globular cluster x-ray sources - chandra x-ray...

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