Understanding the spin-down rate changes of PSR B0919+06

Abstract

We study the spin-down properties of PSR B0919+06 based on almost 30 years of radio observations. We confirm that the time derivative of the rotational frequency $\dot \nu$ is modulated quasi-periodically and show that it exhibits a repeating double-peaked structure throughout the entire observation span. We model the $\dot \nu$ variation of the pulsar assuming two spin-down rates with sudden switches between them in time. Our results show that the double-peak structure in $\dot \nu$ has a repetition time of about 630 days until MJD 52000 (April 2001) and 550 days since then. During this cycle, the pulsar spin varies from the lower spin-down rate to the upper spin-down rate twice with different amounts of time spent in each state, resulting in a further quasi-stable secondary modulation of the two-state switching. This particular spin-down state switching is broadly consistent with free precession of the pulsar, however, a strong evidence linked with this mechanism is not clearly established. We also confirm that the pulsar occasionally emits groups of pulses which appear early in pulse phase, so-called "flares", and these events significantly contribute to the pulse profile shape. We find the $\dot \nu$ modulation and the pulse shape variations are correlated throughout the observations. However, the flare-state is not entirely responsible for this correlation. In addition to the flare-state, we detect flare-like events from the pulsar in single pulse observations. During these events, the shift in pulse phase is small compared to that of the main flare-state and clearly visible only in single pulse observations.