2026ApJS..282...42C

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 ${\mathrm{log}}_{10}({A}_{{\rm{gwb}}})\lt -13.4$. 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.