A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background
The Fermi-LAT Collaboration*†
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 −14 at a frequency of 1 year –1 . The sensitivity is expected to scale with t obs , the observing time span, as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:msubsup><mml:mi>t</mml:mi><mml:mrow><mml:mtext>obs</mml:mtext></mml:mrow><mml:mrow><mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>13</mml:mn></mml:mrow><mml:mo>/</mml:mo><mml:mn>6</mml:mn></mml:mrow></mml:mrow></mml:msubsup></mml:mrow></mml:math> . This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.