A Chandra look at the X-ray faint millisecond pulsars in the globular cluster NGC 6752

• Published in: Monthly notices of the Royal Astronomical Society 2014-06, Vol.441 (1), p.757-768
• Main Authors: Forestell, L. M., Heinke, C. O., Cohn, H. N., Lugger, P. M., Sivakoff, G. R., Bogdanov, S., Cool, A. M., Anderson, J.
• Format: Article
• Language: English
• Subjects: Pulsars; Radio; Space telescopes; Temperature; X-ray astronomy
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stu559

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 non-thermal 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) = $1.0^{+0.9}_{-0.5}\times 10^{30}$  erg s−1. We check for optical counterparts of the three isolated MSPs in the core using new Hubble Space Telescope Advanced Camera for Surveys images, finding no plausible counterparts, which is consistent with their lack of binary companions. We compile measurements of L X and spin-down 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.