Research

Two effects can shape the same radio pulse: structure in the emission itself and propagation through material along the line of sight. FAST observations let us study both.

Interstellar scattering

Fluctuations in interstellar electron density scatter radio waves along paths of different lengths. The signals that travel farther arrive later, giving the pulse a tail that generally lengthens at lower frequencies. In a first-author study using FAST at 1.0–1.5 GHz, my colleagues and I fitted the intrinsic profile and its scattering tail in several frequency bands. This let us estimate the dispersion measure and trace how the broadening time changes with frequency.

The broadening varies between sightlines, even for similar electron columns. Our FAST results link stronger scattering along some paths through spiral arms to larger density fluctuations there. In a separate collaboration, we used the Murchison Widefield Array at 80–240 MHz to test how well a smooth shell of ionised gas describes six sightlines towards the Gum Nebula. The different scattering and absorption signatures favour a more structured medium.

Pulsar distances from neutral hydrogen

A pulsar and a supernova remnant can appear close together on the sky even when they lie at different distances. In my first-author study of PSR B0458+46, we used FAST to detect two foreground hydrogen clouds in absorption. Using their velocities and a model of Galactic rotation, we inferred a lower limit of about 2.7 kpc for the pulsar’s distance. This places it beyond the Perseus Arm and argues against its proposed association with the nearby remnant HB9.

A related study led by Jinchen Jiang used the 21-cm line towards the millisecond pulsar B1937+21. Comparing the absorption spectrum with the velocities of Galactic gas tightened the upper distance limit to 9.4 ± 0.5 kpc. These two pulsars illustrate how local gas motions enter a kinematic distance estimate.

Other Research Interests

Pulsar radio emission
Pulsar searches
Pulsar timing
Binary pulsars
Fast radio bursts
Galactic spiral structure
Galactic neutral hydrogen
Extragalactic H I galaxies