Pulsar Timing Arrays
The pulsar timing array group focuses on the detection and characterization of gravitational waves in the nanohertz frequency band, as well as the understanding and modeling of relevant astrophyical sources. A number of observational groups have worked for nearly two decades to detect these gravitational waves via high-precision timing of Galactic millisecond pulsars.
We are entering the era where detection of nanohertz gravitational waves may be possible with pulsar timing arrays (PTAs). In summer 2023, independent publications by four different Pulsar Timing Array collaborations (CPTA, EPTA, NANOGrav, PPTA) have reported varying levels of evidence, ranging from “weak” to “compelling”, for a stochastic gravitational-wave background in this frequency range.
This requires robust data analysis techniques and source models. Combining data from radio telescopes from around the world is challenging: the data sets are affected by systematics and contaminated by noise sources. While methods have been developed for the coherent analysis required for gravitational-wave observations, many open questions remain. In addition, to interpret the results and draw reliable conclusions, the gravitational-wave sources in the nanohertz band must to be correctly modeled. The PTA group focuses on these challenges.
Measuring gravitational waves with pulsars
Gravitational waves stretch and squeeze space-time, an effect which is currently observed with laser-interferometric methods in current ground-based detectors. The same effect also influences electromagnetic waves travelling through our Galaxy, including the radio pulses from pulsars. They arrive a little early or late, because gravitational waves affect space-time between the pulsar and Earth.
The expected variations in pulse arrival times are very small. This means that pulsars with very stable and predictable rotation properties and precisely measureable time of pulse arrivals are needed. Millisecond pulsars (rotating hundreds of times per seconds) satisfy both requirements Pulsar Timing Arrays regularly observe (“time”) a large number of milliseconds pulsars distributed over the entire sky over a long period of time. This minimizes disturbances and optimizes directional sensitivity.
Pulsar Timing Arrays
Currently, there are four pulsar timing arrays on the planet. The Parkes Pulsar Timing Array, the European Pulsar Timing Array, the North American Nanohertz Observatory for Gravitational Waves, and the Indian Pulsar Timing Array. The first three of them collaborate in the International Pulsar Timing Array, which regularly observes more than 60 millisecond pulsars.