PUB - Publikationen an der Universität Bielefeld

Recycled pulsars are old pulsars which have been spun up to very high rotational frequencies through the transfer of angular momentum by stellar material accreted from a companion via Roche-lobe overflow. These pulsars consist of matter at extreme densities, subject to some of the strongest electromagnetic fields and while the study of these objects has remained a challenging task, recycled pulsars are also extremely sensitive probes for studying fundamental physics.
In this thesis I present results from three investigations related to the observation of recycled pulsars. I present an overview of the most significant artifacts that affect digital data recording systems for pulsar observations. I show that for the special case of a coherent dedispersion pulsar backend, the action of the dedispersion filter is non-linear in phase and therefore the resultant signal cannot be reconstructed perfectly by an analysis-synthesis filterbank which is built using simple digital filters, if the channel bandwidths are very high. I review a least-mean-squares based filter optimisation algorithm with the aim of addressing this issue.
I then present an updated pulsar timing solution for PSR J2051$-$0827, which was the second black-widow pulsar to be discovered. For the first time for this system, we are able to detect a decrease in the dispersion measure of $\sim 2.5 \times 10^{-3} cm^{-3} pc$. The extended timing also results in the most precise measurements to date of the mean proper motion ($6.1(1) mas yr^{-1}$) and the 2-D transverse velocity ($30(9) km s^{-1}$) of this system. Secular variations in the orbital period are recovered for more than one complete cycle and we detect previously unknown short-term variations, on timescales of $\sim 150$ days. The 21-year dataset results in a weighted timing residual of of $\sim 5 \mu s$, which is comparable to that of sources already in PTAs suggesting some black-widow pulsars _may_ indeed be useful PTA sources.
Finally, I measure the spectral indices of 12 recycled pulsars for 11 of which flux densities at less than two frequency bands were known, using multi-epoch flux density measurements at three frequency bands carried out with the Arecibo Radio Observatory. We add rederived spectral indices for 43 recycled pulsars which had preexisting spectral index measurements by adding flux density measurements at other frequency bands and 19 recycled pulsars for which only flux densities were available in literature to increase the sample size to 74, which is almost two-thirds of the known Galactic population of 195. The measured spectral indices suggest that while no obvious difference exists between isolated recycled pulsars and those in binaries, redback systems appear to have steeper spectral indices. Recycled pulsars which are also visible in the $\gamma$-ray regime appear to have a steeper spectral index than those which are visible only radio frequencies. This may be the reason why targeted searches for such sources at high radio frequencies have been less successful compared to concurrent searches at lower frequencies. The two-tailed Kolmogorov-Smirnov test shows that the spectral index distribution of recycled pulsars is similar to that of classical pulsars, as well.