The North American Nanohertz Observatory for Gravitational Waves

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a pulsar timing array collaboration based in the USA and Canada, and is part of the International Pulsar Timing Array (IPTA) consortium. In this article we focus on the history of the collaboration and the milestones that have been achieved. Data sets, analysis methods and interpretation have been done by NANOGrav since 2007, leading to the announcement in June 2023 of evidence of a gravitational wave (GW) background by several IPTA members. Using the 15 year data set, NANOGrav has reported a nominal amplitude of around 2.4×10^-15 for a common red signal and a significance of ~3.5σ for the characteristic spatial correlations required for a GW background. The expected increase in sensitivity through combination of data sets, such as the upcoming IPTA DR3, and future data sets will confirm the GW origin of the signal, which should constrain the properties of the source and the implications of various theories of gravity.