Pulsar Radio Emission Modulation in Relation to Rotational Instability

Abstract

The magnetospheric conditions responsible for radio emission in pulsars are still not clearly understood. Through studying the modulation of this emission, in relation to the rotational properties of these stars, the observer can obtain insight into the mechanism which governs the radio emission in pulsars, as well as their magnetospheric environments. Nulling pulsars are instrumental in this study due to their meta-stable configurations, which result in abrupt cessation or re-activation of their radio emission. As the observed modulation timescales in these sources vary from seconds to years, several theories have been proposed to explain their behaviour. However, no consensus has been reached on what triggers their magnetospheric reconfigurations, nor whether they are all governed by the same processes. In this work, we have studied three extreme nulling pulsars to help determine how these objects can be best classified, and what the most viable processes responsible for their behaviour could be.Using approximately 13 years of high-cadence observations, we confirm that the prototype 'intermittent' pulsar, PSR B1931+24, has a variable spin-down rate which is correlated with the presence (absence) of radio emission. The spin-down rates of the pulsar, attributed to the radio-on and -off emission phases, do not exhibit any evidence for time evolution and, as such, retain a constant difference of ~50 % throughout the 13-year data-set. We find that the radio-on and -off emission phases last approximately 8 days and 22 days on average respectively. They are also found to repeat quasi-periodically, over an average timescale of 36 days. We find no evidence for any interruptions to a given emission phase, that is the pulsar appears to switch between emission phases over long timescales (~days) only. Using our unprecedented data span, we find the object has an overall nulling fraction of 74 ± 6 %.We use an unparallelled 153-day set of observations to show that the pulsar B0823+26 exhibits a bimodal distribution of nulling timescales, that is short nulls (~1-3 pulse periods) and long nulls (~100-10 000 pulse periods) that are not periodic in nature. Remarkably, the longest nulls observed in PSR B0823+26 can be as extreme as those found in rotating radio transients (RRATs). The pulsar is shown to undergo pre-ignition periods of emission bursts and nulls before transitioning to a reasonably steady radio-on phase. Through characterising the emission variability of the source in the radio-on phases, we find that the post-cursor emission component exhibits a remarkable degree of variability. We do not obtain any direct evidence for spin-down rate variation in this pulsar, but we do place an upper limit of smaller or equal to 6 % on any change.We also investigate the radio emission characteristics of PSR J1107-5907, and find that it exhibits a wide range of nulling timescales (i.e. seconds to many hours). Coupled with its highly irregular bright emission, these nulling properties suggest a strong link between this object and RRAT-like sources. Although the pulsar undergoes both nulls and dramatic pulse shape changes, no evidence for spin-down rate variation is obtained.The nulling properties of the sources studied suggest a close relationship between conventional nulling pulsars, RRATs and intermittent pulsars. We find that the existing characterisation of the different types of nulling pulsars, based on their initial detection properties, is unsuitable. We also predict that, in addition to those yet undetected, a significant number of nulling objects are located in the existing known pulsar population, but have not yet been fully characterised.

Date of Award	31 Dec 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Benjamin Stappers (Supervisor)