Here are summaries of the publications that I have led. You can find my full publication list here.
Figure from McKee et al. (2020)
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arXiv link: A precise mass measurement of PSR J2045+3633
I used 6 years of radio timing data obtained from observations with the Arecibo (Puerto Rico), Lovell (UK), Nançay (France), and Effelsberg (Germany) telescopes to measure the masses of the pulsar and white dwarf in the PSR J2045+3633 binary system. To do this, I first had to detect the small effects that arise from the General Theory of Relativity, each of which provides a range of possible values for the pulsar and white dwarf masses. In the plot, the lines show the constraints imposed by the relativistic effects I measured (and their associated 1-sigma uncertainties): the rate of advance of periastron time (red), the amplitude of the Shapiro delay (solid blue), and the ratio of the Shapiro delay (dashed blue). The grey region is excluded by our knowledge of the mass function of the binary system. All of the constraints intersect at a single point on the plot, indicating that the binary companions are a 1.251 ± 0.021 Solar mass pulsar and a 0.873 ± 0.015 Solar mass white dwarf - a very precise measurement! |
Figure from McKee et al. (2019)
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arXiv link: A detailed study of giant pulses from PSR B1937+21 using the Large European Array for Pulsars
The single pulses from most pulsars, especially millisecond pulsars, are too weak to be detected. There are some pulsars, however, that occasionally emit 'giant pulses' - single pulses that are extremely bright, short-duration, and originate from a very small phase window in the pulsar rotation. I searched for giant pulses in 21 observations of the millisecond pulsar B1937+21 with the Large European Array for Pulsars, and found 4265 - the largest giant pulse data set ever gathered for this pulsar. Such a large data set allowed me to show that the giant pulses from the main pulse component (MGPs in the plot) follow a broken power law - very bright giant pulses are much less rare than we thought in this pulsar! |
Figure from McKee et al. (2018)
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arXiv link: Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision
The Crab Pulsar is located within the Crab Nebula - the supernova remnant from the birth of the pulsar. The ionised material that comprises the nebula is highly turbulent, and causes radio waves to be dispersed and scattered, and the relative strength of these effects to vary much more rapidly than other pulsars. Using low-frequency observations from the 42-ft telescope at Jodrell Bank, and higher-frequency observations from the Lovell Telescope, I measured the variations in scattering and dispersion in this pulsar over a time span of 30 years. In recent years, where the Crab is observed regularly, I was able to demonstrate that the two effects track each other very closely. In the plot, the upper panel shows the variations in dispersion measure, and the lower panel shows the variations in scattering time scale - you can see by eye that they are highly correlated. The high degree of correlation between these quantities suggests that they are caused by discrete structures within the nebula, which cut through the line of sight to the pulsar as it moves. We estimate from the typical duration of these variations that they must be caused by structures with sizes of approximately 6 astronomical units (900 million kilometres). |
Figure from McKee et al. (2016)
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arXiv link: A glitch in the millisecond pulsar J0613−0200
Although pulsars are extremely stable rotators, a small number experience rotational glitches: sudden increases in spin frequency. Glitches are usually associated with young pulsars that rotate slowly (spin periods ~ 1 second), and are thought to be caused by superfluid neutron vortices within the neutron star interior interacting with the solid component of the crust and transferring angular momentum. When combining pulsar timing data from four telescopes in the European Pulsar Timing Array with earlier data from the Lovell Telescope at Jodrell Bank, I discovered that the millisecond pulsar J0613−0200 had experienced a glitch in early 1998 which had gone unnoticed for 17 years. This is notable as PSR J0613−0200 is among the most stable rotators of the known millisecond pulsars, and is considered one of the best timers of pulsars used in pulsar timing array experiments. Also, there was previously only one millisecond pulsar that was known to have glitched. In the plot, the top panel shows the PSR J0613−0200 timing residuals as a function of date (i.e. the difference between the data and a timing model that assumes there was no glitch). The middle panel shows the variation in measured spin frequency, and the bottom panel shows the variation in measured spin-down rate. This confirms that millisecond pulsars do experience glitches. I measured the fractional change in spin period to be just 0.0025 ± 0.0001 nano-Hertz - by far the smallest glitch ever recorded! |
Banner image credit: Sam W of Simple Desktops