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

Abstract

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.