The Binary Pulsar — Physical Processes, Possible Companions, And Evolutionary Histories
L. Smarr and R. Blandford, Astrophysical Journal, 207, 574-588 (1976).
Abstract
A study of some aspects of the binary pulsar, PSR1913+16, is presented in the light of recently reported observational results. If the companion to the pulsar gives a Newtonian contribution to the observed apsidal motion (a helium star or a rotating white dwarf), then it will probably be observable: the former either optically or through the dispersive effects of a stellar wind, and the latter through a secular change in the observed inclination of the orbit. If alternatively, the companion behaves dynamically as a point mass (apsidal motion caused solely by general relativity), observations of the 0(v/c)2 frequency shift will furnish a measurement of the masses of the components. In addition, with a ten-fold improvement in timing accuracy, the post-Newtonian corrections to the Keplerian ellipse and the effects of aberration could in principle be measured. This would then provide a determination of the orientation of the pulsar spin axis and allow the observation of geodetic spin precession to become a test of general relativity. We also examine the test based on the detection of orbital period changes due to gravitational radiation. Possible evolutionary histories are discussed. The most probable present system consists of two neutron stars, the end result of an X-ray binary phase followed by a double core star phase. In this scenario, the long timing age and short period of PSR1913+16 are shown to be interrelated if the pulsar was the first neutron star formed. To show that other companions are not ruled out on evolutionary grounds, we construct alternate histories leading to either Hestar, white dwarf, or black hole companions.