Observation of discontinuities in the periodic modulation of PSR B1828-11

Abstract

PSR B1828-11 is a radio pulsar that undergoes periodic modulations (~500 days) of its spin-down rate and beam width, providing a valuable opportunity to understand the rotational dynamics of neutron stars. The periodic modulations have previously been attributed to planetary companion(s), precession, or magnetospheric effects and have several interesting features: they persist over 10 cycles, there are at least two harmonically related components, and the period is decreasing at a rate of about 5 days per cycle. PSR B1828-11 also experienced a glitch, a sudden increase in its rotation frequency, at 55 040.9 Modified Julian Day(MJD). By studying the interaction of the periodic modulations with the glitch, we seek to find evidence to distinguish explanations of the periodic modulation. Using a phenomenological model, we analyse a recently published open data set from Jodrell Bank Observatory, providing the longest and highest resolution measurements of the pulsar's spin-down rate data. Our phenomenological model consists of step changes in the amplitude, modulation frequency, and phase of the long-term periodic modulation and the usual spin-down glitch behaviour. We find clear evidence with a (natural-log) Bayes factor of 1486 to support that not only is there a change to these three separate parameters but that the shifts occur before the glitch. Finally, we also present model-independent evidence which demonstrates visually how and when the modulation period and amplitude change. Discontinuities in the modulation period are difficult to explain if a planetary companion sources the periodic modulations, but we conclude with a discussion on the insights into precession and magnetospheric switching.