Constraining the nHz Gravitational-wave Background with an X-Ray Pulsar-timing Array from NICER Observations

Abstract We present constraints on the nHz gravitational-wave background (GWB) using X-ray pulsar-timing data from the Neutron Star Interior Composition Explorer (NICER). By analyzing six millisecond pulsars over a 6 yr observational baseline, we employed a Bayesian framework to model noise components and search for a common red signal consistent with a GWB from supermassive black hole binaries (assuming a spectral index γ gwb = 13/3). Our results show no significant evidence for a GWB, yielding a 95% upper limit of log 10 ( A gwb ) < − 13.4 . Weak evidence for Hellings–Downs spatial correlations was found ( S = 2.5), though the signal remains statistically inconclusive. Compared to radio and γ -ray pulsar-timing arrays, the NICER constraint is currently less stringent but demonstrates the feasibility of X-ray timing with NICER for GWB studies and highlights the potential for improved sensitivity with future X-ray missions.