Neutron stars in globular clusters : XMM-Newton results

1. Neutron stars in globular clusters : XMM-Newton results Bruce Gendre, Didier Barret, Natalie A. Webb Centre d’Etude Spatiale des Rayonnements, Toulouse, France Contact : bruce.gendre@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 ω 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 : ρ1.5rc2 Successive encounter rate from 12 : ρ0.5rc-1 Fig. 2 : Spectrum of the qNS candidate, presented with a neutron star atmosphere model fit (TNSA= 67 ± 2 eV, χ2ν=1.00, 32 d. o. f.) Fig. 3 : Number of qNS LMXB within a cluster versus his collision rate (ρ1.5rc2). LX(0.1-5.0keV) = (3.2 ± 0.2) 1032erg.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 (ρ0.5rc-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, χ2ν=0.55, 15 d. o. f.) CONCLUSIONS. We have detected a qNS LMXB in both ω Cen and M13.  qNS LMXBs can form even within globular clusters with low collision rates LX(0.1-5.0keV) = (7.3 ± 0.6) 1032erg.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 References 1 : Becker W., Trümper J., 1998, A&A, 341, 803 2 : Becker W., et al., 2003, ApJ, in press 3 : Gendre B., Barret D., Webb N. A., 2003, A&A in press 4 : Gendre B., Barret D., Webb N. A., 2003, A&A submitted 5 : Grindlay J. E., et al., 2001, Sci., 292, L45 6 : Grindlay J. E., et al., 2002, ApJ, 581, 470 7 : Narayan R., et al., 2002, in press (astro-ph 0107387) 8 : Pooley D., et al., 2002, ApJ, 569, 405 9 : Pooley D., et al., 2002, ApJ, 573, 184 10 : Rutledge R.F., et al, 2001, ApJ, 559, 1054 11 : Rutledge R.F., et al, 2002, ApJ, 578, 405 12 : Verbunt F., 2002, to appears in ASP Conf. Ser. (astro-ph 0210057) 13 : Webb N.A., Gendre B., Barret D., 2001, A&A, 381, 481 14 : White N.E., Angelini L., 2001, ApJ, 561, L101