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Timing and analysis of eclipsing black widow pulsar PSR J2256-1024 Crowter, Kathryn

Abstract

Pulsars are rapidly spinning neutron stars emitting radiation about their magnetic field axes. Misalignment of the spin and magnetic axes causes a “lighthouse” effect where we observe radiation pulses, in time with the pulsar’s rotation. Millisecond pulsars are those which have accreted material from a companion star, spinning themselves up to rotate faster. Some millisecond pulsars occur in tight orbits with low mass companions; this combination can lead to the companion losing material, due to bombardment by energetic particles from the pulsar, presumably eventually destroying the companion. These pulsars are known as Black Widows (BWs). This thesis is an analysis of radio observations of PSR J2256-1024, a BW pulsar with a spin period of 2.294531816964939(10)ms. Observing pulsars, we can calculate the arrival times of individual pulses and compare these with those predicted from various models to find the best-fitting one. This process is known as pulsar timing. We present the timing solution for PSR J2256-1024. We find it has a 5.1091831284(9)hr binary orbit with a semimajor axis of 4.1(3)ltsec and a 0.0312(9)M⊙ companion. PSR J2256-1024 shows a radio eclipse over 7.8% of its orbit - approximately twice the size of the Roche lobe calculated for the companion. This confirms the picture of a Black Widow pulsar with material being stripped from the companion and forming a trailing cloud which blocks the pulsar signal. We also find evidence for variable clumps of material in the system. We present polarization profiles and mean flux densities at 350 MHz, 820MHz and 1500 MHz. We discuss polarization changes in the post-eclipse region, where the pulsar signal is transmitted through eclipsing material in the system, and find evidence of Faraday rotation. At one epoch, synchronous measurements of excess dispersion and rotation measure lead to a detection of a 3.9(0.6)mG line-of-sight magnetic field. This field occurs an estimated minimum 3.3(0.3) companion-Roche-lobe-radii from the companion. We believe this is the first successful detection of a magnetic field component in eclipsing material within a Black Widow system.

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