1. Investigating the Faint X-ray Sources in
Globular Clusters with XMM-Newton
Natalie Webb
Centre d’Etude Spatiale
des Rayonnements,
Toulouse
D. Barret, B. Gendre

2. Globular clusters - Dense group of old stars
- Stable on dynamical timescales
- Unstable on thermal timescales
- Binaries may be responsible for delaying core collapse
- Binaries are difficult to detect because of high stellar density
M 22 - optical
M 22 - X-rays

3. Two types of X-ray source :
Bright X-ray sources (Lx > 1036 ergs s-1) – X-ray binaries
Faint X-ray sources (Lx < ergs s-1)
- X-ray binaries
- cataclysmic variables
- active binaries
- back- and foreground sources
- millisecond pulsars

4. Globular clusters:
- large cores
- close
- low absorption
Observations with
XMM-Newton
MOS + PN imaging
 Centauri
M 22
NGC 6366
M 13

5. RS CVn
 Centauri
CV, bremsstrahlung fit
kT=17.3±6.2 keV
2=0.84 (30 dof)
Lx=4.7±0.2x1032 erg s-1
( keV)

6. M 22 =2.03±0.21 Gaussian, centre=1.0 keV L=8.1x1031ergs s-1
2=1.08, 36
 Pulsar like 1E
MEKAL, z=0.10±0.02
kT=2.25±0.21 keV
Abundance=0.44±0.18 solar
Lbol=5.4x1042ergs s-1
R= Mpc

7. M 13 T = 76±3 eV R = 12.8±0.4 km With a mass of 1.4Msolar

8. Do we expect so many Neutron Star Low Mass X-ray Binaries?
Theory predicts that the number of NSLMXBs scales with the
collision rate (Verbunt 2002)
XMM-Newton and Chandra observations of globular clusters
are deep enough to observe all NSLMXBs
NSLMXBs easily identified by VERY soft spectrum and well
fitted by hydrogen atmosphere model

9. Number of
neutron star
X-ray
binaries
Normalised collision rate

10. Results and Conclusions
Faint X-ray sources are many different types of binaries (and
fore-/background objects)
Confirmed Neutron Star Low Mass X-ray Binary (NSLMXB)
in  Centauri + detected a new one in M 13
Observations support that NSLMXBs are formed via collisions
NSLMXBs in globular clusters are excellent objects to use to
eventually constrain the NS equation of state