A New, Low Braking Index For the LMC Pulsar B0540-69

TL;DR

This study presents a 16-month monitoring of the young pulsar B0540-69, revealing an exceptionally low braking index of approximately 0.031, which challenges previous understanding of pulsar spin-down behavior.

Contribution

It provides the first measurement of the braking index in the pulsar's high spin-down state, using extensive Swift and RXTE data to ensure accuracy and rule out timing noise effects.

Findings

Braking index is 0.031 +/- 0.013, much lower than typical values.

Timing noise unlikely to affect the measurement.

First measurement in the high spin-down state.

Abstract

We report the results of a 16-month monitoring campaign using the Swift satellite of PSR B0540-69, a young pulsar in the Large Magellanic Cloud. Phase connection was maintained throughout the campaign so that a reliable ephemeris could be determined, and the length of the campaign is adequate to accurately determine the spin frequency and its first and second derivatives. The braking index is 0.031 +/- 0.013 (90% confidence), a value much lower than previously reported for B0540-69 and almost all other young pulsars. We use data from the extensive monitoring campaign with RXTE to show that timing noise is unlikely to significantly affect the measurement. This is the first measurement of the braking index in the pulsar's recently discovered high spin-down state. We discuss possible mechanisms for producing the low braking index.