The High Time Resolution Universe Pulsar Survey IV: Discovery and polarimetry of millisecond pulsars

We present the discovery of six millisecond pulsars (MSPs) in the High Time Resolution Universe (HTRU) survey for pulsars and fast transients carried out with the Parkes radio telescope. All six are in binary systems with approximately circular orbits and are likely to have white dwarf companions. PSR J1017-7156 has a high flux density and a narrow pulse width, making it ideal for precision timing experiments. PSRs J1446-4701 and J1125-5825 are coincident with gamma-ray sources, and folding the high-energy photons with the radio timing ephemeris shows evidence of pulsed gamma-ray emission. PSR J1502-6752 has a spin period of 26.7 ms, and its low period derivative implies that it is a recycled pulsar. The orbital parameters indicate it has a very low mass function, and therefore a companion mass much lower than usually expected for such a mildly recycled pulsar. In addition we present polarisation profiles for all 12 MSPs discovered in the HTRU survey to date. Similar to previous observations of MSPs, we find that many have large widths and a wide range of linear and circular polarisation fractions. Their polarisation profiles can be highly complex, and although the observed position angles often do not obey the rotating vector model, we present several examples of those that do. We speculate that the emission heights of MSPs are a substantial fraction of the light cylinder radius in order to explain broad emission profiles, which then naturally leads to a large number of cases where emission from both poles is observed.

💡 Research Summary

The paper reports the discovery of six new millisecond pulsars (MSPs) identified in the High Time Resolution Universe (HTRU) survey conducted with the Parkes radio telescope, and presents polarimetric observations for all twelve MSPs discovered by the survey to date. All six newly found pulsars are members of binary systems with nearly circular orbits, indicating a recycling history through mass transfer from low‑mass companions, most likely white dwarfs.

Key timing results are derived using the “ELL1” binary model, which is appropriate for low‑eccentricity systems. The measured spin periods range from 2.06 ms to 26.7 ms, and orbital periods span from 0.28 days (≈ 6.6 h) to 6.5 days. Minimum companion masses, inferred from the mass function assuming a 1.4 M⊙ neutron star, lie between 0.019 M⊙ and 0.78 M⊙, placing the objects into three conventional categories: low‑mass binary pulsars (LMBPs), intermediate‑mass binary pulsars (IMBPs), and ultra‑low‑mass “black‑widow”‑type systems.

One of the discoveries, PSR J1017‑7156, stands out because of its high flux density (≈ 0.9 mJy at 1369 MHz) and an exceptionally narrow pulse width (≈ 0.16 ms). These properties yield timing residuals as low as 0.8 µs, making the source an excellent candidate for high‑precision timing experiments such as pulsar‑timing arrays (PTAs) aimed at detecting nanohertz gravitational waves.

Two of the MSPs, PSR J1446‑4701 and PSR J1125‑5825, are spatially coincident with unidentified Fermi‑LAT gamma‑ray sources. By folding the LAT photon data with the radio timing ephemerides, the authors detect pulsed gamma‑ray emission, confirming these objects as gamma‑ray pulsars. Their spin‑down power (Ė) and distance‑scaled figure of merit (Ė¹ᐟ²/d²) exceed the empirical threshold for LAT detection, reinforcing the link between high spin‑down luminosity and gamma‑ray visibility in MSPs.

The polarimetric component of the work provides high‑quality Stokes I, Q, U, and V profiles for all twelve MSPs, obtained with the BPSR and PDFB3 back‑ends (340 MHz and 256 MHz bandwidths, respectively). The authors find that MSPs generally exhibit very broad emission windows, often covering more than half of the rotational phase, and display a wide range of linear polarisation fractions (∼1 % to >50 %). Circular polarisation is also detected in many cases, sometimes changing sign across the pulse.

Position angle (PA) swings frequently deviate from the simple rotating vector model (RVM) that successfully describes many young pulsars. Nevertheless, for several objects (e.g., J1017‑7156, J1337‑6423) the PA evolution can be fitted with an RVM, allowing estimates of the magnetic inclination (α) and impact angle (β). The authors argue that the frequent RVM failures are a natural consequence of emission occurring at a substantial fraction of the light‑cylinder radius (10 %–30 % of R_LC). At such heights, emission from both magnetic poles can be visible within a single rotation, producing the observed wide profiles and complex PA behaviour.

Timing analysis also accounts for kinematic contributions to the observed spin‑frequency derivative (˙ν), notably the Shklovskii effect arising from transverse motion and the Galactic acceleration term. For PSR J1017‑7156, the Shklovskii correction may account for up to 30 % of the measured ˙ν, highlighting the importance of proper motion measurements for accurate intrinsic spin‑down estimates.

Overall, the study demonstrates that the high time and frequency resolution of the HTRU survey dramatically improves sensitivity to MSPs with high dispersion‑measure‑to‑period ratios, which were often missed by earlier surveys due to intra‑channel smearing. The newly discovered MSPs enrich the sample of precision clocks available for PTA projects, contribute to multi‑wavelength studies of pulsar emission, and provide valuable constraints on the geometry and altitude of radio emission in recycled pulsars. The polarisation results, in particular, suggest that MSP emission regions are extended and that traditional low‑altitude models may be insufficient to explain their complex profiles.