arXiv subjects

Paul C. W. Lai

Publications and source records attributed to Paul C. W. Lai.

6 recordsLinked to original sources

arXiv2026

High-resolution Very Large Array Radio Observations of the Boomerang Pulsar Wind Nebula

We present a radio polarimetric study of the Boomerang pulsar wind nebula G106.65+2.96 with Very Large Array (VLA) observations at the 6 GHz band. Our high-resolution image discovers new small-scale features in the nebula, including an elliptical core of $40''\times20''$ surrounding the central pulsar and a $2'$-long arc wrapping around the core in the north. The latter shows a gap from the core, and it consists of a bright lobe in the northwest and a tongue-like structure in the northeast. These could be resulting from the pulsar wind interaction with the environment. Our polarization measurement reveals a highly ordered magnetic field with toroidal geometry. The small scale features are all highly linearly polarized. In particular, the lobe has a polarization fraction of $\sim$60%, close to the synchrotron limit. This is also much higher than the value measured at a lower frequency, implying significant depolarization. We show that this can be explained by Faraday rotation in the nebula, and we constructed a simple 3D model accordingly to estimate a magnetic field strength of $\sim$25-70$ μ$G assuming a median inclination angle of $60^\circ$, which uncertainty arises from the uncertainty in the flux measurement and its distance.

astro-ph.HE↗

arXiv2026

Bridging Theory and Observation in the SKA Era: A Cosmological Polarized Radiative Transfer Framework for Point-to-Point Polarized Sky Comparisons

Realizing the full scientific potential of the SKA requires not only revolutionary instrumentation but also accurate modeling of light propagation in an evolving, expanding Universe, in order to translate intensity and polarization data into physical insight about magnetic fields and cosmic plasma. When all-sky cosmological polarized radiative transfer (CPRT) calculations meets SKA observations, theory and data interlock to deliver a predictive, and testable picture of the evolving magneto-ionic Universe. This synergy transforms polarization observations -- assembled into empirical maps of diffuse emission and rotation-measure (RM) grids of discrete sources -- from descriptive data products into powerful astrophysical probes, advancing our understanding of cosmic magnetism across space and time. The CPRT formalism -- derived from fundamental conservation laws and incorporating relativistic, cosmological, and full radiative-transfer effects -- provides a robust platform and a common framework for observers, theorists, and simulation experts to pursue shared scientific goals. Observers gain synthetic templates to interpret RM grids and polarization maps; theorists can directly confront models of magnetogenesis and magnetic-field evolution with data; and simulation experts obtain a post-processing tool to transform cosmological magneto-hydrodynamic (MHD) outputs into observable skies. Furthermore, CPRT serves as a powerful testbed when traditional RM-based methods reach their limitations -- for example, in interpreting complex Faraday spectra, disentangling multiple intervening magnetized media, or achieving a coherent picture when diverse observational diagnostics -- such as dispersion measure, synchrotron emission and spectral index, and dust polarization -- are combined.

astro-ph.CO↗

arXiv2026

Neutrinos as a new tool to characterise the Milky Way Centre

The Central Molecular Zone (CMZ), a star-forming region rich in molecular clouds located within hundreds of parsecs from the centre of our Galaxy, converts gas into stars less efficiently than anticipated. A key challenge in refining star-formation models is the lack of precise mapping of these dense molecular hydrogen clouds, where traditional tracers often yield inconsistent results. We demonstrate how, in the near future, neutrinos will emerge as a robust mass tracer due to worldwide advancements in neutrino telescopes. Neutrinos are produced alongside gamma-rays when cosmic-rays interact with molecular clouds. The neutrino production rate is proportional to the gas density without dependence on the complex properties of a cloud. Neutrinos also have the advantage of negligible absorption and unambiguous production channels, making it a method with the lowest systematic uncertainties. In an optimistic case where most gamma-ray emission from the Galactic Centre region originates from pion decays, we expect several tens of muon neutrinos to be detected in about two decades. Neutrinos from the CMZ will provide indications on the biases of traditional mass tracers and thus indirectly enhance the accuracy of gas measurements in far galaxies from which a neutrino signal is not detectable.

astro-ph.HE↗

arXiv2025

Spectropolarimetry of synchrotron radiation from relativistic electrons with anisotropic pitch-angle and various energy distributions

For synchrotron radiation from relativistic electrons having a power-law energy distribution with a power-law index $p$ in an optically thin medium with a locally uniform magnetic field, it is generally adopted that the spectral index $α= (p-1)/2$ and the degree of linear polarisation $Π_{\rm L,pl} = (p +1)/(p+7/3)$, and hence that $Π_{\rm L,pl} = (α+1)/(α+5/3)$. These ($α$, $Π_{\rm L,pl}$; $p$) relations are derived assuming that the electrons have an isotropic momentum distribution and a power-law energy distribution, and they have been used, almost universally, in the interpretation of polarimetry observations. In this study, we assess the validity of the ($α$, $Π_{\rm L,pl}$; $p$) relations in different scenarios, such as anisotropic electron momenta and non-power-law distributions of electron energy. We calculate the synchrotron radiation and polarisation spectra for power-law, kappa and log-parabola electron-energy distributions, with isotropic and two anisotropic (beamed and loss cone) electron-momentum distributions. Our calculations show that when the electron momenta are isotropic, the usual ($α$, $Π_{\rm L,pl}$; $p$) relations are generally applicable. However, if the electrons are anisotropic, the usual ($α$, $Π_{\rm L,pl}$; $p$) relations could break down. Applying the usual ($α$, $Π_{\rm L,pl}$; $p$) relations indiscriminately without caution on anisotropy in the electron momentum distribution, would lead to incorrect interpretations of polarimetry data.

astro-ph.HE↗

arXiv2023

Neutrino Imaging of the Galactic Centre and Millisecond Pulsar Population

In this work, we consider the possible presence of a large population of millisecond pulsars in the Galactic Centre. Their direct detection would be challenging due to severe pulse broadening caused by scattering of radiation. We propose a new method to constrain their population with neutrino imaging of the Galactic Centre. Millisecond pulsars are proposed cosmic-ray accelerators. The high-energy protons they produce will collide with the baryonic matter in the central molecular zone to create charged and neutral pions that decay into neutrinos and $γ$-rays, respectively. The specific neutrino and $γ$-ray fluxes must be below their corresponding observed values, allowing us to put a conservative upper limit on the millisecond pulsar population of N_MSP < 10,000 within a galacto-centric radius of 20 pc. This upper limit is sensitive to the proton acceleration efficiency of the pulsars, but is less dependent on the particle injection spectral index and the choice of mass tracers. The population will be better constrained when high resolution neutrino observations of the Galactic Centre become available. The presence of these millisecond pulsars can account for the $γ$-ray excess in the Galactic Centre.

astro-ph.HE↗

arXiv2022

Magnetic Field Structure and Faraday Rotation of the Plerionic Supernova Remnant G21.5$-$0.9

We present a polarimetric study of the pulsar wind nebula (PWN) in supernova remnant G21.5$-$0.9 using archival Very Large Array (VLA) data. The rotation measure (RM) map of the PWN shows a symmetric pattern that aligns with the presumed pulsar spin axis direction, implying a significant contribution of RM from the nebula. We suggest that the spatial variation of the internal RM is mostly caused by non-uniform distribution of electrons originated from the supernova ejecta. Our high-resolution radio polarization map reveals an overall radial $B$-field. We construct a simple model with an overall radial $B$-field and turbulence in small scale. The model can reproduce many of the observed features of the PWN, including the polarization pattern and polarized fraction. The results also reject a large-scale toroidal $B$-field which implies that the toroidal field observed in the inner PWN cannot propagate to the entire nebula.

astro-ph.HE↗