High precision timing

Abstract

Pulsars are highly magnetised, fast spinning neutron stars, formed after a supernova explosion. Pulsars have been used to study a wide range of physical and astrophysical phenomena. The majority of science that we can do with pulsars comes from a technique called, pulsar timing. Millisecond pulsars (MSPs), pulsars with spin periods of only few milliseconds, are the ideal objects to apply timing, since their rotational stability on long timescales approaches that of atomic clocks.<br /> Probably one of the most interesting applications of pulsar timing is the detection of gravitational waves (GWs) in the nHz regime. Main sources of these low frequency GWs are inspiraling super-massive black hole binaries (SMBHBs). GW signal can be from individual sources or from the gravitational wave background (GWB) made from the superposition of the cosmic population of the SMBHBs. A GW signal can be detected as a correlated red noise signal in the timing residuals of a pulsar timing array (PTA), formed by timing stable MSPs, almost homogeneously distributed on the sky. The main source of red noise in pulsars that potentially blocks the detection of GWs with PTAs is chromatic variations produced by perturbations in the interstellar medium. In the first science chapter of this thesis (Chapter 3) we investigate the usage of Effelsberg and MeerKAT high-frequency receivers (both those presently used and those being commissioned) for measuring DM variations and increasing the PTA sensitivity to continuous GWs and GWBs. For our analysis we used Effelsberg real observations, contacted for the European PTA (EPTA), as well as simulated Effelsberg and MeerKAT observations. We concluded that the combination of Effelsberg 1.4 GHz and MeerKAT S-band observations, for flux density spectral indices less equal to -1.6, provides the best precision for DM variation measurements as well as the lowest uncertainty on the time-of arrival corrected from DM variations. Moreover, for the currently used Effelsberg observing combination for measuring DM variations existing from the 21 cm and the 11 cm receiver, an addition of 6.7 years for a GWB and another 80 years of observations for continuous GWs, on the top of 10, are needed in order to achieve the same sensitivity as we expect for the Effelsberg 21-cm combined with MeerKAT 11-cm observations.<br /> Even though GWs have been directly detected with LIGO, the first observational evidence of their existence came from the study of the first binary neutron star system ever discovered, PSR B1913+16. In the Chapter 5 of this thesis, we present the effect of the relativistic geodetic precession on the total intensity profile and polarisation properties on this pulsar. As well as our study about the shape of the emission beam. For our analysis we used observations taken with Arecibo and Effelsberg radio telescopes at observing wavelength of 21 cm. The effect of geodetic precession on the separation and the relative amplitude of the two main pulse components were obvious. After applying two independent to each other models, one fitted to the total intensity and the other to our polarisation data, we concluded that the inclination angle of the PSR B1913+16 system is 132.9 degrees and the misalignment angle ~21 degrees, and also that the emission beam is probably hollow-cone with the magnetic axis not being positioned at the center of the beam but shifted around 10 degrees from the center.<br /> Finally, in Chapter 4, we present the timing analysis of a pulsar-white dwarf system, PSR J1933-6211. Based on scintillation and timing analysis we concluded that the pulsar has, as have very few other fully recycled MSPs, a CO companion. The formation of this system can be explained with an intermediate-mass X-ray binary. In this case the transfer of masses started when the WD companion was still in main sequence stage and thus the transfer of mass through a Roche-lobe overflow lasted long enough time scale to fully recycle the pulsar.