Improved Mass and Radius Constraints for Quiescent Neutron Stars in Omega Cen and NGC 6397

We use Chandra and XMM observations of the globular clusters $omega$ Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favored by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS.

A Chandra Look at the X-ray Faint Millisecond Pulsars in the Globular Cluster NGC 6752

We combine new and archival Chandra observations of the globular cluster NGC 6752 to create a deeper X-ray source list, and study the faint radio millisecond pulsars (MSPs) of this cluster. We detect four of the five MSPs in NGC 6752, and present evidence for emission from the fifth. The X-rays from these MSPs are consistent with thermal emission from the neutron star surfaces, with significantly higher fitted blackbody temperatures than other globular cluster MSPs (though we cannot rule out contamination by nonthermal emission or other X-ray sources). NGC 6752 E is one of the lowest-L_X MSPs known, with L_X(0.3-8 keV)=1.0+0.9-0.5*10^30 ergs/s. We check for optical counterparts of the three isolated MSPs in the core using new HST ACS images, finding no plausible counterparts, which is consistent with their lack of binary companions. We compile measurements of L_X and spindown power for radio MSPs from the literature, including errors where feasible. We find no evidence that isolated MSPs have lower L_X than MSPs in binary systems, omitting binary MSPs showing emission from intrabinary wind shocks. We find weak evidence for an inverse correlation between the estimated temperature of the MSP X-rays and the known MSP spin period, consistent with the predicted shrinking of the MSP polar cap size with increasing spin period.

An Examination of the X-ray Sources in the Globular Cluster NGC 6652

We observed the globular cluster NGC 6652 with Chandra for 47.5 ks, detecting six known X-ray sources, as well as five previously undetected X-ray sources. Source A (XB 1832-330) is a well-known bright low-mass X-ray binary (LXMB). The second brightest source, B, has a spectrum that fits well to either a power-law model (Gamma ~ 1.3) or an absorbed hot gas emission model (kT ~ 34 keV). Its unabsorbed 0.5-10 keV luminosity (L_X = 1.6+-0.1*10^34 erg/s) is suggestive of a neutron star primary; however, Source B exhibits unusual variability for a LMXB, varying by over an order of magnitude on timescales of ~ 100 s. Source Cs spectrum contains a strong low-energy component below 1 keV. Its spectrum is well fit to a simplified magnetic cataclysmic variable (CV) model, thus the soft component may be explained by a hot polar cap of a magnetic CV. Source D has an average L_X (0.5-10 keV) ~ 9*10^32 erg/s, and its spectrum is well fit to a neutron star atmosphere model. This is indicative of a quiescent neutron star LXMB, suggesting Source D may be the third known LMXB in NGC 6652. Source E has L_X (0.5-10 keV) ~ 3*10^32 erg/s, while Source F has L_X (0.5-10 keV) ~ 1*10^32 erg/s. Their relatively hard X-ray spectra are well-fit by power-law or plasma emission models. Five newly detected fainter sources have luminosities between 1-5*10^31 erg/s. NGC 6652 has an unusually flat X-ray luminosity function compared to other globular clusters, which may be connected to its extremely high central density.

Chandra X-Ray Observatory Observations of the Globular Cluster M71

We observed the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-ray Observatory to study its faint X-ray populations. Five X-ray sources were found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A. The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2 h. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources were found within the half-mass radius, reaching to a limiting luminosity of 1.5 10^30 erg/s (0.3-8 keV). From a radial distribution analysis, we find that 18+/-6 of these 29 sources are associated with M71, somewhat more than predicted, and that 11+/-6 are background sources, both galactic and extragalactic. M71 appears to have more X-ray sources between L_X=10^30--10^31 erg/s than expected by extrapolating from other studied clusters using either mass or collision frequency. We explore the spectra and variability of these sources, and describe the results of ground-based optical counterpart searches.