Pulsars are highly magnetized, rapidly rotating neutron stars. Radio pulsars, so called since they are seen through their radio emission, given off from their magnetic poles. Usually the magnetic axis is not aligned with the pulsar's spin axis, and so every time it passes by our line of sight, we see a pulse of radio emission, as with a lighthouse beam. Pulsars have been seen to emit at pretty much all wavelengths, though not as commonly as in the radio or x-ray.

Pulsars are famous for being extremely accurate clocks; that is, consecutive rotations of the neutron star are separated by very regular time intervals. Most radio pulsars spin about once per second or so. There is another population of pulsars, however, that spin much faster, at millisecond periods. These are often found in binary systems with other stars; it is these systems in which I am most interested.

Some of these recycled pulsars are mysteriously isolated; most are found in binary systems, and most frequently with a white dwarf or neutron star companions, the remnants of the stars that once donated matter to the pulsar that we now see with our radio telescopes. These binary systems can be used for many areas of study.

By studying the pulse arrival times from binary pulsars, we can thus learn about its orbit. Where things get really interesting is if the system displays relativistic orbital effects, such as orbital precession (the actual path of the orbit rotates; this is seen in Mercury's orbit around the Sun), or orbital decay (where the size and period of the orbit shrinks over time). These are described by Einstein's theory of general relativity. By measuring the magnitude of some of these relativistic effects in systems which exhibit them, we can call on general relativity to tell us what the individual masses of the neutron star and its companion are. In addition, if we determine enough of these parameters (three or more to be exact), we can use those measurements to actually test general relativity! I have been involved in a study to do just this for the double-neutron-star system PSR J0737-3039, unique among pulsar binaries: for one thing, it is the most relativistic known to date; as well, it is the first system found so far in which both neutron stars are observable as radio pulsars. It has, and continues to provide researchers with many opportunities, from studies of pulsar and binary system formation, to plasma and magnetospheric studies, to the most precise study of general relativity ever performed in the strong field: within 0.05%, it was shown that the measured parameters of this system are consistent with the predictions of Einstein's theory. I was fortunate enough to play a role in this study.

Pulsar binaries can also be used to study binary stellar evolution. Since these systems represent those at or near the end of their lives, we can, through their observation and measurement, investigate just how they came to arrive at their present states. This is accomplished mainly through the mass measurements mentioned above. Knowing the present binary component masses, along witht he system parameters and the spin behaviour of the pulsar, gives us a big clue as to how long the mass transfer episode occurred when the pulsar was spun up, and perhaps how much matter was donated by the companion. There are relatively very few pulsar systems with known or measurable masses. However, those for which these quantities are determined show a wide variety of values. This tells us that the evolutionary histories of these systems also vary significantly. For example, one system I am currently studying, PSR J1802-2124 has a light pulsar and heavy companion white dwarf star. It also has a compact orbit (17 hours) and the pulsar is only mildly spun up (13 miilliseconds). The most probable scenario that would produce such a system involves the binary having gone through a phase of common-envelope evolution, in which the neutron star had spiraled into the gas envelope of its companion. Before it had enough time to accrete much matter, and spin up to millisecond periods, the common envelope was ejected from the system (to conserve angular momentum). This would leave behind a light pulsar in a tight orbit---precisely what we see.

Check out my publications page to link to the above studies.