Pulsars

celestial clocks

R. N. Manchester

L. Guo

G. Hobbs

W. A. Coles

pp. 253-265

in: Elisa Felicitas Arias, Ludwig Combrinck, Pavel Gabor, Catherine Hohenkerk (eds), The science of time 2016, Berlin, Springer, 2017

Abstract

Pulsars are rapidly rotating neutron stars, with most of the known examples located within our Milky Way Galaxy. The class of "millisecond pulsars' (MSPs) have remarkably stable pulse periods, with a stability over long intervals comparable to that of the best atomic frequency standards. Timing observations of such pulsars in binary systems with another star have been used to make the most stringent tests of general relativity in strong gravitational fields. Observations of many MSPs, widely distributed across the sky and forming a "pulsar timing array" (PTA), have been used to set limits on the strength of the gravitational-wave background at nanohertz frequencies. These limits are beginning to constrain current ideas about the formation and evolution of supermassive binary black holes in the cores of distant galaxies. PTA observations can also be used to define a "pulsar timescale" that can limit or measure instabilities in terrestrial timescales over intervals of years and decades. Pulsar timescales form a useful 'secondary standard" that is totally independent of terrestrial time and frequency standards and will be continuous for billions of years.