How Pulsars Became Some of the Universe’s Best Clocks

The useful observation is a pulse arrival time

A telescope records a repeating signal when the emission region of a rotating neutron star passes through our line of sight. Observers combine many rotations to obtain a stable average pulse shape, then estimate when that profile arrived. The measurement is compared with a timing model. The resulting difference, called a residual, contains both possible astrophysical information and unwanted measurement effects.

Earth and interstellar gas alter the timetable

The observatory itself moves as Earth rotates and orbits the Sun. Timing analysis accounts for that motion so measurements from different dates can be compared consistently. Free electrons between a pulsar and Earth also delay lower radio frequencies more strongly. Observing at several frequencies helps separate that dispersive delay from a change that affects arrival times across the radio band.

Stability does not mean a perfectly uniform spin

Pulsars gradually lose rotational energy, and some undergo sudden spin changes called glitches. Binary companions introduce orbital motion and, in suitable systems, measurable relativistic effects. Stable millisecond pulsars are especially useful for precise timing, but each object still needs its own model. Telescope clocks, instrumental changes and variable pulse shapes can also produce residuals that require investigation.

An array looks for relationships between different clocks

A gravitational wave can change the apparent travel time of pulses across the distance between Earth and a pulsar. Timing arrays monitor many pulsars for years and compare their residuals. The sought signal includes a particular pattern of correlations across sky directions. One star arriving early is insufficient: the shared spatial pattern helps distinguish a gravitational-wave background from several sources of timing noise.

Sources and further reading

1. NASA Hubble — Pulsars
2. NANOGrav — Pulsars as cosmic clocks
3. NANOGrav — Detector noise and timing