Alessandro Ridolfi

Context . Globular clusters are efficient factories of recycled pulsars, but searches towards high dispersion-measure (DM) clusters can be strongly limited around 1 GHz – which is where most pulsar surveys are typically conducted – due to dispersive smearing and interstellar scattering. Observations at higher radio frequencies provide a complementary route to mitigate these effects and improve sensitivity. Aims . We present the first results from a MeerKAT S-band ( ν ≃ 2.4 GHz) pulsar survey of 14 globular clusters. Our aims were to discover new pulsars, re-detect known pulsars with a uniform high-frequency dataset, and assess practical S-band sensitivity and DM-search requirements for future globular cluster pulsar surveys. Methods . High time and frequency resolution S-band observations were searched in the Fourier domain using segmented acceleration and jerk techniques to maintain sensitivity to pulsars in compact binaries, followed by candidate identification and folding. For selected detections and non-detections, we additionally folded the S-band data together with archival MeerKAT L-band observations using available timing ephemerides to derive flux-density measurements and spectral-index constraints. Results . We re-detected 39 previously known pulsars across 14 clusters and discovered four new millisecond pulsars in Glimpse-C01 (J1848–0129C, D, E, and F). Multi-epoch follow-up enabled preliminary Keplerian orbital fits for two of the new systems based on their observed barycentric spin-period modulation. J1848–0129C is an eclipsing millisecond pulsar with P _b ∼ 5 d, placing it among the long-period eclipsing systems known as ‘huntsman’ binaries. J1848–0129D is in a P _b ∼ 3.4 d nearly circular orbit with a ∼1 M _⊙ massive white dwarf companion. Two Glimpse-C01 pulsars exhibit substantial DM offsets relative to the average cluster value. Comparison with other Galactic globular clusters showed that large intracluster DM spreads become increasingly common at high foreground DMs, implying that searches confined to narrow DM windows may be sub-optimal for high-DM clusters. From detections and non-detections, we inferred a practical single-epoch S-band tied-array detectability scale of ∼10–20 μJy, which serves as an operational benchmark rather than a strict completeness limit. Conclusions . MeerKAT S-band observations are highly effective for probing globular clusters affected by strong dispersion and scattering, enabling both reliable re-detections and the discovery of new millisecond pulsars in high-DM environments such as Glimpse-C01. The results of our work highlight the importance of high-frequency surveys for expanding the known pulsar population in obscured clusters.

We report on the identification of the optical counterpart to the recently discovered millisecond pulsar (MSP) NGC 362D in the Galactic globular cluster NGC 362 based on deep, multi-band, and multi-epoch Hubble Space Telescope observations. Our analysis robustly shows that this object is a very low-mass (∼0.18 M _⊙ ) He white dwarf (WD) still in the pre-cooling phase. Interestingly, a detailed comparison with updated binary evolution models indicates that this object completed the mass-transfer phase only recently (∼0.6 Gyr ago), thus strongly suggesting that COM-NGC 362D is the youngest WD companion to a MSP identified to date. Remarkably, in this respect, the photometric properties of NGC 362D show, for the first time in this class of objects, significant wavelength-dependent variations that are consistent with the presence of residual circumstellar material. This system therefore provides a valuable test case to directly probe the immediate aftermath of the MSP recycling process and to constrain the early evolutionary stages of proto-WD companions and their radio and optical properties. Our results suggest that residual material can produce radio and optical signatures that mimic those of systems with non-degenerate companions, potentially leading to the misclassification of young MSPs.

Binary millisecond pulsars (MSPs) in globular clusters (GCs) are key for binary and stellar evolution studies under extreme conditions. The identification of their optical companion stars is instrumental in order to characterise these systems and to constrain the possible recycling mechanisms. For this work, we searched for the optical counterpart to PSR J1717+4308A (hereafter M92A) in the GC M92. To this end, we exploited a multi-epoch, multi-wavelength dataset obtained with the Hubble Space Telescope. We constructed colour–magnitude diagrams, investigated proper motions to assess cluster membership, and modelled the observed light curves. We identified an object located at only 0.02 arcsec from the nominal radio position as the likely optical companion to M92A. The star is significantly bluer than the main sequence at the same luminosity level and exhibits clear photometric variability with a periodicity in agreement with the orbital motion of the binary. The light curve displays two maxima and two minima, indicative of strong tidal distortion and only mild irradiation. Such mild irradiation is consistent with the ratio of the pulsar spin-down to the companion flux ( f _sd ), which for M92A lies close to the boundary between ellipsoidal- and irradiation-dominated regimes ( f _sd ≈ 2.71). From the light curve modelling we inferred the main physical properties of the companion star. The best-fit model indicates a high-inclination system with a relatively low-mass companion and a massive neutron star. With a base temperature of ~7200 K, the companion ranks among the hottest redbacks known to date. This object therefore adds additional pieces to the puzzle of MSP companion properties and contributes to outlining the characteristics of redbacks across the different classes.

Context. 47 Tucanae is one of the largest, brightest, and closest globular clusters to Earth. It hosts an exotic stellar population with stellar dynamics that indicate a complex evolution history. The cluster contains a large number of X-ray binaries and millisecond pulsars. Aims. Searching for pulsars in this cluster has been an ongoing effort over more than two decades with fruitful results. However, given its large distance relative to the known pulsar population, previous surveys have found only the very brightest sources. Therefore, surveys with increased sensitivity should find many additional pulsars. Increasing the number of pulsars is crucial to investigate the dynamics of this globular cluster and could also lead to the discovery of unusual types of system. Methods. With a significantly increased sensitivity compared to earlier telescopes, MeerKAT is the natural choice to perform new surveys. We carried out two campaigns with different observational cadences to account for the high scintillation along the line of sight to this cluster. All observations were carried out using multi-beam filter-banking user-supplied equipment to increase efficiency and localisation. Results. Here we report the discovery of fifteen new pulsars in 47 Tucanae with MeerKAT. These discoveries bring the total number of known pulsars in this globular cluster to 42, and the MeerKAT discoveries in this cluster to 17. We discuss some of their characteristics, which include preliminary localisations and estimates of orbits for most systems. Highlights include the discovery of 47 Tuc af, a ‘black widow’ pulsar with a short orbital period that was identified optically in 2002 as a candidate binary pulsar, and 47 Tuc ai, an eccentric binary pulsar with a massive companion, a unique system in 47 Tuc to date. Apart from the new systems, we also re-detect and localise 47 Tuc P and V, two elusive, seldom-detected systems that had no precise localisation from a phase-connected timing solution. The localisation of 47 Tuc V places it in a position consistent with a continuum source detected earlier in MeerKAT imaging data.

In this paper we present PSR J0024−7204ai, a 13.026-ms binary pulsar recently discovered in the globular cluster 47 Tucanae by the MeerKAT radio telescope. This is the slowest-spinning pulsar known in this globular cluster. It has a ∼1.67-day orbit with an eccentricity of e ≈ 0.18. Although it is not yet possible to derive an unambiguous phase-connected timing solution, by combining detections obtained from MeerKAT and archival Parkes data, we were able to measure the rate of advance of periastron to a high significance: ω ˙ = 0.1601 ± 0.0046 deg yr ⁻¹ . This value implies a total system mass of 2.41 ± 0.11 M _⊙ (68.3% C.L.), which, when combined with the binary mass function, gives a maximum pulsar mass of ∼1.7 M _⊙ and a minimum companion mass of ∼0.7 M _⊙ . Apart from being the slowest pulsar in 47 Tucanae, its orbit is by far the most eccentric, and its companion is the most massive among all known binary pulsars in this globular cluster. It is possible that the system is an old millisecond pulsar–carbon-oxygen white dwarf binary whose orbit was perturbed by stellar dynamical interactions in the cluster core. Further follow-up observations of this system will be essential to obtaining a more detailed characterisation of this system and its evolution.

Aims . The mysterious Galactic center (GC) gamma-ray ( γ -ray) excess (GCE) could be explained by a large population of millisecond pulsars (MSPs) hiding in the Galactic bulge, too faint to be detected as individual high-energy point sources by the Fermi Large Area Telescope and too fast and dispersed to be detected in shallow radio pulsation surveys. Methods . Motivated by an innovative candidate selection method, we aim to detect millisecond pulsars associated with the GCE by carrying out deep radio pulsation searches toward promising candidates detected in the inner Galaxy in X rays by Chandra and in radio or γ rays by the Very Large Array or Fermi . Results . We conducted deep radio observation and follow-up campaigns with MeerKAT, as well as the Murriyang and Green Bank telescopes toward nine X-ray candidate sources. Conclusions . We report the detection of two new MSPs, including a black widow candidate, toward the Galactic bulge: PSRs J1740-2805 and J1740-28. These discoveries double the number of MSPs discovered within the innermost 2° from the GC.

We report the discovery of 12 new radio pulsars in the Large Magellanic Cloud (LMC) as part of the TRAPUM (TRAnsients and PUlsars with MeerKAT) Large Survey Project, using the MeerKAT L-band receivers (856–1712 MHz). These pulsars, discovered in 18 new pointings with 2 h integration times, bring the total number of pulsars identified by this ongoing survey to 19 (yielding a total of 44 LMC radio pulsars now known), representing an 80 per cent increase in the LMC radio pulsar population to date. These include PSR J0454–6927, the slowest extragalactic radio pulsar discovered to date, with a spin period of 2238 ms, and PSR J0452–6921, which exhibits the highest dispersion measure (DM) among extragalactic radio pulsars, at 326 pc cm$^{-3}$. The fastest spin period among the new discoveries is 245 ms, and the lowest DM is 62 pc cm$^{-3}$. We also present timing results for our first pulsar discoveries with MeerKAT and the Murriyang radio telescope, obtaining phase-connected solutions for seven pulsars in the LMC. These results indicate that the pulsars are isolated, canonical radio pulsars with characteristic ages up to 8.8 Myr.