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Dead Stars Do Tell Tales: Chandra Observations of Dense Stellar Systems. Michael Muno (UCLA/Hubble Fellow). What Are We Looking For?. Black hole and neutron star X-ray binaries. L x = 10 30 to 10 39 erg s -1. Accreting white dwarfs (intermediate polars). L x = 10 29 to 10 33 erg s -1.

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Dead stars do tell tales chandra observations of dense stellar systems
Dead Stars Do Tell Tales:Chandra Observations of Dense Stellar Systems

Michael Muno (UCLA/Hubble Fellow)


What Are We Looking For?

Black hole and neutron star X-ray binaries.

Lx= 1030 to 1039 erg s-1.

Accreting white dwarfs (intermediate polars).

Lx= 1029 to 1033 erg s-1.

WR 124;

HST/WFPC

Wolf-Rayet and OB stars in colliding-wind binaries.

Lx= 1029 to 1034 erg s-1.

Pulsars.

Lx= 1029 to 1038 erg s-1.


The era before chandra

N

E

The Era Before Chandra

The central 300 pc of the Galaxy

(

30 pc


Chandra observations of dense stellar systems

N

E

Chandra Observations of Dense Stellar Systems

The central 300 pc of the Galaxy: 30 x 12 ks exposures

Wang, Gotthelf, & Lang 2002; NASA/UMass

30 pc


Chandra observations of dense stellar systems1

N

E

Chandra Observations of Dense Stellar Systems

Sgr A

complex

BH LMXB

LMXB

pulsar

The central 300 pc of the Galaxy: 30 x 12 ks exposures

Wang, Gotthelf, & Lang 2002; NASA/UMass

30 pc


Chandra observations of dense stellar systems2

N

E

Chandra Observations of Dense Stellar Systems

Sgr A

complex

Sgr B

GMCs

Sgr C

GMC

The central 300 pc of the Galaxy: 30 x 12 ks exposures

Wang, Gotthelf, & Lang 2002; NASA/UMass

30 pc


Chandra observations of dense stellar systems3

N

E

Chandra Observations of Dense Stellar Systems

Foreground

star cluster

Arches &

Quintuplet

Sgr A

complex

Foreground

star cluster

The central 300 pc of the Galaxy: 30 x 12 ks exposures

Wang, Gotthelf, & Lang 2002; NASA/UMass

30 pc


X ray observations of accreting black holes and neutron stars

N

E

X-ray Observations of Accreting Black Holes and Neutron Stars

The central 300 pc of the Galaxy: 30 x 12 ks exposures

Wang, Gotthelf, & Lang 2002; NASA/UMass

30 pc


N

E

5 pc

1 Msec over 7 yr

Baganoff et al. (2003);

Muno et al. (2003a, ApJ, 589, 225)


N

E

5 pc

Iron Fluorescence

Sgr A East

SNR

Sgr A*

Outflow from

Sgr A*

1 Msec over 7 yr

Baganoff et al. (2003);

Muno et al. (2003a, ApJ, 589, 225)


N

E

0.5 pc


N

E

0.5 pc

Pulsar?

Sgr A*

Small star

cluster IRS 13

LMXB


The population of x ray sources
The Population of X-ray Sources

  • What are the X-ray sources?

  • How do dense stellar environments affect the population of close binaries?

  • What are the masses of stars that form black holes and neutron stars?

The central 300 pc of the Galaxy:

Wang, Gotthelf, & Lang 2002; NASA/UMass

30 pc


The surprises
The Surprises

  • X-ray transients are concentrated in the central parsec of the Galaxy.

  • There is an X-ray pulsar in the star cluster Westerlund 1.


The population of x ray sources1
The Population of X-ray Sources

Full survey: 1% of the Galactic stellar mass

Deep survey: 0.1% of the Galactic stellar mass

The central 300 pc of the Galaxy:

Wang, Gotthelf, & Lang 2002; NASA/UMass

30 pc


Galactic center x ray sources
Galactic Center X-ray Sources

  • Shallow survey:

    • Lx>5x1032 erg s-1

    • 549 foreground sources

    • 1352 Galactic center sources

    • <130 background AGN

  • Deep Sgr A* field:

    • LX>1031 erg s-1

    • 200 foreground sources

    • 2287 Galactic center sources

    • <40 background AGN

Sgr A*


X ray sources trace the stellar population
X-ray Sources Trace the Stellar Population

In the central 20 pc, the surface density of X-ray sources falls off as R-1, just like the stellar population in the infrared.

(Muno et al. 2003a, 2006)


Spectra of the point sources
Spectra of the Point Sources

Color = (H-L/H+L).

L = 3.3-4.7 keV

H = 4.7-8.0 keV

keV

Muno et al. (2004b, ApJ, 613, 1179)


Spectra of the point sources1
Spectra of the Point Sources

mCV

HMXB

pulsar

WR/O

LMXB

CV

Muno et al. (2004b, ApJ, 613, 1179)


Most x ray sources are magnetic cvs
Most X-ray Sources are Magnetic CVs

  • The spectra of the point sources are consistent with those of mCVs.

  • Seven sources exhibit periodic flux modulations, as expected from mCVs.

  • 104 CVs are expected in the field, and about 10% of CVs are magnetic, so 1000 should be observed.

Muno et al. (2003c) Ruiter et al. (2006)


High mass x ray binaries
High Mass X-ray Binaries

  • Pfahl, Rappaport, & Podsiadlowski (2002) predict on order 100 HXMBs.

  • These will have infrared counterparts with K<17.

  • IR observations indicate up to 10% of sources (i.e., <100) could be HMXBs (e.g., Laycock et al. 2005).

1”

Diffraction-limited Keck

Image (K band).


Sources with radio counterparts
Sources with Radio Counterparts

G. Bower; Muno et al. (2006)

  • Found two X-ray sources with radio and IR matches.

  • One is a known massive, young star in an HII region.

8.4 GHz


Hmxbs or colliding winds
HMXBs or Colliding Winds?

A. Burgasser;

Muno et al. (2006)

Paschen

He I

Br g

  • IR spectra reveal lines from H and He characteristic of winds from stars with T >15,000 K, often classified as B[e], Of, and LBV stars (also, Mikles et al. 2006).

  • These are only bright in X-rays when in binaries.

IRTF/SPEX. R~130


Low-Mass X-ray Binaries

One Solar Radius

Location of

Compact Object

Image from binsim, by Rob Hynes

  • Belczynski & Taam (2004) predict hundreds of LMXBs with low mass transfer rates.

  • The disks will be unstable, producing outbursts with LX>1036 erg s-1.

  • About 5 per year should be in outburst.


Searching for x ray binaries

Sgr A*

5 pc

Searching for X-ray Binaries

  • We identified accreting black holes and neutron stars by looking for sources that:

    • varied by at least a factor of 10, and

    • had peak luminosities >1034 erg s-1.

  • We found 7 transients in 5 years of data.

Muno et al. (2005)


Searching for x ray binaries1

Sgr A*

5 pc

Searching for X-ray Binaries

Neutron Star LMXB

GRS 1741.9-2858

  • We identified accreting black holes and neutron stars by looking for sources that:

    • varied by at least a factor of 10, and

    • had peak luminosities >1034 erg s-1.

  • We found 7 transients in 5 years of data.

Muno et al. (2005)


Conclusions
Conclusions

  • The population of X-ray sources at the Galactic center is dominated by magnetic CVs, with small populations of X-ray binaries and WR/O stars.

  • X-ray transients are concentrated in the central parsec of the Galaxy.

  • There is an X-ray pulsar in the star cluster Westerlund 1.


An overabundance of transients in the central parsec
An Overabundance of Transients in the Central Parsec

  • Four lie within 1 pc of Sgr A*. The enclosed stellar mass is 2 106 Mo.

  • Three lie between 1-25 pc of Sgr A*. The enclosed stellar mass is >3 107 Mo.

  • Transients are over-abundant by >20x in the inner parsec!

Muno et al. (2005)

1 pc


1 pc

47 Tuc

LMXBs Are Also Concentrated in Globular Clusters

Keel et al.

Grindlay et al. 2001; Pooley et al. 2003

Optical: 1.5 m telescope in Chile

X-ray: Chandra

In globular clusters, LMXBs are over-abundant by a factor of 100 per unit stellar mass.


Dynamical friction
Dynamical Friction

  • Lighter objects tend to collect in the wakes of heavier ones.

  • As a result, the heavier object is slowed down.

  • The heavier object loses energy, and falls deeper into the gravitational potential.


1 pc

Dynamically Forming LMXBs

Grindlay et al. 2001; Pooley et al. 2003

47 Tuc

simulation by

E. Pfahl

In globular clusters, LMXBs are over-abundant by a factor of 100 per unit stellar mass.


Dynamically forming lmxbs

1 pc

47 Tuc

Dynamically Forming LMXBs

  • rc = 6 104 Mo pc-3

  • s = 12 km s-1

  • rc = 7 106 Mo pc-3

  • s = 70 km s-1


Dynamically forming lmxbs1
Dynamically Forming LMXBs

  • 104 black holes have dynamically settled into the central pc (Morris 1993, Miralda-Escudé & Gould 2000).

  • Pfahl & Loeb (in prep.) estimate that these form LMXBs via binary-single interactions at a rate of 10-6 yr -1.

  • Over the dynamical time scale of 1 Gyr, 103 LMXBs could form.

simulation by E. Pfahl;

Muno et al. (2005)


Young hmxbs

Sgr A*

Young HMXBs

  • Several dozen massive stars formed among 104 stars 7 Myr ago.

  • Up to 300 black holes may have already formed.

  • At most 10% of these could be in HMXBs, or on order 30 systems.

1 pc

Muno et al. (2005)

Infrared laser guide-star image

courtesy W.M. Keck Observatory.


Conclusions1
Conclusions

  • The population of X-ray sources at the Galactic center is dominated by CVs, with small populations of X-ray binaries and WR/O stars.

  • X-ray transients in the central parsec of the Galaxy formed through three-body interactions from a population of ~10,000 black holes.

  • The discovery of an X-ray pulsar in the star cluster Westerlund 1.


The unusual stellar population in the central parsec
The Unusual Stellar Population in the Central Parsec

Pulsar?

Sgr A*

IRS 13

Sgr A*

IRS 13

1 pc

(plus diffuse X-rays)

Infrared laser guide-star image (Keck Observatory; Ghez et al. 2005).


What affects on the x ray population

Sgr A*

What Affects on the X-ray Population?

  • The gravitational potential of the supermassive black hole?

  • The recent burst of star formation?

    • 22 Wolf-Rayet stars.

    • ~60 main sequence and supergiant OB stars.

    • A handful of red supergiants.

1 pc

Infrared laser guide-star image (Keck Observatory; Ghez et al. 2005).

e.g., Genzel et al. 2003, Paumard et al. 2005


The unusual stellar population in westerlund 1
The Unusual Stellar Population in Westerlund 1

  • Main sequence 06 stars.

  • Over 25 Wolf-Rayet stars.

  • Over 80 OB supergiants

  • One confirmed LBV.

  • Several red supergiants.

  • Five yellow hypergiants.

1 pc

(e.g., Westerlund 1987, Clark et al. 2005)

VRI from 2.2m MPG/ESO+WFI

Clark et al. (2005)


A galactic super star cluster
A Galactic Super Star Cluster

  • 150 stars with M>35 Msun

  • Mass: 105 Msun

  • Extent: ~6 pc across

  • Distance: 5 kpc

  • Age: 4 +/- 1 Myr

    The cluster is coeval, and old enough to have produced supernovae

1 pc

VRI from 2.2m MPG/ESO+WFI

Clark et al. (2005)


Chandra observations
Chandra Observations

1 pc

VRI from 2.2m MPG/ESO+WFI

Clark et al. (2005)

Chandra ACIS

We see diffuse X-rays from the cluster wind and unresolved pre-main-sequence stars, stellar emission from colliding wind binaries, and black holes.


Chandra observations1
Chandra Observations

pulsar

1 pc

VRI from 2.2m MPG/ESO+WFI

Clark et al. (2005)

Chandra ACIS

We see diffuse X-rays from the cluster wind and unresolved pre-main-sequence stars, stellar emission from colliding wind binaries, and a pulsar!


Pulsar cxo j164710 2 455216
Pulsar CXO J164710.2-455216

  • Period: 10.6107(1) s

  • Spin-down: <2x10-10 s s-1

  • LX = 3x1033 erg s-1 (not a radio pulsar)

  • Spectrum: kT = 0.6 keV blackbody (not a cooling NS)

  • No IR counterpart, so K>18.5 (Mcount. < 1Msun; not an X-ray binary)

This pulsar is almost certainly a magnetar.


A massive progenitor
A Massive Progenitor

pulsar

1 pc

VRI from 2.2m MPG/ESO+WFI

Clark et al. (2005)

Chandra ACIS

With 35 Msun stars still on the main sequence, only stars with initial masses >40 Msun could have exploded.


Other neutron stars with 30 m sun progenitors
Other Neutron Stars with >30 Msun Progenitors

1E 1048.1-5937

SGR 1806-20

  • A HI shell around 1E 1048.1-5937 was interpreted as the wind-blown bubble from a 30-40 Msun progenitor (Gaensler et al. 2005)

  • SGR 1806-20 is the member of a star cluster ~3 Myr old, and so had a ~50 Msun progenitor (Figer et al. 2005).


WhichStars Form Black Holes?

solar

White Dwarf

Metallicity

Heger et al. 2003

metal-free

9

25

40

100

140

260

Initial Mass (Solar Masses)


WhichStars Form Black Holes?

Wd 1

solar

White Dwarf

Metallicity

Heger et al. 2003

metal-free

9

25

40

100

140

260

Initial Mass (Solar Masses)


Massive progenitors to neutron stars
Massive Progenitors to Neutron Stars

  • These pulsars show that massive stars can lose 95% of their mass:

    • Through winds (e.g., Heger et al 2003),

    • Via binary mass transfer (Wellstein & Langer 1999),

    • Or during supernovae (Akiyama & Wheeler 2005).

  • As magnetars, B-fields appear important:

    • Massive stars could produce rapidly-rotating cores (e.g., Duncan & Thomas 1992; Heger et al. 2005).

    • Or magnetars could form from highly-magnetic progenitors (e.g., Ferrario & Wickramasinghe 2005).


Conclusions2
Conclusions

  • The population of X-ray sources at the Galactic center is dominated by CVs, with small populations of X-ray binaries and WR/O stars.

  • X-ray transients in the central parsec of the Galaxy formed through three-body interactions from a population of ~10,000 black holes.

  • The discovery of an X-ray pulsar in the star cluster Westerlund 1 implies that the progenitor to the neutron star had an initial mass of >40 Msun.


What s next
What’s Next:

  • How many high mass X-ray binaries are in the Galactic center? How about pulsars?

    • Multi-wavelength survey, including deeper Chandra observations.

  • Which stars form black holes?

    • Chandra observations of clusters being discovered using 2MASS and Spitzer/ GLIMPSE.


Why no supernova remnant
Why No Supernova Remnant?

Westerlund 1

RCW 49 (Westerlund 2)

2 pc

3.6, 4.5, and 8.0 mm:

Spitzer/GLIMPSE

(courtesy R. Indebetouw)

3.6, 4.5, 5.8 and 8.0 mm:

Spitzer/GLIMPSE

(E. Churchwell et al.)


The progenitor was 40 m sun
The Progenitor Was >40 Msun

  • The Pulsar is in Wd 1 (99.95% confidence)

    • A search of 300 archival Chandra and XMM fields reveals no new 5-30 s pulsars, so there is a <0.5% chance of finding one in any field (Nechita, Gaensler, Muno, et al. in prep).

    • The pulsar is well within the cluster, with a <10% chance of being an unrelated X-ray source.

Position of pulsar

Expected density of interlopers (dashed line, very small number)


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