The Binary Pulsar

In 1974, Joseph Taylor and Robert Hulse discovered a pulsar, designated PSR 1913+16, that is a member of a binary system in which the other star appears to be a neutron star (but not a pulsar). This system has come to be known as the Binary Pulsar.

The discovery and subsequent study of the Binary Pulsar was of such fundamental importance that Taylor and Hulse were awarded the 1993 Nobel Prize in Physics for their work. Chief among the reasons for this importance is that the Binary Pulsar has provided the most stringent tests yet of the general theory of relativity and has provided the first (indirect) evidence for a key prediction of that theory: the existence of gravitational waves.

Periodic Variations
Because the repetition period for a pulsar is clock-like in its precision, variations in that period as observed from Earth must be associated with orbital motion in the binary. Thus, these variations can be used to give very precise information about the orbit. For example, when the pulsar is moving toward us, the repetition rate of the pulses as observed from Earth will be higher than when the pulsar is moving away (Doppler effect), and this can be used to measure the radial velocity. The top right figure illustrates this shift in frequency for the Binary Pulsar.

Likewise, the pulse arrival times vary as the pulsar moves through its orbit because it takes three seconds longer for the pulses to arrive from the far side of the orbit than from the near side. From this, the Binary Pulsar orbit can be inferred to be about a million kilometers further away from Earth when on the far side of its orbit than when on the near side (1 million kilometers is about three light seconds).

Orbital Characteristics
The orbits determined for the binary system are shown in the above left diagram. Both neutron stars have masses of about 1.4 solar masses and the orbits are very eccentric (an eccentricity of about 0.6 has been deduced). The minimum separation (periastron) is about 1.1 solar radii, the maximum separation (apastron) is about 4.8 solar radii, and the orbital plane is inclined by about 45 degrees as viewed from Earth.

By Kepler's laws, the radial velocity of the pulsar varies substantially as it moves around its elliptical orbit, as illustrated in the top right diagram. As we shall discuss shortly, these orbits are not quite closed ellipses because of precession effects associated with general relativity. This causes the location of the periastron to shift a small amount for each revolution. The adjacent diagram illustrates this precession schematically. The points P1, P2, and P3 are periastrons on three successive orbits (with the amount of precession greatly exaggerated for clarity).