A Chandra X-ray study of millisecond pulsars in the globular cluster Omega Centauri: a correlation between spider pulsar companion mass and X-ray luminosity

Abstract

Millisecond pulsars (MSPs) are faint X-ray sources commonly observed in Galactic globular clusters (GCs). In this work, we investigate 18 MSPs newly found in the GC Omega Centauri (ω Cen) and search for their X-ray counterparts using Chandra observations with a total exposure time of 290.9 ks. We identify confident X-ray counterparts for 11 of the MSPs, with 9 of them newly identified in this work based on their positions, spectral properties, and X-ray colours. The X-ray spectra of nine MSPs are well described by a neutron star hydrogen atmosphere model, while two MSPs are well fitted by a power-law model. The identified MSPs have X-ray luminosities ranging from 1.0 × 10³⁰ to 1.4 × 10³¹ erg s^-1. Additionally, for population comparison purposes, we study the X-ray counterpart to MSP E in the GC M71, and find its X-ray spectrum is well described by blackbody-like models with a luminosity of 1.9 × 10³⁰ erg s^-1. We investigate the empirical correlations between X-ray luminosities and minimum companion masses, as well as mass functions, of spider pulsars. Clear correlations are observed, with best-fitting functions of log₁₀L_X = (1.0 ± 0.1)log₁₀M_{c, min} + (32.5 ± 0.2) and (0.35 ± 0.04)log₁₀MF + (32.71 ± 0.20), respectively, with an intrinsic scatter of log₁₀L_X of ~0.3, where L_X is the 0.5-10 keV X-ray luminosity, M_{c, min} is the minimum companion mass, and MF represents the mass function, in solar masses.