Chinese Pulsar Timing Array Upper Limits on Microhertz Gravitational Waves from Supermassive Black-hole Binaries Using PSR J1713+0747 FAST Data

Abstract We derive the gravitational-wave (GW) strain upper limits from resolvable supermassive black-hole binaries using the data from the Five-hundred-meter Aperture Spherical radio Telescope, in the context of the Chinese Pulsar Timing Array project. We focus on circular orbits in the μ Hz GW frequency band between 10 −7 and 3 × 10 −6 Hz. This frequency band is higher than the traditional pulsar timing array band and is less explored. We used the data of the millisecond pulsar PSR J1713+5307 observed between 2019 August and 2021 April. A dense observation campaign was carried out in 2020 September to allow for the μ Hz band coverage. Our sky-averaged continuous source upper limit at the 95% confidence level at 1 μ Hz is 1.26 × 10 −12 , while the same limit in the direction of the pulsar is 4.77 × 10 −13 .