New Heavyweight Champion Neutron Star
Figure caption: The Shapiro Time Delay in the J1614-2230 binary system. As the companion white dwarf star passes in front of the pulsar at orbital phase 0.25, the radio pulses experience an excess delay of about 40 us. The large signal strength is mainly due to the orbital inclincation, which is less than 1 degree away from edge-on as viewed from Earth. The amplitude and shape of this curve are determined by both the orbital inclincation and white dwarf mass. Knowing these, we can then infer the pulsar's mass as well from orbital dynamics.
The same properties that make millisecond pulsars so useful for gravitational wave detection --- their exceptional stability as cosmic metronomes --- allow them to be used for other purposes as well. The millisecond pulsar J1614-2230 is in a binary system with a white dwarf star as its companion. We see this system close to edge-on from Earth; so, once every orbit the signal from the pulsar must pass very close to the companion white dwarf as it travels toward Earth. Einstein's theory of gravity predicts that, when this happens, the pulse takes longer than expected to cross the binary systems orbit. The effect is small, but the pulsar metronome is so stable that this delay --- the so-called Shapiro Time Delay --- can be measured quite precisely. By how this delay increases and decreases the orbits inclination and the white dwarf mass can be measured and, together with other orbital parameter measurements (which can also be measured by monitoring the variations in pulse arrival times at Earth), the mass of the neutron star can be inferred. In this way, NANOGrav scientists Paul Demorest and Scott Ransom (NRAO) have recently measured the largest known neutron star mass: nearly twice the mass of the Sun. There is a limit to how massive a neutron star can be. That limit is set by the properties of matter at densities far in excess of what can be created or explored in terrestrial laboratories. By measuring neutron star masses astronomer's are thus able to explore properties of matter that high energy physicists, working with the most sophisticated and expensive accelerators that can be built,cannot hope to probe. By measuring such a high-mass neutron star, Demorest and Ransom's observations have ruled-out many theoretical previously viable theoretical models for high-density nuclear matter, helping us to home-in on Nature's Truth.
To learn more, please see http://www.nrao.edu/pr/2010/bigns/ and http://www.nature.com/nature/journal/v467/n7319/full/nature09466.html. Figure caption: The Shapiro Time Delay in the J1614-2230 binary system. As the companion white dwarf star passes in front of the pulsar at orbital phase 0.25, the radio pulses experience an excess delay of about 40 us. The large signal strength is mainly due to the orbital inclination, which is less than 1 degree away from edge-on as viewed from Earth. The amplitude and shape of this curve are determined by both the orbital inclination and white dwarf mass. Knowing these, we can then infer the pulsar's mass as well from orbital dynamics.
