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 (2309.13189v1)

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 $1.0\times10^{30}$ erg s$^{-1}$ to $1.4\times10^{31}$ 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 $1.9\times10^{30}$ 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 $\log_{10}{L_X} = (1.0\pm0.1) \log_{10}{M_{c, min}} + (32.5\pm0.2)$ and $\log_{10}{L_X} = (0.35\pm0.04) \log_{10}{\rm MF} + (32.71\pm0.20)$, 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.

• The paper demonstrates that deep Chandra observations of 18 MSPs in Omega Centauri uncover a clear correlation between companion mass and X-ray luminosity.
• It finds that 11 MSPs have X-ray counterparts, with most spectra fitting a neutron star hydrogen atmosphere model and two requiring a power-law fit.
• The derived empirical relationships suggest that MSP binary system parameters can reliably predict X-ray emissions, guiding future research directions.

X-ray Study of Millisecond Pulsars in Omega Centauri

In the paper titled "A X-ray study of millisecond pulsars in the globular cluster Omega Centauri: a correlation between spider pulsar companion mass and X-ray luminosity," Jiaqi Zhao and Craig O. Heinke present a detailed X-ray analysis of recently discovered millisecond pulsars (MSPs) in Omega Centauri. Using deep X-ray observations, the authors illuminate the characteristics and correlations of this MSP population, offering insights into their X-ray emission properties and companion star dynamics.

The authors utilized Chandra X-ray Observatory data with a total exposure time of 290.9 kiloseconds to investigate 18 MSPs identified in the globular cluster Omega Centauri. They found X-ray counterparts for 11 of these MSPs, with nine being new identifications. Notably, the analysis is augmented by profiling the MSP population across other globular clusters for comparative purposes. The X-ray emission of the MSPs is characterized, with most spectra well-suited to a neutron star hydrogen atmosphere model, while two MSPs required a power-law fit. The detected X-ray luminosities span from $1.0 \times 10^{30}$ erg/s to $1.4 \times 10^{31}$ erg/s.

A significant contribution of the paper is the investigation of correlations between X-ray luminosity and companion star masses for so-called spider pulsars, which include both redbacks and black widows. The authors derive empirical relationships suggesting that Y-ray luminosity increases with companion mass. Specifically, they find $$\log_{10}L_X = (1.0\pm0.1) \log_{10}M_{c, min} + (32.5\pm0.2)$$ and $$\log_{10}L_X = (0.35\pm0.04) \log_{10}{\rm MF} + (32.71\pm0.20)$$, with intrinsic scatter around $\log_{10}{L_X}$ of approximately 0.3. Here, $L_X$ represents the X-ray luminosity, $M_{c, min}$ the minimum companion mass, and MF the mass function.

The findings from Omega Centauri are placed in the context of prior studies with MSP populations in other globular clusters. This comparative approach not only reinforces the identified correlations but also suggests promising avenues for future research. The correlations between companion masses and X-ray brightness indicate potential for predicting the X-ray luminosity of MSPs from binary system parameters, offering a tool for exploring MSP demographics and behavior within dense stellar environments. The implications of such correlations are significant, suggesting alterations in binary system properties may be traced through X-ray observations, revealing evolutionary aspects of binary MSP systems.

For future research prospects, addressing variability in inclinations and other orbital parameters are anticipated to refine the current models. Further, with enhanced sensitivity from next-generation observatories, deeper insights into the emissions from faint companion-dominated systems could augment understanding of intrabinary shock processes. Additionally, novel detections of isolated systems might expand context for understanding the role dynamics play post binary disruption in cluster environments. While this study firmly establishes correlations, leveraging advanced modeling could further elucidate underlying physical processes influencing these emission properties.

In summary, the paper makes a substantive contribution to the field of MSP research, particularly in globular environment contexts, and highlights potential predictive frameworks to anticipate X-ray emissions based on binary characteristics. Such insights can influence both practical observational programs and theoretical constructs about MSP evolution and interactions within their environments.