A census of X-ray millisecond pulsars in globular clusters

Abstract

ABSTRACT We present a comprehensive census of X-ray millisecond pulsars (MSPs) in 29 Galactic globular clusters (GCs), including 68 MSPs with confirmed X-ray luminosities and 107 MSPs with X-ray upper limits. We compile previous X-ray studies of GC MSPs, and add new analyses of six MSPs (PSRs J1326–4728A, J1326–4728B, J1518+0204C, J1717+4308A, J1737–0314A, and J1807–2459A) discovered in five GCs. Their X-ray spectra are well described by a single blackbody model, a single power-law model, or a combination of them, with X-ray luminosities ranging from 1.9 × 1030to 8.3 × 1031 erg s−1. We find that most detected X-ray MSPs have luminosities between ∼1030 and 3 × 1031 erg s−1. Redback pulsars are a relatively bright MSP population with X-ray luminosities of ∼2 × 1031–3 × 1032 erg s−1. Black widows show a bimodal distribution in X-ray luminosities, with eclipsing black widows between ∼7 × 1030 and 2 × 1031 erg s−1, while the two confirmed non-eclipsing black widows are much fainter, with LX of 1.5–3 × 1030 erg s−1, suggesting an intrinsic difference in the populations. We estimate the total number of MSPs in 36 GCs by considering the correlation between the number of MSPs and stellar encounter rate in GCs, and suggest that between 600 and 1500 MSPs exist in these 36 GCs. Finally, we estimate the number of X-ray-detectable MSPs in the Galactic bulge, finding that 1–86 MSPs with LX > 1033 erg s−1, and 20–900 MSPs with LX > 1032 erg s−1 should be detectable there.