The IPTA’s Search for Nanohertz Gravitational Waves

Detection of the radio emission from the astronomical object in 1930+ by Karl Jansky started an era of Radio Astronomy followed by many discoveries enriching our understanding of the Universe we live in. Similarly, we are on the verge of opening a new window (ultra-low frequencies) in Gravitational Wave Astronomy. We are watching the rolling history of scientific discoveries in slow motion.

In the quest to unravel the nature of gravitational waves (GWs) with pulsar timing arrays (PTAs), the International Pulsar Timing Array (IPTA) is a consortium of PTA collaborations. The current IPTA members include the European Pulsar Timing Array (EPTA), the Indian Pulsar Timing Array (InPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and the Parkes Pulsar Timing Array (PPTA) using the largest radio telescopes in Europe, India, United States, Canada and Australia. There are additionally emergent PTAs in China and South Africa. Through the combination of the individual data sets resulting from the PTAs, the IPTA focuses on the search for GWs in the very low-frequency part of their spectrum (in contrast with previous detections of GWs reported by the LIGO, Virgo and KAGRA collaborations).

About two years ago, the IPTA consortium put in place an arrangement to coordinate the efforts of its constituent PTAs for the release of their latest data sets and the results of GW searches. The objective of this agreement, called the 3P+ agreement, was to provide cross-PTA checks and validation while at the same time allowing independent analysis of the respective data sets by each PTA.

After almost a six-month process of such a review, NANOGrav, the PPTA, the Chinese Pulsar Timing Array (CPTA), and the EPTA, along with the InPTA, are releasing new results regarding their ongoing search for low-frequency GWs on June 29, 2023. These results from independent data and analyses from CPTA, EPTA+InPTA, NANOGrav, and PPTA represent a significant milestone in the search for gravitational waves.

We use ultra-stable millisecond pulsars as Galactic-scale gravitational wave detectors. Monitoring pulsars for over a decade has finally paid off: long-term radio observations, advanced technology, and sophisticated data analysis techniques led to the result reported today around the globe.

The four constituent PTAs of the IPTA are now combining their latest data with the data from a pulsar timing experiment using the MeerKAT Telescope in South Africa, MeerTime, to form the most sensitive data set under the auspices of the IPTA. The cross-PTA platform provided by the IPTA enables scientists from across the globe to combine their talents in this search, which is likely to open a new window to the Universe.

These data are promised to be the most sensitive and hopefully will confirm the nature of the observed signal as gravitational waves. A long road is still ahead of us to understand the source of these GWs: do we observe a signal from supermassive black hole binaries or a signal from the early Universe? We are entering an era of great discoveries. The future is bright in gravitational waves.