Hunt for millisecond pulsars around the sky region of OJ287 - a supermassive black hole binary

This thesis presents the outcomes of a pulsar survey conducted around the sky region of the blazar OJ287. OJ287 hosts a supermassive black hole binary at its core and is one of the few well-studied systems. By precisely timing millisecond pulsar signals, any modulation in the arrival time of pulses caused by gravitational waves can potentially be detected. If the angular separation in the sky between the millisecond pulsars and the gravitational wave source is small (but not zero), then the induced modulation will be stronger than for larger separations. However, there are no known millisecond pulsars in the sky region of OJ287. A new pulsar survey was carried out in 2020 using the multibeam receiver of the Parkes 64-m radio telescope with an integration time 1000 s, to search for such millisecond pulsars.

We developed a pulsar-searching pipeline based on the frequency-domain search technique (searching for periodic signals instead of individual pulses), using PRESTO. We describe the pipeline in detail and how we applied the pipeline to 2,353 individual observations leading to four promising pulsar candidates and the successful detection of a test pulsar PSR J0613- 0200. However, follow-up observations could not confirm these pulsar candidates so we only successfully detected the test pulsar PSR J0613-0200.

We hoped to detect at least one-millisecond pulsar in the sky area around OJ287. The null detection means that the detection of the gravitational waves from the OJ287 will be slightly harder than it would have been if we had found one or more bright millisecond pulsars. To investigate the reasons behind this null detection, we first determined the sensitivity of our pulsar-searching pipeline using pulsar injection. simulateSearch was utilised to inject millisecond pulsars with varying flux values in the pulsar survey data. The analysis revealed that our pipeline was sensitive to pulsars approximately three times brighter than our predicted minimum flux value which was calculated purely based on the telescope system noise. To further investigate the null detection, a simulation was performed using PSRPoppy to model the population of millisecond pulsars and their detectability by both the Parkes and FAST radio telescopes in the vicinity of the OJ287 sky region. Based on 100 simulated universes, it was observed that the Parkes multibeam receiver failed to detect millisecond pulsars in approximately 55% of cases and hence our results are not surprising. The simulation results indicated a high probability of finding two to three millisecond pulsars using the FAST telescope. This finding prompted us to consider a new pulsar survey using the FAST telescope. However, the FAST telescope has a narrow beam, therefore, FAST cannot cover (in a reasonable time) the necessary sky region. So, we submitted an ASKAP proposal to identify sky positions with a point source which would likely be a pulsar. If likely candidates are identified then we will submit a proposal in order to carry out a sensitive periodicity search to confirm these candidates as pulsars using the FAST telescope.