http://physicstoday.scitation.org/doi/10.1063/PT.3.3621
Radio observatories are accumulating data to detect mergers of supermassive black holes.
Pulsar timing arrays monitor millisecond pulsars in the Milky Way. As a pulsar rotates, it emits radio waves that sweep by Earth with the period of rotation.
The shorter the period, the more and sharper the incident radio-wave ticks, so the better the clock. A gravitational wave passing between a pulsar clock and
an observer distorts spacetime and causes the signals to arrive either later or earlier. “That’s the fingerprint,” says EPTA member Alberto Sesana of
the University of Birmingham.
The distortions due to gravitational waves are faint; if they arise from galaxy mergers, they occur with periods of decades. Data are collected for each pulsar
for about a half hour every few weeks. The hundreds of thousands of pulses from a given observation are summed to extract the signal from the noise.
Extragalactic gravitational waves wash over all the pulsars in the Milky Way. Because the pulsars are independent, and each has its own timing and its own
interstellar medium, the main giveaway for detecting a gravitational wave is a correlated signal between the pulsars. “Pulsars do their own thing, and it’s hard
to dig out a tiny signal from a large number of sources of noise,” says Sesana. “The main way to overcome this is by timing an array of pulsars.”
Each experiment keeps tabs on around 50 pulsars. Often an individual astronomer is responsible for specific clocks. For example, NANOGrav member Maura
McLaughlin of West Virginia University monitors five. Assessing the data “is not completely deterministic,” she says. “It’s a bit of an art form.”
