with slopes correlating with
• galaxy type

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XLF slopes and galaxy types
spirals
starbursts

• XLF shapes seem to
• correlate with SFR and
• age
• Older populations have
• steeper slopes,
• but is the correlation
• monotonic ?

ellipticals bulges
from Kilgard et al. 2002 (astro-ph/0203190)
XLF slope
from Sarazin et al. 2001
SFR
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X-Ray Binary Populations
other galaxies post-Chandra ...
cont

• existence of Ultra-Luminous X-ray sources
  (ULXs)
• established, although not yet understood
• (formerly known as super-Eddington sources)
• ?
• LX gt 1040 erg/s gt MBH gt 50 Mo
• ?
• or beaming ?
• elliptical galaxies high incidence of sources
  in
• globular clusters ? (Sarazin et al. 2001
  Kundu et al. 2002)

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XLF observations some of the puzzles

• What determines the shape of XLFs ?
• Is it a result of a blend of XRB populations
  ?
• How does it evolve ?
• Are the reported breaks in XLFs real
• or due to incompleteness effects ?
• If they are real, are they caused by
• gt different XRB populations ? (Sarazin et
  al. 2000)
• gt age effects ? (Wu 2000 Kilgaard
  et al. 2002)
• gt both ? (VK, Jenkins, Belczynski 2003)

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Theoretical Modeling

• Current status observationally-driven
• Chandra observations provide an excellent
  challenge
• and opportunity for progress in the study of
  global
• XRB population properties.
• Population Synthesis Calculations necessary
• Basic Concept of Statistical Description
• evolution of an ensemble of binary
  and
• single stars with focus on XRB
  formation and
• their evolution through the X-ray
  phase.

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primordial binary
How do X-ray binaries form ?
Common Envelope orbital contraction and mass loss
NS or BH formation
X-ray binary at Roche-lobe overflow
courtesy Sky Telescope Feb 2003 issue
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Population Synthesis Elements

• Star formation conditions
• gt time and duration, metallicity, IMF, binary
  properties
• Modeling of single and binary evolution
• gt mass, radius, core mass, wind mass loss
• gt orbital evolution e.g., tidal
  synchronization and
• circularization, mass loss, mass transfer
• gt mass transfer modeling
• stable driven by nuclear evolution or
  angular momentum loss
• thermally unstable or dynamically unstable
• gt compact object formation masses and
  supernova kicks
• gt X-ray phase evolution of mass-transfer rate
  
• and X-ray luminosity

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Population Synthesis withStarTrack
Belczynski et al. 2001,2003

• Single-star models from Hurley et al. 2000
• Tidal evolution of binaries included
• gt important for wind-fed X-ray binaries
• tested with measured Porb contraction
• (e.g., LMC X-4 Levine et al. 2000)
• Mass transfer calculations ( M and Lx )
• gt wind-fed Bondi accretion
• gt Roche-lobe overflow
• M based on radial response of donor and
  Roche lobe
• to mass exchange and possible loss from the
  binary
• (tested against detailed mass-transfer
  calculations)
• gt also included Eddington-limited accretion
  (testable)
• thermal-time scale mass transfer,
  transient behavior

?
?
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Example of Mass-Transfer Calculation
Comparison between a detailed caclulation with a
full stellar evolution code (N. Ivanova) and the
semi-analytic treatment implemented in StarTrack
semi-analytic calculation most appropriate for
statistical modeling of large binary populations
BH mass 4.1Mo donor mass 2.5Mo
log M / (Mo/yr)
?
choice of masses from Beer Podsiadlowski
2002 Results in very good agreement ( within
20-50)
time (yr)
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NGC 1569
(post-)starburst galaxy at 2.2Mpc with
well-constrained SF history gt
100Myr-long episode, probably
ended 5-10Myr ago, Z 0.25 Zo
gt older population with
continuous SF for 1.5Gyr, Z
0.004 or 0.0004, but weaker in SFR
than recent episode by factors of
gt10
courtesy Schirmer, HST
courtesy Martin, CXC,NOAO
Vallenari Bomans 1996 Greggio et al.
1998 Aloisi et al. 2001 Martin et al. 2002
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XLF dependence on age
(cf. Grimm et al. Wu Kilgaard et al.)
Normalized Model XLFs
non-monotonic behavior 10 Myr strong winds
from most massive stars
50 Myr 100 Myr 150 Myr 200 Myr Roche-lobe
overflow XRBs become
important
log N( gt Lx )
log Lx / (erg/s)
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XLF dependence on model parameters
Normalized Model XLFs
all XRBs at 100 Myr std model no BH kicks at
birth Z Zo stellar winds reduced by 4
log N( gt Lx )
log Lx / (erg/s)
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Old 1.5 Gyr Young 110 Myr SFR Y/O 20
NGC 1569 XLF modeling
Belczynski, VK, Zezas, Fabbiano 2003
Old 1.5 Gyr Young 70 Myr SFR Y/O 20

• Hybrid of
• 2 populations
• underlying old
• starburst young

Old 1.3 Gyr Young 70 Myr SFR Y/O 40
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XLF slopes and breaks
Normalized XLFs Models match NGC1569 SF history
all XRBs Eddington-limited accretion no
Eddington limit imposed
log N( gt Lx )
Arons et al. 1992... Shaviv 1998... Begelman et
al. 2001...
log Lx / (erg/s)
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Observational Diagnostic for ULXs
IMBH or thermal-timescale mass transfer
with anisotropic emission ?
VK, Henninger, Ivanova, King 2003
In young ( gt100Myr ) stellar environments transie
nt behavior is shown to be associated with
accretion onto an IMBH
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Conclusions

• 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
• 'Broken' power-laws seem to be due to
  
• Eddington-limited accretion
• Transient behavior can distinguish between
• IM and stellar-mass BH

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What's coming next ...

• Choose a sample of galaxies with relatively
  well-understood star-formation histories and
  
• gt indentify XRB models that best describe the
  XLF shape
• gt use the results to 'calibrate' population
  models for
• different galaxy types (spirals, starburst,
  ellipticals) and
• derive constraints on the star-formation
  history of
• other galaxies
• Use the number of XRBs, to examine correlation
  with SFR
• and constrain binary evolution parameters that
  affect the absolute normalization of the XLF but
  not its shape

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What's coming next ...

• How are XLFs different if dynamical processes
  are
• important ?
• If IMBH form, how do they acquire binary
  companions that can initiate mass transfer ?
  
  

(work with N. Ivanova C. Belczynski)
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ULX source in M82
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NGC 1316
elliptical galaxy at XXXMpc with a recent merger
gt short SF episode 1-3Gyr ago, Z
Zo gt older population with and age of
11.5Gyr Z 0.29
courtesy Kim, Fabbiano CXC,DSS
Goudfrooij et al. 2001 Trager et al. 2000
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NGC 1316
Normalized XLFs Model matches NGC1316 SF history
data 55 sources (Kim Fabbiano
2002) all XRBs at 1Gyr
log N( gt Lx )
log Lx / (erg/s)
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Source Identification based on X-ray Colors
Prestwich et al 2002 astro-ph/0206127
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XLF observations questions and puzzles

• Can the XLF properties (shapes, numbers) be used
  as
• star-formation indicators ?
• e.g., IMF, metallicity, star-formation rate,
  or age ?
• What is the origin of the ULXs ?
• Can we explain them as normal' BH-XRBs or
• the hypothesis of intermediate-mass BH is
• necessary ?
• What is the role of XRB formation in globular
  clusters ?
• Do dynamically formed XRBs have different XLF
  
• characteristics ?

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NGC 1569
Normalized XLFs Models match NGC1569 SF history
data 14 sources all XRBs at 110Myr NS
XRBs wind-fed XRBs wind-fed NS XRBs
log N( gt Lx )
log Lx / (erg/s)