Radio study of the lighthouse pulsar wind nebula

Abstract

Pulsar wind nebulae (PWNe) are created by magnetized relativistic outflows of pulsars that are shocked upon interaction with the ambient medium. A bow-shock PWN is formed when a pulsar moves supersonically in a medium. Previous radio polarization studies of bow-shock PWNe showed various mag- netic field configurations. It was claimed that the B-field preferentially aligns with the PWN elongation for systems traveling at a high speed of ∼ 1000 km/s. In this thesis, I report radio observations of the Lighthouse PWN using the Australia Telescope Compact Array in 3cm and 6cm bands. This bow-shock PWN moving at a speed > 1000 km/s and has a ∼ 10 kpc-long tail trailing the pulsar motion. My new radio image shows that the PWN consists of a faint cube-like structure near the pulsar, a cone-like head with two bright cores inside, and a tail. These features resemble two other bow-shock systems, G319.9−0.7 and the Guitar Nebula. The morphology of the head and the two-core structure suggest that there could be flow instabilities driving bubbles into the interstellar medium (ISM) or the PWN just encountered several ISM density discontinuities. I developed a simple model with a bow-shock front and two expanding shocks to fit the PWN structure. Radio polarimetric measurements reveal a helical B-field in the head of the Lighthouse and a possible switch of the field direction at the second core. This is the first time that such a field configuration is found in a fast-moving system. A direct comparison with the Frying Pan, G319.9−0.7, and the Mouse PWNe suggests that the alignment angle between the pulsar velocity and its spin axis direction could not decide the magnetic field configuration. The speed and axis should be both considered. Future polarimetric studies of bow-shock PWNe and magnetohydrodynamics simulations are needed to further confirm and explain this idea.