Fifteen new millisecond pulsars in 47 Tucanae

Technical Summary: Fifteen new millisecond pulsars in 47 Tucanae

Problem and Context
The globular cluster 47 Tucanae (47 Tuc) is a dense stellar environment with a high rate of stellar interactions, leading to the formation of exotic binary systems, including low-mass X-ray binaries and millisecond pulsars (MSPs). Prior to this work, 27 pulsars were known in the cluster, primarily discovered using the Murriyang (Parkes) 64-m radio telescope. However, the cluster's distance and the strong interstellar scintillation along the line of sight have historically limited surveys to detecting only the brightest sources. The high stellar density in 47 Tuc suggests that "secondary exchange encounters" could produce unusual systems, such as MSPs with massive companions or eccentric orbits, which are crucial for testing fundamental physics and stellar evolution theories. Previous surveys lacked the sensitivity to detect fainter, sporadically visible pulsars or to precisely localise elusive systems lacking phase-connected timing solutions.

Methodology
The authors utilized the MeerKAT radio telescope, which offers an order-of-magnitude increase in sensitivity over previous instruments, to conduct targeted surveys of 47 Tuc. The study employed two distinct observational campaigns to mitigate the effects of strong interstellar scintillation:

1. First Campaign (May 2020 – Feb 2021): Ten observations spanning L-band (856–1712 MHz) and UHF-band (544–1088 MHz) with varying durations (0.8 to 4 hours).
2. Second Campaign (Jan 2022): A single, exceptionally long observation (>19 hours) split into multiple segments, conducted exclusively in the UHF band to maximize integration time.

Data were processed using the Filter-Banking User-Supplied Equipment (FBFUSE) backend, which generated coherently synthesized beams (CBs) covering the cluster. The search pipeline utilized PULSAR_MINER and PRESTO tools for Radio Frequency Interference (RFI) cleaning, DM trials (23.5–25.5 pc cm⁻³), and periodicity searches (including acceleration searches). To address the lack of precise positions for new or elusive sources, the authors employed the SeeKAT software, which uses multi-beam signal-to-noise ratios and the synthesized beam point spread function to derive precise localisations without requiring additional dedicated observations.

Key Contributions and Results
The paper reports the discovery of 15 new pulsars in 47 Tuc, bringing the total known population in the cluster to 42. Of these 15 new systems:

• 3 are isolated pulsars: 47 Tuc aj, ao, and ap. Notably, 47 Tuc ao has a short spin period (1.88 ms) and a dispersion measure (DM) at the lower edge of the search range (23.64 pc cm⁻³).
• 12 are binary pulsars: These include systems with varying characteristics:
  • 47 Tuc af: A "black widow" or "redback" candidate with a very short orbital period (1.62 h) and a companion mass lower limit of 0.0679 M⊙. It exhibits distinct pulse profile differences between L and UHF bands and shows eclipses.
  • 47 Tuc ai: A unique system with a slow spin period (13.03 ms), a massive companion (minimum mass 0.628 M⊙), and a significant orbital eccentricity (e = 0.18). This is the first eccentric binary with a massive companion found in 47 Tuc.
  • 47 Tuc al: A black widow system with an unusually low-mass companion (0.0091 M⊙).
  • 47 Tuc ae, ag, ah, ak, am, an, aq, ar, as: Various other binary systems, some with poorly constrained orbits due to sporadic detections.

Re-detections and Localisations
The increased sensitivity and multi-beam techniques allowed for the re-detection and precise localisation of four previously known but elusive pulsars that lacked phase-connected timing solutions:

• 47 Tuc P and V: Localised for the first time with arcsecond precision. The position of 47 Tuc V aligns with a continuum source (J002402.7−720539.4) previously identified in MeerKAT imaging data by Heywood (2023).
• 47 Tuc ac and ad: Previously discovered by TRAPUM, these systems were re-detected with improved localisation constraints.

The study also re-analyzed archival Parkes data (1997–2013), successfully re-detecting several of the new pulsars (e.g., 13 detections of 47 Tuc al), confirming their existence and aiding in orbital characterization.

Significance
The paper asserts that these discoveries represent the first significant increase in the known pulsar population of 47 Tuc since the late 1990s. The new binary fraction in the cluster has risen to approximately 69%. The findings highlight the efficacy of MeerKAT's sensitivity in overcoming scintillation limitations to reveal faint, sporadic sources.

The authors emphasize that the discovery of 47 Tuc ai is particularly significant as it breaks the trend of low-mass, low-eccentricity binaries typical of 47 Tuc, suggesting a system formed through a rare exchange encounter. The localisation of 47 Tuc af provides a candidate for the optical source W34opt, identified in 2002, though the authors note a small positional offset (approx. 5 arcseconds) that requires a phase-connected timing solution to confirm the association. Similarly, the localisation of 47 Tuc V confirms its identity as the variable source seen in imaging data.

The paper concludes that while strong scintillation continues to make detections sporadic, the combination of UHF band observations, long integrations, and advanced localisation techniques provides a robust framework for future characterisation. These results lay the groundwork for determining precise orbits, measuring proper motions, and probing the gravitational field and ionised gas content of the globular cluster.