Aris Karastergiou

Abstract:

We report on the subpulse modulation properties of 1198 pulsars using the Thousand-Pulsar-Array programme on MeerKAT. About 35 per cent of the analysed pulsars exhibit drifting subpulses that are more pronounced towards the death line, consistent with previous studies. We estimate that this common phenomenon is detectable in 60 per cent of the overall pulsar population if high-quality data were available for all. This large study reveals the evolution of drifting subpulses across the pulsar population in unprecedented detail. In particular, we find that the modulation period P₃ follows a V-shaped evolution with respect to the characteristic age τ_c, such that the smallest P₃ values, corresponding to the Nyquist period P₃ ≃ 2, are found at τ_c ≃ 10^7.5 yr. The V-shaped evolution can be interpreted and reproduced if young pulsars possess aliased fast intrinsic P₃, which monotonically increase, ultimately achieving a slow unaliased P₃. Enhancement of irregularities in intrinsic subpulse modulation by aliasing in small-τ_c pulsars would explain their observed less well defined P₃’s and weaker spectral features. Modelling these results as rotating subbeams, their circulation must slow down as the pulsar evolves. This is the opposite to that expected if circulation is driven by E × B drift. This can be resolved if the observed P₃ periodicity is due to a beat between an E × B system and the pulsar period. As a by-product, we identified the correct periods and spin-down rates for 12 pulsars, for which harmonically related values were reported in the literature.

Abstract:

Context. PSR J1910-5959A is a binary millisecond pulsar in a 0.837 day circular orbit around a helium white dwarf (HeWD) companion. The position of this pulsar is 6.3 arcmin (∼74 core radii) away from the optical centre of the globular cluster (GC) NGC 6752. Given the large offset, the association of the pulsar with the GC has been debated. Aims. We aim to obtain precise measurements of the masses of the stars in the system along with secular orbital parameters, which will help identify if the system belongs to the GC. Methods. We have made use of archival Parkes 64 m 'Murriyang'telescope data and carried out observations with the MeerKAT telescope with different backends and receivers over the last two decades. Pulse times of arrival were obtained from these using standard pulsar data reduction techniques and analysed using state-of-the-art Bayesian pulsar timing techniques. We also performed an analysis of the pulsar's total intensity and polarisation profile to understand the interstellar scattering along the line of sight, and we determined the pulsar's geometry by fitting the rotating vector model to the polarisation data. Results. We obtain precise measurements of several post-Keplerian parameters: the range, r = 0.202(6) TȮ, and shape, s = 0.999823(4), of the Shapiro delay, from which we infer: the orbital inclination to be 88.9-0.14+0.15 deg; the masses of the pulsar and the companion to be 1.55(7) MȮ and 0.202(6) MȮ, respectively; a secular change in the orbital period Pb = -53-6.0+7.4 × 10-15 s s-1 that proves the GC association; and a secular change in the projected semi-major axis of the pulsar, x = -40.7-8.2+7.3 × 10-16 s s-1, likely caused by the spin-orbit interaction from a misaligned HeWD spin, at odds with the likely isolated binary evolution of the system. We also discuss some theoretical models for the structure and evolution of white dwarfs in neutron star-white dwarf binaries, using PSR J1910-5959A's companion as a test bed. Conclusions. PSR J1910-5959A is a rare system for which several parameters of both the pulsar and the HeWD companion can be accurately measured. As such, it is a test bed for discriminating between alternative models of HeWD structure and cooling

Abstract:

The radio polarization properties of the pulsar population are only superficially captured by the conventional picture of pulsar radio emission. We study the broadband polarization of 271 young radio pulsars, focusing particularly on circular polarization, using high quality observations made with the Ultra-Wideband Low receiver on Murriyang, the Parkes radio telescope. We seek to encapsulate polarization behaviour on a population scale by defining broad categories for frequency- and phase-dependent polarization evolution, studying the co-occurrences of these categorizations and comparing them with average polarization measurements and spin-down energy (⁠E˙⁠). This work shows that deviations of the linear polarization position angle (PA) from the rotating vector model (RVM) are linked to the presence of circular polarization features and to frequency evolution of the polarization. Polarization fraction, circular polarization contribution and profile complexity all evolve with E˙ across the population, with the profiles of high-E˙ pulsars being simple and highly linearly polarized. The relationship between polarization fraction and circular contribution is also seen to evolve such that highly polarized profiles show less variation in circular contribution with frequency than less strongly polarized profiles. This evolution is seen both across the population and across frequency for individual sources. Understanding pulsar radio polarization requires detailed study of individual sources and collective understanding of population-level trends. For the former, we provide visualizations of their phase- and frequency-resolved polarization parameters. For the latter, we have highlighted the importance of including the impact of circular polarization and of E˙⁠.