Abstract
Evidence has emerged for a stochastic signal correlated among 67 pulsars
within the 15-year pulsar-timing data set compiled by the NANOGrav
collaboration. Similar signals have been found in data from the European,
Indian, Parkes, and Chinese PTAs. This signal has been interpreted as
indicative of the presence of a nanohertz stochastic gravitational wave
background. 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 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 gravitational wave background in
the NANOGrav 15-year 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.