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Updated constraint for the gravitational wave background from the gamma-ray Pulsar Timing Array

Serena Valtolina1,2,3,*, Colin J. Clark1,2, Rutger van Haasteren1,2, Aurélien Chalumeau4,5, H. Thankful Cromartie6,†, Matthew Kerr7, Lars Nieder1,2, and Aditya Parthasarathy4,8,3

  • *Contact author: svaltolina@mpifr-bonn.mpg.de
  • †Present address: Resident at Naval Research Laboratory, Washington, DC 20375, USA.
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Phys. Rev. D 113, 063061 – Published 30 March, 2026

DOI: https://doi.org/10.1103/rrz1-mdkg

Abstract

Fermi Large Area Telescope observations of gamma-ray pulsars can be used to build a pulsar timing array (PTA) experiment to search for gravitational wave (GW) signals at nanohertz frequencies. At those frequencies, the dominant signal is expected to be a stochastic gravitational wave background (GWB) produced by the incoherent superposition of the quasimonochromatic GW emissions from a population of supermassive black hole binaries. While the radio PTAs have recently announced compelling evidence for a GWB signal with a power law spectrum of strain amplitude ≈2–3×10−15 (at the frequency of 1  yr−1), in 2022, an analysis of 12.5 years of Fermi data for 35 pulsars led to an upper limit of 1×10−14 for the GWB amplitude. The analysis was carried out on times of arrival obtained by folding from six months up to one year of photon observations. A photon-by-photon approach was also tested to infer constraints on the GWB amplitude from individual pulsars, but without accounting for the cross-pulsar correlations that a GWB would induce. Here, we reanalyze the same dataset using a regularized likelihood method that correctly models cross-pulsar correlations directly from the photons, while additionally marginalizing over the uncertain pulse profile shape. While the two methods are not expected to have significant differences in sensitivity, we prove through simulations of gamma-ray PTA datasets that the photon-by-photon method for GWB recoveries is, statistically, more robust. The resulting upper limit obtained for the GWB strain amplitude is 1.2×10−14, indicating that the improved method yields a consistent result with the previous analyses.

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Phys. Rev. D 113, 063061– Published 30 March, 2026
  • Received 18 November 2025
  • Accepted 13 February 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

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