Timing measurements of the binary millisecond pulsar in the globular cluster M4

Measurements of the periods of pulsars, and of their rate of change with time, are essential in understanding pulsar evolution. For binary pulsars, such as PSR1620-26 in the core of the globular cluster M4, orbital parameters can also be derived from timing measurements. Here we report the results of timing observations of PSR1620 -26, including a first determination of its period derivative P = 3.2+/-0.4x 10^-19ss^-1 and its orbital eccentricity e = 0.025316 +/-0.000003. The age of this millisecond pulsar is less than 2.2 × 10⁸ years. The relatively high eccentricity is probably a result of gravitational encounters with other stars in the cluster during this time.