David PooleyUniversity of Wisconsin
2009 Sep 24
Globular Cluster X-ray Sources
Chandra’s First Decade of Discovery
September 24, 1759
with thanks to friends, colleagues, and collaborators:
Walter LewinFrank Verbunt
Cees BassaLee Homer
Scott AndersonBruce Margon
Vicky KaspiBrian Gaensler
Derek FoxRudy Wijnands
Ed CackettJeroen Homan
Josh GrindlayCraig Heinke
Peter EdmondsSteve MurrayHaldan CohnPhyllis Luger
Adrienne CoolDaryl Haggard
Didier BarretBruce GendreNatalie Webb
Mathieu Servillat
Piet HutSimon Portegies Zwart
John FregeauNatalia Ivanova
Harvey Tananbaum and the entire CXC staff
Albert KongTing-Ni Lu
Shih Hao Lan
X-ray astronomy & globular clusters
Luminous X-ray sources (LX > 1036 erg s 1)Discovered by Uhuru and OSO-7; argued to be formed via cluster dynamics
Stimulated flurry of theoretical work
12 globular clusters thought to contain one each
All but one show Type-I X-ray bursts NS-LMXBs
Gursky 1973, Clark 1975, Katz 1975
e.g., Kuulkers et al. 1996
Fabian, Pringle, & Rees 1975, Sutantyo 1975, Hills 1975, 1976, Heggie 1975, Verbunt & Hut 1983
Bright X-ray Sources: New Chandra discoveries
White & Angelini 2001
M15 NGC 6440
in’t Zand et al. 2001
DP et al. 2002
Heinke et al. 2009
X-ray astronomy & globular clusters
Low Luminosity X-ray sources (LX < 1034 erg s 1)
Discovered by Einstein Hertz & Grindlay 1983 More found with ROSAT e.g., Verbunt 2001
No secure identifications
Suggested to be CVs (Hertz & Grindlay 1983), qLMXBs (Hertz & Grindlay 1983, Verbunt et al. 1984), radio
MSPs (Saito 1997), magnetically active binaries (Bailyn et al. 1990)
Extragalactic globular cluster X-ray sources (T. Maccarone)
Detailed MSP studies (S. Bogdanov)
Intermediate Mass Black Holes
Discovered by Uhuru and OSO-7; argued to be formed via cluster dynamics
Stimulated flurry of theoretical work
12 globular clusters thought to contain one each
All but one show Type-I X-ray bursts NS-LMXBs
Gursky 1973, Clark 1975, Katz 1975
e.g., Kuulkers et al. 1996
Fabian, Pringle, & Rees 1975, Sutantyo 1975, Hills 1975, 1976, Heggie 1975, Verbunt & Hut 1983
Luminous X-ray sources (LX > 1036 erg s 1)
Rcore = 0.74 parsec = 2.4 lightyears = 2.3 1018 cm
core = 4 104 star pc 3
us = 1 star pc 3
Globular Cluster NGC 2808
0 1 2 3
30
25
20
15
log (r/arcsec)
NGC 2808
corr
CGB1ADH-6829A-11944UP-6817SB-1721Illi. scanIlli. ann.Cheb. poly.
Trager, King, & Djorgovski 1995credit: S. Juchnowskiadapted from Servillat et al. (2008)
see poster by M. Servillat
Trager, King, & Djorgovski 1995
Surface Brightness Profiles
0 1 2 3
30
25
20
15
log (r/arcsec)
NGC 2808
corr
CGB1ADH-6829A-11944UP-6817SB-1721Illi. scanIlli. ann.Cheb. poly.
“core collapsed”
“normal”80% of globular clusters
20% of globular clusters
Simulating Globular Clusters
Fregeau et al. 2003
Globular Cluster Life Stages
Fregeau et al. 2003
Core contraction
Binary burning
Gravothermal oscillations
Deep core collapse
20% Core collapse80% Binary burning
X-ray Sources in Globular ClustersChandra image of 47 Tuc Low-mass X-ray Binaries
Millisecond pulsars
Cataclysmic Variables
Active main-sequence binaries
adapted from Grindlay et al. 2001
NGC 6397 M4 47 Tuc
47 Tuc in X-rays
Einstein (8 ksec) ROSAT (77 ksec) Chandra (240 ksec)
CX4CX4CX4CX4CX4
N
E
CX7CX7CX7CX7CX7
N
E
CX1CX1CX1CX1CX1
N
E
CX5CX5CX5CX5CX5
N
E
CX2CX2CX2CX2CX2
N
E
CX6CX6CX6CX6CX6
N
E
N
E
N
E
CX3CX3CX3CX3CX3
N
E
CX10CX10CX10CX10CX10
N
E
N
E
CX11CX11CX11CX11CX11
N
E
DP et al. (2002)
NGC 6752
Edmonds et al. (2003)
47 TucIdentifying the X-ray Sources
DP et al. (2002)
Grindlay et al. (2001)
Identifying the X-ray SourcesNGC 6752NGC 6397
Webb & Barret 2004 (from Gendre et al. 2003)
Cen qLMXB Cen CV
kT = 67 ± 2 eVR = 12.6 ± 3 km kT = 23 +39/-10 keV
Source Identification via X-rays
DP et al. 2003
A Link to Stellar Dynamics
Heinke et al. 2003
Gendre et al. 2003
77 GCs
114 ACIS Obs.
3 Msec
>1500 sources
~250 background
X-ray CMD
-2 -1 0 1 2 3log10 (C0.5-2 keV / C2-8 keV)
1028
1030
1032
1034
L 0
.5-8
keV
(er
g/s)
To Date:
-2 -1 0 1 2log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)
-2 -1 0 1 2log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)
-2 -1 0 1 2log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)
-2 -1 0 1 2log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)
NS A
tmosphere+ 20%
PL+ 50%
PL
X-ray CMD
22 GCs
~200 sources
~15 background
Uniform:
Lx > 4 1031 erg s-1
qLMXB
CVActive Binary
Pulsar
Field Burster
Field Pulsar
L 0
.5-6
keV
(erg
s-1
)
log 10 (F 0.5-2 keV /F 2-6 keV )
adapted from DP & Hut 2006
Specific units: n = N/M = /MM in units of 106 M
Cluster LMXBs
Can rule out constant n at 4
n = a s + c
s = 1.8 ± 0.4c = 0.4 ± 0.5
1 10 100 1000
1
10
100
nL
MX
B
1 10 100 1000
1
10
100
nL
MX
B
DP & Hut 2006
building on Gendre et al. 2003, Heinke et al. 2003, DP et al 2003
Are Globular Cluster CVs overabundant?
They should be:Hut & Verbunt 1983
Di Stefano & Rappaport 1994
Are Globular Cluster CVs overabundant?
They’re not:
Ivanova et al. 2006
With our simulations, we predict that the formation rates of CVsand AM CVn systems in GCs are not very different from those inthe field population. The numbers of CVs and AM CVn systems permass unit are comparable to numbers in the field if the whole clusterpopulation is considered, and they are only two to three times largerin the core than in the field. Dynamical formation is responsibleonly for 60–70 per cent of CVs in the core. This fraction decreases
Townsley & Bildsten 2005
Maybe a little?
Shara 1996,
elliptical galaxies. Likewise, we predict that there should be 60–180 CVs for every 106 L�, K in an old stellarpopulation. The population of X-ray–identified CVs in the globular cluster 47 Tuc is similar to this number, showingno overabundance relative to the field. The observed CN Porb distribution also contains evidence for a CV population
X-ray CMD
22 GCs
~200 sources
~15 background
Uniform:
Lx > 4 1031 erg s-1
qLMXB
CVActive Binary
Pulsar
Field Burster
Field Pulsar
-2 -1 0 1 2log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)
NS A
tmosphere+ 20%
PL+ 50%
PL
L 0
.5-6
keV
(erg
s-1
)
log 10 (F 0.5-2 keV /F 2-6 keV )
Can rule out constant n at 6
n = a s + c
s = 0.8 ± 0.2c = 0.3 ± 1.4
Bright, hard sources
Specific units: n = N/M = /MM in units of 106 M
(mostly CVs)
1 10 100 1000
1
10
100
nC
V
1 10 100 1000
1
10
100
nC
V
DP & Hut 2006
Dynamical Formation
Specific units: nx = Nx/M = /MM in units of 106 M
n = a s + c
s = 0.45 ± 0.17c = 5.3 ± 7.7
DP & Hut 2006
1 10 100 1000
0
50
100
150
200
n x
Cen
47 Tuc
1 10 100 1000
0
50
100
150
200
n x
Cen
47 Tuc
“Core collapsed”
1 10 100 1000
0
50
100
150
200
n x
Cen
47 Tuc
All sources withLx > 4 1030 erg s-1
Globular Cluster Life Stages Revisited
Fregeau et al. 2003
Core contraction
Binary burning
Gravothermal oscillations
Deep core collapse
Fregeau (2008) pointed out:
Better simulations reveal rc was over-estimated by 10 in binary-burning phase
Production of X-ray sources in binary burning phase is 2–20 higher than in core contraction phase
Chandra reveals that “core collapsed” clusters have many more binaries than the N- relation predicts
80% Binary burning+ 20% Core collapsed
80% Core contraction+ 20% Binary burning
Paradigm shift?
20% Core collapse80% Binary burning
10 100 1000 / M6
1
10
100
NX /
M6
15 "core-collapsed" GCs48 "normal" GCs
Dynamical FormationAll sources withLx > 4 1031 erg s-1
PRELIMINARY
DP et al. in prep.
Future Work
Individual identificationsSubpopulation dynamicsInvestigate importance of other parameters (e.g., metallicity)
Fast (~3 day) transients Heinke et al.
Low density clusters (primordial binaries) Kong, Lu, Lan et al.
Deep exposures of M4 (DP et al.) and NGC 6397 (Grindlay et al.)
Extending into rich open clusters: see poster by N. Gosnell
Fermi survey to determine overall MSP population
Summary
It’s Guinness’s 250th birthday!
X-ray sources are dynamically formed Great tracers for large scale simulations (especially LMXBs)
CVs are finally being found in large numbers in globular clusters and are overabundant
Chandra is the most efficient and effective means of finding the important close binaries in a globular cluster
Possible revolution in our understanding of the current dynamically states of globular clusters
Understanding Globular Clusters
No Thermodynamic Limit
M R3 Ekin M but Epot M2/R M5/3
Negative Heat Capacity
Mass segregation stratified system dense core
Encounters extract energy from core, increasing temperature
Nearly Unlimited Reservoir of Binary Binding Energy
3-body encounters tap into binary energy
Feedback between Stellar Dynamics and Stellar Evolution
No Easy Way to Compare Theory to ObservationInput theory to simulations
Compare simulations to observations
⇒ “Infrared Divergence”
⇒ “Ultraviolet Divergence”
(is tough)
Discovered by Uhuru and OSO-7; argued to be formed via cluster dynamics
Stimulated flurry of theoretical work
12 globular clusters thought to contain one each
All but one show Type-I X-ray bursts NS-LMXBs
Good evidence that many have ultra-short periods: Porb < 80 min
Gursky 1973, Clark 1975, Katz 1975
e.g., Deutsch et al. 1996
e.g., Kuulkers et al. 1996
Bright X-ray Sources: Neutron Star LMXBs
courtesy L. Homer
Kuulkers et al. 2003
Ultrashort Period
Normal Period
Unknown Period
X=0
X=0.73
Fabian, Pringle, & Rees 1975Sutantyo, 1975Hills, 1975, 1976Heggie 1975Verbunt & Hut 1983
GLAST Observations of Globular Cluster MSPs
10-4 10-3 10-2 10-1 100 101 102 103 104 105
Energy (GeV)
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
GeV
/(cm
2 s)
PSR J0437-4715
H.E.S.S.
BPS: R6=0.8, I45=0.6FP: R6=1.0, I45=1.0PS: R6=1.6, I45=2.2
ICS
MAGIC
B0 = 5.8 x 108 G
P = 5.8 ms
GLAST
EGRETCR
Harding, Usov, & Muslimov 2005
d = 0.18 kpc
FCR (1 GeV) = 0.145 10 8 cnts cm 2 s 1 MeV 1
Glob. clusters @ 2–10 kpc
BUT with 100s of MSPs!
Fclus F0437= 1000.185( )
2
F0437= 0.13
Cluster NLMXB d (kpc)MSP Flux
Fig. of Merit
Terzan 5 15.3 8.7 0.203NGC 6440 10.2 8.4 0.144
47 Tuc 2.7 4.5 0.132NGC 6388 13.0 10.0 0.130NGC 6540 1.6 3.7 0.120
Liller 1 10.5 9.6 0.114NGC 6266 5.4 6.9 0.113NGC 6544 0.5 2.7 0.064
Most Promising GLAST Clusters
Specific units: n = N/M = /MM in units of 106 M
Cluster LMXBs
Can rule out constant n at 4
n = a s + c
s = 1.8 ± 0.4c = 0.4 ± 0.5
1 10 100 1000
1
10
100
nL
MX
B
1 10 100 1000
1
10
100
nL
MX
B
DP & Hut 2006
Simulation by S. Portegies Zwart
GRAPE-6 special purposesupercomputer: 64 Tflops
See www.manybody.orgfor more information
“Kitchen Sink” simulations coming soon...
M67 Simulation by J. Hurley
“Kitchen Sink” simulations coming soon...
1960s, 70s: Theory predicted the inevitable collapse of cluster of single stars e.g., Hénon 1961, 1965
1980s: Computer simulations confirmed this and gave rich understanding of collapse e.g., work by Goodman, Heggie, Hut, Spitzer ...
1970s, 80s: Observers found no binaries e.g., Gunn & Griffin 1979
1990s: Observers found some binaries see Hut et al. 1992
1990s: Simulations showed how binaries postpone core collapse e.g., Goodman & Hut 1989
Brief History of Binaries in Globular Clusters
Low Mass X-ray Binary (LMXB)
low mass star + neutron star
Cataclysmic Variable (CV)
low mass star + white dwarf
Active Main Sequence Binary
two stars with strong magnetic fields
What is a “close” binary?
We can divide globular cluster binaries into two broad groups, based on binding energy:
Gm1m2
a>
1
2
⟨mv
2⟩ Gm1m2
a<
1
2
⟨mv
2⟩
“Hard” or “Close” binaries “Soft” binaries
Heggie (1975)
R = n1n2vrelσ
⇒ R ∼ ρ2/v
Γ =
∫rc
0
RdV ≈ ρ2
0r3
c/v = ρ
1.5
0r2
cΓ =
∫rh
0
RdV
(“Encounter Frequency”)
σ = πd2
(1 +
2G(m1 + m2)
v2
reld
)≈ πd
2G(m1 + m2)
v2
rel
“Heggie’s Law” in action (numerically):Perform scattering experiments for different initial parameters.
Hut & Bahcall (1983) from A. Gualandris (http://staff.science.uva.nl/~alessiag/)
“Heggie’s Law” in action (numerically):
Hut (1983)
Fregeau et al. 2003
Key Role of Binaries in GCs2% Binaries 20% Binaries
Verbunt 2004
ROSAT grayscale + Chandra point sources
The Revolutionary Chandra X-ray Observatory
The Search for Black Holes
Muno et al. 2005
Westerlund 1
Related Work: Young Massive Clusters
Cataclysmic Variable (CV)
low mass star + magnetic white dwarf
Intermediate Polar
-3 -2 -1 0 1 2 3log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1029
1030
1031
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)Very low Lx
4 1029 – 4 1030 erg s-1
3 GCs
243 sources
~35 background
Cluster / 6121 LV/LV,6121 Nsrcs Nsrcs /Nsrcs,6121
47 Tuc 34 10 180 – 200 12 – 13
NGC 6121 1 1 12 – 18 1
NGC 6397 0.45 0.77 4 – 6 0.3 – 0.4
Heinke et al. 2005, Bassa et al. 2004, Grindlay et al. 2001
(Mostly Active Main-sequence Binaries)
Cluster Obs. Nmin – Nmax <N> ± N Nunique
47 Tuc 14 70 – 88 78.9 ± 6.4 180
NGC 6121 5 7 – 14 10.2 ± 2.1 22
NGC 6397 11 10 – 16 13.3 ± 2.1 25
X-ray Sources in Globular Clusters
-3 -2 -1 0 1 2 3log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1029
1030
1031
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)
L 0
.5-6
keV
(erg
s-1
)
log 10 (F 0.5-2 keV /F 2-6 keV )
22 GCs
~200 sources
~15 background
18 GCs
~500 sources
~100 background
Uniform:
Lx > 4 1031 erg s-1
Uniform:
Lx > 4 1030 erg s-1
X-ray CMD
X-ray CMD
-2 -1 0 1 2log10 (Flux [0.5-2 keV] / Flux [2-6 keV])
1031
1032
1033
1034
LX [
0.5-
6 ke
V]
(erg
s-1)
21 GCs
~500 sources
~100 background
Uniform:
Lx > 4 1030 erg s-1
Primordial vs. Dynamical Formation
Compare number of sources (N) per cluster to
Primordial quantity: mass of cluster (M)
Dynamical quantity: encounter frequency of cluster ( )
Problem — M and are correlated
Solution — use “specific” units n N/M /M
Method — primordial: n = c dynamical: n( ) = a s + c
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