A Gamma-ray Pulsar Timing Array Constrains the Nanohertz Gravitational Wave Background

TL;DR

This paper uses 12.5 years of gamma-ray data from Fermi to establish limits on the nanohertz gravitational wave background, providing an independent verification method alongside traditional radio pulsar timing arrays.

Contribution

It introduces a gamma-ray pulsar timing array that independently constrains the gravitational wave background, complementing radio-based methods.

Findings

Set a 95% credible limit on the GWB strain of 1.0×10⁻¹⁴ at 1 yr⁻¹

Demonstrated gamma-ray data can independently probe the GWB

Provided a check on radio noise models for pulsar timing

Abstract

After large galaxies merge, their central supermassive black holes are expected to form binary systems whose orbital motion generates a gravitational wave background (GWB) at nanohertz frequencies. Searches for this background utilize pulsar timing arrays, which perform long-term monitoring of millisecond pulsars (MSPs) at radio wavelengths. We use 12.5 years of Fermi Large Area Telescope data to form a gamma-ray pulsar timing array. Results from 35 bright gamma-ray pulsars place a 95\% credible limit on the GWB characteristic strain of $1.0\times10^{-14}$ at 1 yr$^{-1}$, which scales as the observing time span $t_{\mathrm{obs}}^{-13/6}$. This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.