A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background

Abstract

After large galaxies merge, their central supermassive black holes are expected to form binary systems. Their orbital motion should generate a gravitational wave background (GWB) at nanohertz frequencies. Searches for this background use pulsar timing arrays, which perform long-term monitoring of millisecond pulsars at radio wavelengths. We used 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 × 10−14at a frequency of 1 year–1. The sensitivity is expected to scale withtobs, the observing time span, astobs−13/6. This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.