Prsentation PowerPoint

1
Neutron stars in globular clusters XMM-Newton
results
Bruce Gendre, Didier Barret, Natalie A.
Webb Centre dEtude Spatiale des Rayonnements,
Toulouse, France
Contact bruce.gendre_at_cesr.fr
Abstract We present results from an XMM-Newton
survey of galactic globular clusters. We have so
far observed four globular clusters Omega
Centauri, M13, M22, and NGC 6366. We have
detected scores of sources in each field of view,
and many sources within each field of view can be
associated with the respective globular cluster.
With the spectroscopic abilities of XMM-Newton,
we can classify these sources into four
categories of objects. In this work, we present
the sources that we have classified as neutron
star binaries. We have detected one low mass
X-ray binary in quiescence with a neutron star
primary within both M13 and ? Cen. We have not
detected such objects in M22 or NGC 6366. We
discuss the number of low mass X-ray binaries
with neutron star primaries in globular clusters
and the number of detected neutron stars in the
light of our observations and recent Chandra
results.
Introduction This work is part of a project to
constrain the nature of low luminosity X-ray
sources in GCs from spectral studies. A low mass
X-ray binary with a quiescent neutron star
primary (qNS LMXB) has a soft spectrum10 and a
luminosity of 1032-1033 erg s-1.7 A millisecond
pulsar (MSP) has a luminosity of 1030-1031 erg
s-1.1, 6 With the luminosity limits we reached,
we cannot detect all MSPs but we can obtain the
total number of qNS LMXBs. The MSP and qNS LMXB
numbers are expected to scale with the stellar
collision and/or encounter rates. We have tested
this hypothesis.
Neutron star number relationships
The qNS LMXB within Omega Centauri the spectrum

Collision rate from 12 ?1.5rc2
Successive encounter rate from 12 ?0.5rc-1
Fig. 2 Spectrum of the qNS candidate, presented
with a neutron star atmosphere model fit (TNSA
67 2 eV, ?2?1.00, 32 d. o. f.)
Fig. 3 Number of qNS LMXB within a cluster
versus his collision rate (?1.5rc2).
LX(0.1-5.0keV) (3.2 0.2) 1032 erg.s-1 Neutron
star radius R 13.6 0.3 km
Good correlation between the qNS LMXB number and
the collision rate

• M13 discovery of a new qNS LMXB
• Luminosity limit of 2.6 x 1031 erg s-1 4
• One qNS LMXB detected within the core
• No other qNS LMXB detected within the field of
  view

• M22
• Luminosity limit of 8.6 x 1030 erg s-1 13
• No qNS LMXB detected within the field of view

Fig. 4 Number of neutron stars within a cluster
versus his successive encounter rate (?0.5rc-1).

• NGC 6366
• Luminosity limit of 8.3 x 1030 erg s-1
• No qNS LMXB detected within the field of view

Summary of qNS LMXB and MSP number detected in
GC
Large dispersion of point observed at low
encounter rates
Fig. 1 Spectrum of the qNS candidate, presented
with a neutron star atmosphere model fit (TNSA
76 3 eV, ?2?0.55, 15 d. o. f.)
CONCLUSIONS.
We have detected a qNS LMXB in both ? Cen and
M13. ? qNS LMXBs can form even within globular
clusters with low collision rates
LX(0.1-5.0keV) (7.3 0.6) 1032 erg.s-1 Neutron
star radius R 12.8 0.4 km
There is a relationship between the qNS LMXB
number and the collision rate The number of
detected neutron stars in GC might be related to
the encounter rate, but the statistic is very low

• Omega Centauri confirmation of the existence of
  a qNS LMXB
• Luminosity limit of 1.3 x 1031 erg s-1 3
• One qNS LMXB detected at the edge of the half
  mass radius
• No other qNS LMXB detected within the field of
  view

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