Improving timing sensitivity in the microhertz frequency regime: Limits from PSR J1713+0747 on gravitational waves produced by supermassive black hole binaries

We search for continuous gravitational waves (CGWs) produced by individual supermassive black hole binaries in circular orbits using high-cadence timing observations of PSR J1713+0747. We observe this millisecond pulsar using the telescopes in the European Pulsar Timing Array with an average cadence of approximately 1.6 d over the period between 2011 April and 2015 July, including an approximately daily average between 2013 February and 2014 April. The high-cadence observations are used to improve the pulsar timing sensitivity across the gravitational wave frequency range of 0.008-5 mu Hz. We use two algorithms in the analysis, including a spectral fitting method and a Bayesian approach. For an independent comparison, we also use a previously published Bayesian algorithm. We find that the Bayesian approaches provide optimal results and the timing observations of the pulsar place a 95 per cent upper limit on the sky-averaged strain amplitude of CGWs to be less than or similar to 3.5 x 10(-13) at a reference frequency of 1 mu Hz. We also find a 95 per cent upper limit on the sky-averaged strain amplitude of low-frequency CGWs to be less than or similar to 1.4 x 10(-14) at a reference frequency of 20 nHz.