Reciprocating Magnetic Fields in the Pulsar Wind Observed from the Black Widow Pulsar J1720-0534

TL;DR

This study presents radio observations of the black widow pulsar J1720-0534, revealing oscillatory magnetic fields and electron density variations near the companion, advancing understanding of pulsar wind interactions.

Contribution

First detection of reciprocating magnetic fields and polarization changes in a black widow pulsar system using multiple radio telescopes.

Findings

Oscillatory polarization angle change observed during eclipse egress.

Magnetic fields oscillate at approximately 10 milliGauss over 5000 km.

Rapid electron density increase at the shock boundary during ingress and egress.

Abstract

We report the radio observations of the eclipsing black widow pulsar J1720-0534, a 3.26 ms pulsar in orbit with a low mass companion of mass 0.029 to 0.034 M$_{\odot}$. We obtain the phase-connected timing ephemeris and polarization profile of this millisecond pulsar (MSP) using the Five-hundred-meter Aperture Spherical Radio Telescope (FAST), the Green Bank Telescope (GBT), and the Parkes Telescope. For the first time from such a system, an oscillatory polarisation angle change was observed from a particular eclipse egress with partial depolarization, indicating 10-milliGauss-level reciprocating magnetic fields oscillating in a length scale of 5000 km (assuming an orbital inclination angle of 90 degrees) outside the companion's magnetosphere. The dispersion measure variation observed during the ingresses and egresses shows the rapid raising of the electron density in the shock boundary between the companion's magnetosphere and the surrounding pulsar wind. We suggest that the observed oscillatory magnetic fields originate from the pulsar wind outside the companion's magnetosphere.