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 ($\omega$ 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 9 MSPs are well described by a neutron star hydrogen atmosphere model, while 2 MSPs are well fitted by a power-law model. The identified MSPs have X-ray luminosities ranging from $1.0\times 10^{30}$ erg s${}^{-1}$ to $1.4\times 10^{31}$ 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 $1.9\times 10^{30}$ 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-fit functions of ${\log }_{10}{L}_X=(1.0\pm 0.1){\log }_{10}{M}_{c,min}+(32.5\pm 0.2)$ and ${\log }_{10}{L}_X=(0.35\pm 0.04){\log }_{10}\mathrm{MF}+(32.71\pm 0.20)$, respectively, with an intrinsic scatter of ${\log }_{10}{L}_X$ of $\sim$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.

18 pages, 8 figures, 8 tables; accepted for publication in MNRAS