A joint MeerKAT and Parkes view of Omega Centauri: New TRAPUM Searches and Pulsar Timing

Abstract

Millisecond pulsars (MSPs) are powerful probes of globular clusters (GCs), tracing stellar evolution, cluster dynamics, and the local gravitational potential. We investigate the MSP population in GC Omega Centauri. We perform Fourier-domain acceleration and jerk searches on MeerKAT observations, and carry out pulsar timing using MeerKAT and Parkes Murriyang data spanning 2021-2025. We fold Fermi LAT and NICER photons using updated radio ephemerides to search for high-energy pulsations. We discover a new isolated MSP, PSR J1326-4728S (hereafter S), with a spin period of 4.538 ms and a dispersion measure of 96.24 cm$^3$pc. We update the orbital parameters of all known binary systems, with those of I, N, and Q differing significantly from previous estimates, and obtain new timing solutions for G, H, and K. Pulsars B, G, H, K, and L exhibit black widow-like properties, I, N and Q are found in wider binaries, with N and Q having >0.2 M$_\odot$ companions, and N showing a significant orbital eccentricity (e=0.093). Significant spin period derivatives are measured for eight pulsars and interpreted as arising from the cluster gravitational potential. No pulsed high-energy emission is detected from individual pulsars. The inferred line-of-sight accelerations are consistent with a King-model gravitational potential. While our measurements are insensitive to an intermediate-mass black hole with mass 10$^3$-10$^4$ M$_\odot$, they place an upper limit of <10$^5$ M$_\odot$ at 90% confidence. The high fraction of isolated MSPs and black widows systems, and possibly the eccentricity of N, are difficult to reconcile with MSP population predictions based solely on encounter rates. Instead, these properties likely reflect the complex evolutionary history of Omega Centauri, with part of its MSP population having formed in denser environments than the one observed today.

Figures (11)

• Figure 1: prepfold diagnostic plots from 11 February 2025, showing the detection of pulsar S in the three-hour integration. Top left: integrated pulse profile. Bottom left: frequency–phase greyscale. Right: time–phase greyscale.
• Figure 2: SeeKAT localisation map of pulsar S, obtained from detections in multiple neighbouring beams.
• Figure 3: Combined MeerKAT and Murriyang timing residuals for A, B, C, D, E, G, H, and K as a function of time. A global timing jump between the two telescopes has been applied.
• Figure 4: MeerKAT timing residuals for I, L, N, and Q, plotted as a function of orbital phase (0–1). Included observations comprise all TRAPUM MeerKAT epochs, except for the S-band and 28 July 2025 UHF observations for Q, and including the additional PTUSE 14 February 2024 observation for L. Timing jumps have been applied between observations.
• Figure 5: Upper limits on LOS accelerations for pulsars against angular separation from the cluster centre. The plot also shows the maximum and minimum LOS accelerations expected from the King potential, assuming $\sigma_\mathrm{v}$ and $r_\mathrm{c}$ from baumgardt2018catalogueGCs and haberle2025dispersion, as well as the contributions from central compact masses of 6000, 8200, and $4\times10^{4}$ M$_\odot$.