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X-ray Binaries in Nearby Galaxies
Vicky Kalogera Northwestern University
with
Chris Belczynski (NU)
Andreas Zezas and Pepi Fabbiano (CfA)
X-Ray Binaries in Nearby Galaxies
Outline: Observations: Past and Present
Questions and Puzzles
Theoretical models of X-ray Binaries: WhatWhat can they tell us ? HowHow can we use them ?
Population Synthesis Tutorial
Results and Comparisons with Observations
What's next...
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
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 (< 10) > very limited spectral information due to low X-ray counts
long-standing problems with low angular resolution and source confusion > XLF reliably constructed only for M31 and M101 > 'super-Eddington''super-Eddington' sources were tentativelytentatively identified
X-Ray Binary Populations
other galaxies: post-Chandra ... more than ~100 galaxies observed
they cover a wide range of galaxy typesgalaxy types and star-formation historiesstar-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
The Antennae: ~ 80 point sources!
courtesy Fabbiano,Zezas et al.
Chandra ROSAT
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-lawspower-laws with slopes correlatingslopes correlating with galaxy typegalaxy type
XLF slopes and galaxy typesspirals starbursts
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m K
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ard
et
al.
2002
(as
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/020
3190
)
ellipticals & bulges
XLF shapes seem to correlate with SFR and age
Older populations have steeper slopes, but is the correlation monotonicmonotonic ?
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m S
araz
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. 20
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XLF
slo
pe
SFR
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)
?L
X > 1040 erg/s ===> M
BH > 50 Mo
? or beaming ?
elliptical galaxies: high incidence of sources in globular clusters ? (Sarazin et al. 2001; Kundu et al. 2002)
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 > different XRB populations ? (Sarazin et al. 2000)
> age effects ? (Wu 2000; Kilgaard et al. 2002)
> both ? (VK, Jenkins, Belczynski 2003)
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.
courtesy Sky & TelescopeFeb 2003 issue
How do X-ray binaries form ?
primordial binary
X-ray binary at Roche-lobe overflow
Common Envelope:orbital contractionand mass loss
NS or BH formation
Population Synthesis Elements Star formation conditions:
> time and duration, metallicity, IMF, binary properties
Modeling of single and binary evolution > mass, radius, core mass, wind mass loss > orbital evolution: e.g., tidal synchronization and circularization, mass loss, mass transfer > mass transfer modeling: stable driven by nuclear evolution or angular momentum loss thermally unstable or dynamically unstable > compact object formation: masses and supernova kicks > X-ray phase: evolution of mass-transfer rate and X-ray luminosity
Population Synthesis withStarTrack
Single-star models from Hurley et al. 2000 Tidal evolution of binaries included
> 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 )
> wind-fed: Bondi accretion > 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) > also included: Eddington-limited accretion (testable) thermal-time scale mass transfer, transient behavior
Belczynski et al. 2001,2003
●
●
Example of Mass-Transfer Calculation
time (yr)
log[
M /
(M
o/y
r) ]
●
Comparison between a detailed caclulation with a full stellar evolution code (N. Ivanova) and the semi-analytic treatment implemented in StarTrack
BH mass: 4.1Mo
donor mass: 2.5Mo
choice of masses from Beer & Podsiadlowski 2002Results in very good agreement ( within 20-50%)
semi-analytic calculation most appropriate for statistical modelingof large binary populations
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
log [ Lx / (erg/s) ]
log
[ N
( >
Lx
)]
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
XLF dependence on ageXLF dependence on age(cf. Grimm et al.; Wu; Kilgaard et al.)
log [ Lx / (erg/s) ]
log
[ N
( >
Lx
)]
all XRBs at ~100 Myr
std model
no BH kicks at birth
Z = Zo
stellar winds reduced by 4
Normalized Model XLFs
XLF dependence on model parametersXLF dependence on model parameters
Belczynski, VK, Zezas, Fabbiano 2003
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
XLF slopes and breaksXLF slopes and breaks
log [ Lx / (erg/s) ]
log
[ N
( >
Lx
)] all XRBs
Eddington-limitedaccretion
no Eddington limitimposed
Normalized XLFs Models match NGC1569 SF history
Arons et al. 1992...Shaviv 1998...Begelman et al. 2001...
VK, Henninger, Ivanova, & King 2003
Observational Diagnostic for ULXs
In young ( >100Myr ) stellar environmentstransient behavioris shown to be associated with accretion onto an IMBH
IMBH or thermal-timescalemass transfer withanisotropic emission ?
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
What's coming next ... Choose a sample of galaxies with relatively well-understood star-formation histories and
> indentify XRB models that best describe the XLF
shape
> 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
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)
ULX source in M82
NGC 1316NGC 1316
elliptical galaxy at XXXMpcwith a recent merger: > short SF episode 1-3Gyr ago, Z ~ Zo
> older population with
and age of ~11.5Gyr Z ~ 0.29
courtesyKim, Fabbiano CXC,DSS
Goudfrooij et al. 2001Trager et al. 2000
NGC 1316NGC 1316
log [ Lx / (erg/s) ]
log
[ N
( >
Lx
)]
data: ~55 sources (Kim & Fabbiano 2002)
all XRBs at 1Gyr
Normalized XLFs Model matches NGC1316 SF history
Source Identification based on X-ray Colors
Prestwich et al 2002astro-ph/0206127
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 ?
NGC 1569NGC 1569
log [ Lx / (erg/s) ]
log
[ N
( >
Lx
)]
data: 14 sources
all XRBs at 110MyrNS XRBswind-fed XRBswind-fed NS XRBs
Normalized XLFs Models match NGC1569 SF history