The NANOGrav 15 Yr Data Set: Removing Pulsars One by One from the Pulsar Timing Array
Authors: Agazie Gabriella , Anumarlapudi Akash , Archibald Anne M. , Arzoumanian Zaven , Baier Jeremy G. , Baker Paul T. , Bécsy Bence , Blecha Laura , Brazier Adam , Brook Paul R. , Burke-Spolaor Sarah , Casey-Clyde J. Andrew , Charisi Maria , Chatterjee Shami , Cohen Tyler , Cordes James M. , Cornish Neil J. , Crawford Fronefield , Cromartie H. Thankful , Crowter Kathryn , DeCesar Megan E. , Demorest Paul B. , Deng Heling , Dey Lankeswar , Dolch Timothy , Ferrara Elizabeth C. , Fiore William , Fonseca Emmanuel , Freedman Gabriel E. , Gardiner Emiko C. , Garver-Daniels Nate , Gentile Peter A. , Gersbach Kyle A. , Glaser Joseph , Good Deborah C. , Guertin Lydia , Gültekin Kayhan , Hazboun Jeffrey S. , Jennings Ross J. , Johnson Aaron D. , Jones Megan L. , Kaiser Andrew R. , Kaplan David L. , Kelley Luke Zoltan , Kerr Matthew , Key Joey S. , Laal Nima , Lam Michael T. , Lamb William G. , Larsen Bjorn , W. Lazio T. Joseph , Lewandowska Natalia , Liu Tingting , Lorimer Duncan R. , Luo Jing , Lynch Ryan S. , Ma Chung-Pei , Madison Dustin R. , McEwen Alexander , McKee James W. , McLaughlin Maura A. , McMann Natasha , Meyers Bradley W. , Meyers Patrick M. , Middleton Hannah , Mingarelli Chiara M. F. , Mitridate Andrea , Moore Christopher J. , Ng Cherry , Nice David J. , Ocker Stella Koch , Olum Ken D. , Pennucci Timothy T. , Perera Benetge B. P. , Pol Nihan S. , Radovan Henri A. , Ransom Scott M. , Ray Paul S. , Romano Joseph D. , Runnoe Jessie C. , Saffer Alexander , Sardesai Shashwat C. , Schmiedekamp Ann , Schmiedekamp Carl , Schmitz Kai , Shapiro-Albert Brent J. , Siemens Xavier , Simon Joseph , Siwek Magdalena S. , Sosa Fiscella Sophia V. , Stairs Ingrid H. , Stinebring Daniel R. , Stovall Kevin , Susobhanan Abhimanyu , Swiggum Joseph K. , Taylor Stephen R. , Turner Jacob E. , Unal Caner , Vallisneri Michele , Vecchio Alberto , Vigeland Sarah J. , Wahl Haley M. , Witt Caitlin A. , Wright David , Young Olivia 
Published: January 2025

Abstract
Evidence has emerged for a stochastic signal correlated among 67 pulsars within the 15 yr pulsar-timing data set compiled by the NANOGrav collaboration. Similar signals have been found in data from the European, Indian, Parkes, and Chinese pulsar timing arrays. This signal has been interpreted as indicative of the presence of a nanohertz stochastic gravitational-wave background (GWB). To explore the internal consistency of this result, we investigate how the recovered signal strength changes as we remove the pulsars one by one from the data set. We calculate the signal strength using the (noise-marginalized) optimal statistic, a frequentist metric designed to measure the correlated excess power in the residuals of the arrival times of the radio pulses. We identify several features emerging from this analysis that were initially unexpected. The significance of these features, however, can only be assessed by comparing the real data to synthetic data sets. After conducting identical analyses on simulated data sets, we do not find anything inconsistent with the presence of a stochastic GWB in the NANOGrav 15 yr data. The methodologies developed here can offer additional tools for application to future, more sensitive data sets. While this analysis provides an internal consistency check of the NANOGrav results, it does not eliminate the necessity for additional investigations that could identify potential systematics or uncover unmodeled physical phenomena in the data.