Gamma-ray Observations of Millisecond Pulsars Constrain the Low-frequency Gravitational Wave Background

Binary supermassive black holes in the hearts of merged galaxies fill the Universe with a low-frequency gravitational wave background (GWB). Long-term monitoring of millisecond pulsars (MSPs) with radio telescopes has recently revealed a signal which could originate from a nHz GWB with a strain amplitude A_gwb ~ 2-3×10^-15, but which could have other astrophysical or measurement-related origins. The Fermi Large Area Telescope has enabled gamma-ray observations of MSPs, and using 35 bright MSPs and 12.5yr of data, we have searched for a GWB and placed a 95% confidence upper limit on A_gwb <10^-14. This new, independent constraint is also the most direct, because it relies on simple models and gamma-ray data which are free from many of the confounding effects present in radio observations. The sensitivity is expected to improve to A_gwb = 2×10^-15 with ten years of additional data, which will rule out or strengthen the case for a GWB origin of the signal. This result, not envisioned before the launch of Fermi, represents a powerful new capability in gravitational wave science.