Constraining the Nanohertz Gravitational Wave Background with an X-ray Pulsar Timing Array from NICER observations

TL;DR

This study uses six years of NICER X-ray pulsar timing data to set upper limits on the nanohertz gravitational wave background, demonstrating the method's feasibility despite current sensitivity limitations.

Contribution

First application of X-ray pulsar timing with NICER to constrain the nanohertz gravitational wave background, establishing a new observational approach.

Findings

No significant GWB detection; upper limit set at log10(A_gwb)<-13.4

Weak evidence for Hellings-Downs correlations (S=2.5)

NICER constraints are less stringent than radio and gamma-ray pulsar timing arrays

Abstract

We present constraints on the nanohertz gravitational wave background (GWB) using X-ray pulsar timing data from the Neutron Star Interior Composition Explorer(\textit{NICER}). By analyzing six millisecond pulsars over a six-year 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 $\gamma_{\rm gwb}=13/3$). Our results show no significant evidence for a GWB, yielding a 95\% upper limit of $\log_{10}(A_{\rm gwb})<-13.4$. Weak evidence for Hellings-Downs spatial correlations was found (S=2.5), though the signal remains statistically inconclusive. Compared to radio and $\gamma$-ray pulsar timing arrays, the \textit{NICER} constraint is currently less stringent but demonstrates the feasibility of X-ray timing with \textit{NICER} for GWB studies and highlights the potential for improved sensitivity with future X-ray missions.